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Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

Fabio Trevisan
 

Hello folks at Tekscopes,
First of all, apologies for my first message being so long, but I`m excited!
I finally own my first Tektronix!
I just got a Tek 464 with DM44 option!!! S.N. B132839
It's looking really great for a 40 years fellow and, what's better, it was
cheap if you consider that I bought it in Brazil, from a Brazilian and
surprisingly.... It's EVEN WORKING!!!.
Given its price of BRL 465,00 (roughly USD 140,00, to be paid in 12 months,
shipping included).
You might be surprised by my remark, but it comes from the fact that in
Brazil these things are very expensive (even when used, even when in crappy
condition) so, it's very rare to find anything from Tek for this price and
still working.

I plan to restore it to mint condition as much as I possibly can and use it
for my hobbyist projects.

For now, I`ll just share the photo (at the end) that I took after I just
unpack it (last week) .

Since then, I already fixed 3 problems in it and there are basically 2
major problems left:

1. A problem in DM44. It would only show the two Intensified marks on
vertical mode "ALT"... in CH1 mode it would only show the "Delay Time
Position" mark and in CH2 mode it would only show the "Delta time" mark...
but never both of them on the same trace.
I found it to be a missing connection to +5V to the ALT mode switch (it had
the wire going to the DM44 on pin 1, but no +5V in the wiper). I`m still in
doubt if it's missing since it came from factory and nobody noticed it or
missed the functionality of measuring time with only one trace (whether
CH1, CH2, or ADD).

2. A problem is the vertical preamp. CH1 would show superimposed to its own
signal, about 25% of whatever signal that would be displaying at channel 2,
including CH2 position,... Besides... there was a sort of flicker
Problem got fixed by itself after I had to move the vert. preamp board from
my way, to be able to access the Vert. Mode switch board. When I put the
Vert Preamp board back in place, the problem was gone...

3. Calibrator was showing a trapezoidal waveform... More or less like if
the probe would be under compensated, but not caused by the probe...
Problem was a loose ground connection from the 10R resistor that sets the
30mA current. The resistor goes to ground not on the interface board, but
at the back of the chassis, by means of a bolt and interlocked nuts. The
nuts where tightly interlocked, but the bolt/nuts set were not pressing the
chassis and ground wire good enough.

*Problems remaining (that I appreciate any suggestion from you guys)*:

4. DM44 is acting unpredictably... sometimes it freezes up or just show
random numbers... sometimes it works fine for Volts and Ohms... but even
then, when switched to mode "Time", it would only show valid readings in
ranges multiple of 2 (0.2s, 20ms, 2ms, 0.2ms, 20us, 2us and 0.2us). Even
though, the value displayed is 5 times bigger than it should be for that
scale (a 1 division measurement in the 0.2ms range reads, that shoould read
0.2ms, reads actually 1ms on the display).
I`m suspicious of false contacts from all too many ICs that are socketed,
and possibly some flat cable incorrectly connected between the horizontal
timing board and the DM44. I`m still going to look at it further this week
and will let you know.

5. Last but not least, and most worrisome to me... The CRT beam works just
fine for about 30~40 minutes and after that it fades abruptly. It's still
visible for a couple of minutes more with the intensity at MAX and there's
a great increase of the deflection factor in both directions (Horizontal
and Vertical deflections increase by about 30%).
I know already, from these 2 symptoms that the acceleration voltage must be
going down for some reason, after H.V. step up power supply heats up, but
I'm still not sure the actual culprit.
I already did some cleaning work in the whole H.V. area, to eliminate
eventual leakages caused by the black dust deposits and moisture that could
be causing the HV to overheat and give up... but this "care" didn't help
whatsoever.

At this point, of course that I`m suspicious that the H.V transformer is
probably guilty... (I hope not but I know the odds are against me! Sigh!).
I haven't done much troubleshooting so far.... Only thing I was able to do
is to check the Cathode voltage, and it's rock steady at -1476V for several
minutes, from the moment I turn it on up to about 20 minutes of power.
I didn't measure it yet after it's faded because I don't want to leave it
on for too long after that, to avoid frying anything (I`m not sure it would
fry anyway, but better be sure than sorry).

I`m looking forward to sharing with you the results of this restoration,
the problems found and the fixes that will be applied in the process.




Best regards,

Fabio Trevisan

 

On Mon, 12 Sep 2016 20:19:06 -0300, you wrote:

1. A problem in DM44. It would only show the two Intensified marks on
vertical mode "ALT"... in CH1 mode it would only show the "Delay Time
Position" mark and in CH2 mode it would only show the "Delta time" mark...
but never both of them on the same trace.
Someone who has used a DM44 can say for sure but I need to take a
closer look at the schematics to figure it out. The problem may be
the ALT mode switch issue you found; see below.

Normally the 464 delay time control would move both intensified marks
and the DM44 delta delay time control would move the second one only
changing the distance between them but there is an optional change to
the DM44 which makes the marks move independently and it seems that
was done to yours.

I was kind of surprised that there are no videos on Youtube showing
what the DM44 can do. All of them look like people just randomly
pressing buttons.

I found it to be a missing connection to +5V to the ALT mode switch (it had
the wire going to the DM44 on pin 1, but no +5V in the wiper). I`m still in
doubt if it's missing since it came from factory and nobody noticed it or
missed the functionality of measuring time with only one trace (whether
CH1, CH2, or ADD).
The ALT mode switch in the 464/466 is the default position for the
switch bank so without the DM44, it does not connect to anything; when
none of the other switches are pressed, then it defaults in ALT mode.
When the DM44 is installed, it looks like the +5 volt connection needs
to be added but the DM44 schematics do not show it. The late 466
schematics do not show the ALT mode switch at all.

Maybe someone retrofitted the DM44 to your 464 and they missed this
needed change.

2. A problem is the vertical preamp. CH1 would show superimposed to its own
signal, about 25% of whatever signal that would be displaying at channel 2,
including CH2 position,... Besides... there was a sort of flicker
Problem got fixed by itself after I had to move the vert. preamp board from
my way, to be able to access the Vert. Mode switch board. When I put the
Vert Preamp board back in place, the problem was gone...
That was probably a loose mechanical connection.

...

*Problems remaining (that I appreciate any suggestion from you guys)*:

4. DM44 is acting unpredictably... sometimes it freezes up or just show
random numbers... sometimes it works fine for Volts and Ohms... but even
then, when switched to mode "Time", it would only show valid readings in
ranges multiple of 2 (0.2s, 20ms, 2ms, 0.2ms, 20us, 2us and 0.2us). Even
though, the value displayed is 5 times bigger than it should be for that
scale (a 1 division measurement in the 0.2ms range reads, that shoould read
0.2ms, reads actually 1ms on the display).
I`m suspicious of false contacts from all too many ICs that are socketed,
and possibly some flat cable incorrectly connected between the horizontal
timing board and the DM44. I`m still going to look at it further this week
and will let you know.
I would reseat all of the ICs and anything else in a socket. I do not
know if it uses them but the edge wipe TI sockets are not very
reliable.

5. Last but not least, and most worrisome to me... The CRT beam works just
fine for about 30~40 minutes and after that it fades abruptly. It's still
visible for a couple of minutes more with the intensity at MAX and there's
a great increase of the deflection factor in both directions (Horizontal
and Vertical deflections increase by about 30%).

I know already, from these 2 symptoms that the acceleration voltage must be
going down for some reason, after H.V. step up power supply heats up, but
I'm still not sure the actual culprit.
Yep, that is a drop in the CRT acceleration voltage.

I already did some cleaning work in the whole H.V. area, to eliminate
eventual leakages caused by the black dust deposits and moisture that could
be causing the HV to overheat and give up... but this "care" didn't help
whatsoever.

At this point, of course that I`m suspicious that the H.V transformer is
probably guilty... (I hope not but I know the odds are against me! Sigh!).
I haven't done much troubleshooting so far.... Only thing I was able to do
is to check the Cathode voltage, and it's rock steady at -1476V for several
minutes, from the moment I turn it on up to about 20 minutes of power.
I didn't measure it yet after it's faded because I don't want to leave it
on for too long after that, to avoid frying anything (I`m not sure it would
fry anyway, but better be sure than sorry).
I can understand that. There are some things which can be checked
before resorting to a measurement while the high voltage supply has
failed.

I`m looking forward to sharing with you the results of this restoration,
the problems found and the fixes that will be applied in the process.

Best regards,

Fabio Trevisan
Take a close look at C1487 which decouples the unregulated +15 volt
supply for the high voltage inverter. If it is bad, then the inverter
will have trouble supplying enough power.

Fabio Trevisan
 

Hi Dave, Thanks a lot for your comments (so promptly!!)

---In TekScopes@..., <davidwhess@...> you answered :
> Someone who has used a DM44 can say for sure but I need to take a closer look at the schematics to
> figure it out. The problem may be the ALT mode switch issue you found; see below. >Normally the 464 delay time control would move both intensified marks and the DM44 delta delay time >control would move the second one only changing the distance between them but there is an optional >change to the DM44 which makes the marks move independently and it seems that was done to yours. >I was kind of surprised that there are no videos on Youtube showing what the DM44 can do. All of them >look like people just randomly pressing buttons. Indeed you're right about the two different configurations for the "Delta Time" control.
I got to know about it from the manual before opening it up the first time, so I tried it both ways to see if one of the configurations would eliminate or change the problem somehow, but don't...
Both problems were present in either mode of operation.
As for your comment of YouTube, I haven't really searched YouTube for that as I`m not very much into Youtube for technical tutorials.
It's not that I`m that kind of "grumpy" either who wouldn't take anything knew... I like very much the videos from EEVBlog. That guy knows how to present things in video!


You're also right about the ALT mode switch. I wasn't sure of its upper side,which actually has its "pushed" contact connected to +5V and therefore could be very well being used for something else in the scope, but one way or the other, having +5 on the pushed position is the opposite than what the DM44 requires, so I knew that the DM44 no matter what would need to be connected to the lower side of the switch (which, apart from the wire going to the DM44, seemed to be completely unused).
Then, after checking it thoroughly, I connected the wiper of the lower side to the +5V and voilá, I can see two intensified "marks" in all vertical modes. That part of the problem was fixed already when I wrote yesterday but thanks for your comments as they helped me validate my conclusions..


>The ALT mode switch in the 464/466 is the default position for the


>switch bank so without the DM44, it does not connect to anything; when


>none of the other switches are pressed, then it defaults in ALT mode.


>When the DM44 is installed, it looks like the +5 volt connection needs


>to be added but the DM44 schematics do not show it. The late 466


>schematics do not show the ALT mode switch at all.






>Maybe someone retrofitted the DM44 to your 464 and they missed this


>needed change.


Once again, you are "on the spot" about the retrofitting... Today I found the ultimate proof of that!
They didn't just miss that jumper to +5V on the ALT switch!
While looking for why the "Time" function would only work on 2's timebases, I discovered that the 3 resistors that should make up for the 1-2-5 voltage divider ARE MISSING! from the 464's Hor.Timebase board (R1142, R1143 and R1147).
And not just missing... There are clear signs that they were removed.
Now it's getting funny and it reminds me of the story about Jim Williams at his first job at MIT, when he would ask colleagues from other departments to "inflict" defects to their instruments so that he could fix it.
Now anything is possible because I know I`m working on a set that has been obviously tampered with.

> That was probably a loose mechanical connection.
I Agree! The CH2 leak problem into CH1 is gone anyway
> I would reseat all of the ICs and anything else in a socket. I do not
know if it uses them but the edge wipe TI sockets are not very
reliable. I shall reseat the ICs when I have some time... I already reseat most transistors. Once I`m done with the DM44 and know it works in all functions and ranges (when I know I won't have to replace anything else), I`m even thinking of soldering them all to the board...
> Yep, that is a drop in the CRT acceleration voltage.


> I can understand that. There are some things which can be checked


> before resorting to a measurement while the high voltage supply has


> failed.


> Take a close look at C1487 which decouples the unregulated +15 volt
supply for the high voltage inverter. If it is bad, then the inverter will
have trouble supplying enough powerThanks for that last tip. I will check C1487, and the unreg +15V supply for voltage and ripple.
I`m thinking of monitoring the current drawn by the inverter, as it heats-up , to see if there's any noticeable bump or dip when it reaches the "dropout". I have a DMM with RS232 logging that will fit this bill.


Thanks again for your comments... I`ll keep all posted.
Rgrds,
Fabio

Fabio Trevisan
 

Sorry Dave, My reply to you ended up with the formatting completely screwed up.
I`ll try to make it better the next time.
Rgrds, Fabio

 

On 13 Sep 2016 08:46:02 -0700, you wrote:

Sorry Dave, My reply to you ended up with the formatting completely screwed up.
I`ll try to make it better the next time.
Rgrds, Fabio
It was no problem; many replies on the list get mangled for whatever
reason.

On 13 Sep 2016 08:42:37 -0700, you wrote:

Hi Dave, Thanks a lot for your comments (so promptly!!)

I got to know about it from the manual before opening it up the first time, so I tried it both ways to see if one of the configurations would eliminate or change the problem somehow, but don't...
Both problems were present in either mode of operation.
As for your comment of YouTube, I haven't really searched YouTube for that as I`m not very much into Youtube for technical tutorials.
It's not that I`m that kind of "grumpy" either who wouldn't take anything knew... I like very much the videos from EEVBlog. That guy knows how to present things in video!
I had a general idea about how the DM44 delta delayed sweep function
should work but I found the description in the manual was ambiguous so
I wanted to find an example on video. I suspect most users who pick
up a DM44 have no idea what delta delayed sweep is.

You're also right about the ALT mode switch. I wasn't sure of its upper side,which actually has its "pushed" contact connected to +5V and therefore could be very well being used for something else in the scope, but one way or the other, having +5 on the pushed position is the opposite than what the DM44 requires, so I knew that the DM44 no matter what would need to be connected to the lower side of the switch (which, apart from the wire going to the DM44, seemed to be completely unused).
I was surprised that the ALT switch is not used except as a
placeholder in these oscilloscopes. The late 466 manual does not show
it at all and the 464 and early 466 manuals show it but with no
connections.

The DM44 is suppose to be a factory installed option so it does not
quite surprise me that the DM44 service manual does not cover the
needed modification to the ALT switch.

Then, after checking it thoroughly, I connected the wiper of the lower side to the +5V and voilá, I can see two intensified "marks" in all vertical modes. That part of the problem was fixed already when I wrote yesterday but thanks for your comments as they helped me validate my conclusions.
The DM44 has a toggle set/reset flip-flop which determines if the
delayed or delta delayed sweep control is used but it was not
immediately clear to me from only the DM44 schematic under which
conditions it toggles. I would have to study the oscilloscope
schematics as well to figure it out.

The ALT mode switch in the 464/466 is the default position for the
switch bank so without the DM44, it does not connect to anything; when
none of the other switches are pressed, then it defaults in ALT mode.
When the DM44 is installed, it looks like the +5 volt connection needs
to be added but the DM44 schematics do not show it. The late 466
schematics do not show the ALT mode switch at all.
Maybe someone retrofitted the DM44 to your 464 and they missed this
needed change.
Once again, you are "on the spot" about the retrofitting... Today I found the ultimate proof of that!
They didn't just miss that jumper to +5V on the ALT switch!

While looking for why the "Time" function would only work on 2's timebases, I discovered that the 3 resistors that should make up for the 1-2-5 voltage divider ARE MISSING! from the 464's Hor.Timebase board (R1142, R1143 and R1147).

And not just missing... There are clear signs that they were removed.
Now it's getting funny and it reminds me of the story about Jim Williams at his first job at MIT, when he would ask colleagues from other departments to "inflict" defects to their instruments so that he could fix it.
Now anything is possible because I know I`m working on a set that has been obviously tampered with.
I remember that story about Jim Williams. They also had differential
diagnostic competitions where the objective was to figure out the
problem with a minimum number of tests.

I would reseat all of the ICs and anything else in a socket. I do not
know if it uses them but the edge wipe TI sockets are not very
reliable.
I shall reseat the ICs when I have some time... I already reseat most transistors. Once I`m done with the DM44 and know it works in all functions and ranges (when I know I won't have to replace anything else), I`m even thinking of soldering them all to the board...
I do not think soldering the ICs or other socketted parts in is
warranted. If you remove the sockets, they can be replaced with
better sockets. Machined collet pin sockets are very reliable and if
you want to spend a little extra time on it, the collect pins can be
cut out of the replacement sockets and used individually:

<https://s14.postimg.io/3og36ice9/DC505_Lowest_Profile_Collet_Socket_2a.jpg>
<https://s14.postimg.io/l0gfry5vl/DC505_723_Socket.jpg>

Yep, that is a drop in the CRT acceleration voltage.
I can understand that. There are some things which can be checked
before resorting to a measurement while the high voltage supply has
failed.

Take a close look at C1487 which decouples the unregulated +15 volt
supply for the high voltage inverter. If it is bad, then the inverter will
have trouble supplying enough powerThanks for that last tip. I will check C1487, and the unreg +15V supply for voltage and ripple.
I`m thinking of monitoring the current drawn by the inverter, as it heats-up , to see if there's any noticeable bump or dip when it reaches the "dropout". I have a DMM with RS232 logging that will fit this bill.
Some people put a 1 ohm resistor in series with the fuse F1487 to
monitor the collector current of the output transistor.

If you want to monitor the error amplifier, I think the voltage at the
collector of Q1476 is the best spot.

Another failure mode has occurred to me; Q1486 could be failing as it
heats up which is something I have seen before in transistors.

Fabio Trevisan
 

Hi David, Just to give a feedback:
It's a long message and OF COURSE don't feel obliged to either to read it or answer... telling it to someone is my way to organize the ideas and - maybe - the outcome can be disclosed later and help someone.


On the DM44 issues, I installed the missing R1142, R1143 and R1147 on the A&B Timing Switch board, and now it can measure time on all horizontal scales.


Oh... And there's a TEK trick there...


On the 464 service manual "A&B Timing Switch Diagram 7", it shows the 3 resistors above as 2K, 1K and 1K respectively... and I naively installed them exactly as such... just to find out that the measurements were wrong in scale. (but at least, this time it was measuring in all horizontal time-bases, and not just on the 2s multiples).
Went to the Parts list of the 464, and it stated: Check DM44 manual for Resistor Values...!!!
Then, went to DM44 manual and, next to the DM44's parts list, there's an additional section describing all the values of all components that need to be added, removed, or changed values, for each scope model that the DM44 fits in...
Guess what, the correct values are 500R, 250R and 250R..!!! And not 2k, 1k and 1k as it was drawn on 464's diagram.


DM 44 is still sometimes crazy (less now than it was at the beginning), but I assume it's some bad contact somewhere which I will eventually find out.


Now, regarding the HV supply... I did some progress in making various checks but still didn't find the root cause for the drop of HV after the scope heats-up...


1. When the HV drops, cathode voltage drops to around 1000V, and then, in 20~30 seconds, it drops to 920V. when I shut it off.


2. Checked the Unregulated +15 V and it reads about 24V (just a tad higher than what shows in the schematics). The ripple is about 1Vpp while in normal operation and, after the HV drops, the ripple increased to almost 2Vpp until when I shut it off...
2.b. Still at the Unreg +15V, as the manual doesn't state how much is the acceptable ripple, I did the following... I added a brand new 6800uF cap in parallel (to the original which is nominal 5500uF +100% -10%)... and the ripple under normal condition dropped from 1Vpp to 0.5Vpp... I didn't care to look after the HV drops because it doesn't add anything to the conclusion.
So, IMHO, the original cap is performing roughly the same as the brand new 6800uF cap... In other words... C1487 is GOOD.


3. I don't have a temperature probe, but using my finger (with the scope off, and after discharging!), and while the HV is still normal... both the driving transistor and the HV transformer are hot, but not too hot... I can keep my finger on them, so they're about or below 70 Deg Celsius.
But when HV drops and I leave it on for about 20~30 secs which is as far as I went, both the transistor and the HV transformer becomes VERY Hot, to the point that I cannot keep my finger on one of the pins from the HV transformer (I can keep the finger on its plastic body though)


4. I don't have a reference stated on the manual, but the Post Acceleration anode is - while in normal operation - at 7.22kV. I didn't measure it yet after HV is down. I suppose that it will be down by the same amount (33% below the 7.22kV)...


4b. I measured the 5 resistors of 3M Ohms in the HV multiplier and they're all up by 10%, like 3.3M each.
It's not ideal, but I don't think it's the cause for the cathode HV drop, which is the main issue.


4c. I managed to find some ESJA53-12 diodes, which are 12KV, 5mA fast recovery diodes (actually better than the originals, at 100ns, while the originals are 250ns, only the forward voltage is worse, at 37V while the originals are about 25V and this doesn't seem to be a problem here).
I already tested them reverse biased at the first stage of the HV multiplier (3.61kV) and they're quite promising at 1.1nA (an indirect measure... I actually measured how much voltage it allowed to leak to a 10Mohm voltmeter, and it read 11mV!!!)
I will try to replace - first only CR1503 at the cathode - and see if it helps.
Specially this one, which is mounted right under the transformer and soldered very closely to its pins, if it's leaking too much, as it gets heated by the transformer, it may be causing the whole mess.


If replacing the diode doesn't help, however, I'm afraid I will need to start looking for an HV transformer, as I`m not seeing much more that can be ruled out...
The HV caps seem to be in rather good condition, and they're HUGE (I imagine they're very robust)...


Rgrds,


Fabio

 

On 16 Sep 2016 17:07:00 -0700, you wrote:

Hi David, Just to give a feedback:

It's a long message and OF COURSE don't feel obliged to either to read it or answer... telling it to someone is my way to organize the ideas and - maybe - the outcome can be disclosed later and help someone.
Parsing a problem so it can be described to someone else is a powerful
technique.

1. When the HV drops, cathode voltage drops to around 1000V, and then, in 20~30 seconds, it drops to 920V. when I shut it off.

2. Checked the Unregulated +15 V and it reads about 24V (just a tad higher than what shows in the schematics). The ripple is about 1Vpp while in normal operation and, after the HV drops, the ripple increased to almost 2Vpp until when I shut it off...
This supply is unregulated so the variation from nominal is expected.
A change from 1Vpp to 2Vpp indicates that the current draw is
doubling.

2.b. Still at the Unreg +15V, as the manual doesn't state how much is the acceptable ripple, I did the following... I added a brand new 6800uF cap in parallel (to the original which is nominal 5500uF +100% -10%)... and the ripple under normal condition dropped from 1Vpp to 0.5Vpp... I didn't care to look after the HV drops because it doesn't add anything to the conclusion.
So, IMHO, the original cap is performing roughly the same as the brand new 6800uF cap... In other words... C1487 is GOOD.
Yep, doubling the capacitance will halve the ripple. The rule of
thumb for the 120 Hz output from a full wave 60 Hz rectifier is that
8200uF will yield 1 Vpp of ripple at 1 amp.

3. I don't have a temperature probe, but using my finger (with the scope off, and after discharging!), and while the HV is still normal... both the driving transistor and the HV transformer are hot, but not too hot... I can keep my finger on them, so they're about or below 70 Deg Celsius.
But when HV drops and I leave it on for about 20~30 secs which is as far as I went, both the transistor and the HV transformer becomes VERY Hot, to the point that I cannot keep my finger on one of the pins from the HV transformer (I can keep the finger on its plastic body though)

4. I don't have a reference stated on the manual, but the Post Acceleration anode is - while in normal operation - at 7.22kV. I didn't measure it yet after HV is down. I suppose that it will be down by the same amount (33% below the 7.22kV)...
My notes say it is 8.5 kilovolts. That is enough difference that I
would suspect failure of the high voltage multiplier is causing the
problem.

4b. I measured the 5 resistors of 3M Ohms in the HV multiplier and they're all up by 10%, like 3.3M each.
It's not ideal, but I don't think it's the cause for the cathode HV drop, which is the main issue.
The resistors could not cause the problem anyway.

4c. I managed to find some ESJA53-12 diodes, which are 12KV, 5mA fast recovery diodes (actually better than the originals, at 100ns, while the originals are 250ns, only the forward voltage is worse, at 37V while the originals are about 25V and this doesn't seem to be a problem here).
The PDA voltage will decrease a little bit but the effect on
deflection calibration will be minor.

I already tested them reverse biased at the first stage of the HV multiplier (3.61kV) and they're quite promising at 1.1nA (an indirect measure... I actually measured how much voltage it allowed to leak to a 10Mohm voltmeter, and it read 11mV!!!)
I will try to replace - first only CR1503 at the cathode - and see if it helps.
Specially this one, which is mounted right under the transformer and soldered very closely to its pins, if it's leaking too much, as it gets heated by the transformer, it may be causing the whole mess.
Leakage through either of the diodes will cause problems. The
capacitors could also be causing the problem.

I have not ordered from them yet, but these guys look like a good
source for high voltage diodes and capacitors:

<http://hvstuff.com/>

If replacing the diode doesn't help, however, I'm afraid I will need to start looking for an HV transformer, as I`m not seeing much more that can be ruled out...
The HV caps seem to be in rather good condition, and they're HUGE (I imagine they're very robust)...

Rgrds,

Fabio
I would change the high voltage capacitors before messing with the
transformer. Make sure the board around the high voltage multiplier
is clean when you finish working on it.

The oscilloscope and CRT will still work with the PDA missing although
deflection will be lower and brightness and sharpness will be
affected. You could temporarily remove C1582 to verify if the high
voltage multiplier is the problem.

Then I would take a close look at the cathode supply rectifiers and
capacitors around CR1503. And then the +600 volt bias supply built
around CR1512.

Fabio Trevisan
 

Hi Dave,
Thanks again for your feedback... I am suppressing my original post and
leave only your comments and my replies.
I hope this won't mangle the whole thing this time (Ì'm doing it from my
mail client).
Rgrds,
Fabio

2016-09-22 2:15 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

On 16 Sep 2016 17:07:00 -0700, you wrote:
Parsing a problem so it can be described to someone else is a powerful
technique.
Thanks for having the patience to go through it.

This supply is unregulated so the variation from nominal is expected.
A change from 1Vpp to 2Vpp indicates that the current draw is
doubling.
Since our last exchange, I did further progress in measurements, as well as
in troubleshooting resources..
I measured the current drawn by the inverter (while it's still operating
normally), and it's around 200mA.
I didn't have chances to measure it after HV drops, after the inverter runs
hot.

Yep, doubling the capacitance will halve the ripple. The rule of
thumb for the 120 Hz output from a full wave 60 Hz rectifier is that
8200uF will yield 1 Vpp of ripple at 1 amp.
From your rule of thumb, and from my previous conclusion that C1487 is
still at its nominal value,
the current being drawn from the +15V UNREG must be around 0.67A (making up
for a 1Vpp ripple on 5500uF).
Since the Inverter alone, draws just 0.2A, (while in normal operation) and
since the ripple doubles to 2Vpp when the inverter is in "fault"...
the net current out of the +15V UNREG is doubling to ~ 2 x 0.67A = 1.33A.
This means that the current drawn by the inverter alone is raising from
0.2A to 0.87A (almost 4.5 times).

My notes say it is 8.5 kilovolts. That is enough difference that I
would suspect failure of the high voltage multiplier is causing the
problem.
I concur with you that the 464 should be 8.5kV, But I have another theory!
I think the correct anode voltage is indeed in the 7kV ballpark for 2
reasons:
1. because this is exactly what is marked in the 464's HV's Cage Warning.
It says: CAUTION 7000V inside !!!
2. 7kV of PDA plus 1500V of cathode voltage makes up exactly for 8500V
acceleration !!!

Moreover, I`m considering that even the 7.22kV I`m measuring may be already
HIGH by 220V
and this can be exactly an indication of the inverter being driven harder
to compensate for some
deficiency on the cathode circuitry (either the winding or something else).
This rational is consistent with the other winding (600V) which also
measures around 620V
So, it really seems the problem is at the Cathode circuitry...
I will try to confirm that by measuring the HV doubler voltage or the 600V
while the scope
heats up towards to failure.

The resistors could not cause the problem anyway.
Agree, but I replaced them anyway, by glass insulated (3kV class) metal
film resistors. (but no appreciable change in status)

On my suggestion of possible replacements of the HV diodes... you wrote...
The PDA voltage will decrease a little bit but the effect on
deflection calibration will be minor.
Agreed! So much that I decided to take the step and replace all 3 HV diodes
(the 2 at the HV doubler and at the cathode circuit).

Leakage through either of the diodes will cause problems. The
capacitors could also be causing the problem.
I have not ordered from them yet, but these guys look like a good
source for high voltage diodes and capacitors:
<http://hvstuff.com/>
About hvstuff.com... I have seen their website before, I think that I will
ultimately need to source from
them if I need to replace the capacitors, as the sources in Brazil for HV
capacitors are either
unreliable or only affordable for the industries (no hobby market).
We used to have Newark/Element14 in Brasil but they closed their operations
here.

As I mentioned above, I decided to take my chances with the ESJA53-12
diodes I found on a loca store...
After I replaced the HV diodes, I noticed a great improvement... but
unfortunately it wasn't the definitive fix...
Now the HV doesn't seem to fail anymore if I run the scope without the
outer casing (cooler).
While this is good news in some sense, it made things more difficult to
troubleshoot.
When I install the outer casing, the time to fail increased from 20~30
minutes to about 1 hour.
I measured the leakage of the original cathode diode (CR1503) and under the
same voltage of about 3600V,
it's 25 times bigger than the 1.1nA that I measured in the new diodes,
but 28nA is still very small leakage and I think that it may just have
given the inverter a little bit more breath
so now it takes longer to fail.

I would change the high voltage capacitors before messing with the
transformer. Make sure the board around the high voltage multiplier
is clean when you finish working on it.
That's my next step... It will be difficult to find 6.8nF x 5kV around
here...
But at least for testing, I may resort to an association (parallel)
It's not HV friendly to have the uninsulated connections that would be
required
but there's plenty of space where the original capacitors are (they're
huge!)

The oscilloscope and CRT will still work with the PDA missing although
deflection will be lower and brightness and sharpness will be
affected. You could temporarily remove C1582 to verify if the high
voltage multiplier is the problem.
That tip is one that I wouldn't expect!
It would never occurred to me to remove the PDA altogether.
I didn't think it would ever work.
For the reasons I stated before, I don't think my problem in on the HV
or the HV doubler, but I will keep this advice if I need to troubleshoot
this area further.
For now, I will focus on the Cathode Supply circuitry and see if I get to
any conclusion.

Then I would take a close look at the cathode supply rectifiers and
capacitors around CR1503. And then the +600 volt bias supply built
around CR1512.
I've already checked the +600V supply and it seems alright.
This one is simpler to troubleshoot as the components are not that hard to
source.

Thanks again...
I will answer shortly the other message you answered on another topic...
The "double-peak"... That's another story.

Rgrds,

Fabio




 

On Tue, 27 Sep 2016 19:00:14 -0300, you wrote:

Hi Dave,
Thanks again for your feedback... I am suppressing my original post and
leave only your comments and my replies.
I hope this won't mangle the whole thing this time (Ì'm doing it from my
mail client).
Rgrds,
Fabio
I usually reply in line when posts get complicated enough.

I assembled and cleaned up a set of color schematics for the 464 which
Kurt has been nice enough to host at his site:

http://w140.com/tekwiki/wiki/464

2016-09-22 2:15 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

On 16 Sep 2016 17:07:00 -0700, you wrote:

...

My notes say it is 8.5 kilovolts. That is enough difference that I
would suspect failure of the high voltage multiplier is causing the
problem.
I concur with you that the 464 should be 8.5kV, But I have another theory!
I think the correct anode voltage is indeed in the 7kV ballpark for 2
reasons:
1. because this is exactly what is marked in the 464's HV's Cage Warning.
It says: CAUTION 7000V inside !!!
2. 7kV of PDA plus 1500V of cathode voltage makes up exactly for 8500V
acceleration !!!

Moreover, I`m considering that even the 7.22kV I`m measuring may be already
HIGH by 220V
and this can be exactly an indication of the inverter being driven harder
to compensate for some
deficiency on the cathode circuitry (either the winding or something else).
This rational is consistent with the other winding (600V) which also
measures around 620V
So, it really seems the problem is at the Cathode circuitry...
I will try to confirm that by measuring the HV doubler voltage or the 600V
while the scope
heats up towards to failure.
Ah, you are right. My 7000 notes break out PDA (post deflection
acceleration) and cathode acceleration voltages separately but my 465,
22xx, and 24xx notes do not. It is time to add a couple more columns.

The schematic and detailed theory section both say -1470 volts for the
cathode voltage and about 7kV for the PDA voltage. The theory section
says the cathode voltage is altered slightly between normal, variable
persistence, and fast storage modes. This will also affect the PDA
voltage.

The resistors could not cause the problem anyway.
Agree, but I replaced them anyway, by glass insulated (3kV class) metal
film resistors. (but no appreciable change in status)

On my suggestion of possible replacements of the HV diodes... you wrote...
The PDA voltage will decrease a little bit but the effect on
deflection calibration will be minor.
Agreed! So much that I decided to take the step and replace all 3 HV diodes
(the 2 at the HV doubler and at the cathode circuit).
When repairing these types of circuits, I prefer to replace all of the
high voltage resistors, capacitors, or diodes at the same time if they
are of a common type. It is just easier and safer this way.

Leakage through either of the diodes will cause problems. The
capacitors could also be causing the problem.
I have not ordered from them yet, but these guys look like a good
source for high voltage diodes and capacitors:
<http://hvstuff.com/>
About hvstuff.com... I have seen their website before, I think that I will
ultimately need to source from
them if I need to replace the capacitors, as the sources in Brazil for HV
capacitors are either
unreliable or only affordable for the industries (no hobby market).
We used to have Newark/Element14 in Brasil but they closed their operations
here.
You are not the first person in Brazil to mention having problems
finding parts.

As I mentioned above, I decided to take my chances with the ESJA53-12
diodes I found on a loca store...
After I replaced the HV diodes, I noticed a great improvement... but
unfortunately it wasn't the definitive fix...
Now the HV doesn't seem to fail anymore if I run the scope without the
outer casing (cooler).
While this is good news in some sense, it made things more difficult to
troubleshoot.
When I install the outer casing, the time to fail increased from 20~30
minutes to about 1 hour.
So it helped but did not fixed the problem.

I measured the leakage of the original cathode diode (CR1503) and under the
same voltage of about 3600V,
it's 25 times bigger than the 1.1nA that I measured in the new diodes,
but 28nA is still very small leakage and I think that it may just have
given the inverter a little bit more breath
so now it takes longer to fail.
But what is the leakage at higher operating temperature? Whatever
part is causing the problem is only doing so after it warms up.

I would change the high voltage capacitors before messing with the
transformer. Make sure the board around the high voltage multiplier
is clean when you finish working on it.
That's my next step... It will be difficult to find 6.8nF x 5kV around
here...
But at least for testing, I may resort to an association (parallel)
It's not HV friendly to have the uninsulated connections that would be
required
but there's plenty of space where the original capacitors are (they're
huge!)
Usually it is the voltage multiplier capacitors which have problems
and their value is not critical. The value for the capacitors which
filter the cathode supply affect the regulator's frequency
compensation which is partially controlled by the network built around
R1473; I think you can get away with increasing them to 10nF but I am
not positive about this.

 

On Tue, 27 Sep 2016 19:00:14 -0300, you wrote:

As I mentioned above, I decided to take my chances with the ESJA53-12
diodes I found on a loca store...
After I replaced the HV diodes, I noticed a great improvement... but
unfortunately it wasn't the definitive fix...
Now the HV doesn't seem to fail anymore if I run the scope without the
outer casing (cooler).
While this is good news in some sense, it made things more difficult to
troubleshoot.
When I install the outer casing, the time to fail increased from 20~30
minutes to about 1 hour.
I measured the leakage of the original cathode diode (CR1503) and under the
same voltage of about 3600V,
it's 25 times bigger than the 1.1nA that I measured in the new diodes,
but 28nA is still very small leakage and I think that it may just have
given the inverter a little bit more breath
so now it takes longer to fail.
I have not quite decided what to make of this but I have some ideas.
If the diodes were the problem, then I would have expected changing
them to have completely fixed it instead of making it a little better.

What I might try is looking for some other part which is getting hot
causing enough leakage to pull the high voltage supply voltages down.
If you do not have an infrared thermometer to scan with, a cotton swab
dipping in rubbing alcohol could be used to cool suspected parts after
the HV supply starts to collapse to see if it has any effect. Q1486
and Q1484 would be especially good targets for this test.

Freeze spray is what would normally be used for this but I would be a
little leery about using it around high voltages where condensation
might cause problems.

Fabio Trevisan
 

Hi Dave,
Thanks for all your comments / input.
See my comments next to yours...Again, I`m keeping only the last
conversation (to easy up to parse)
Rgrds,
Fabio

2016-09-27 23:20 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

I assembled and cleaned up a set of color schematics for the 464 which
Kurt has been nice enough to host at his site:
http://w140.com/tekwiki/wiki/464
Thanks for that, it was useful already.
I do have a scan of Service Manual that has the diagrams in colors, like
yours, but each diagram is split in 2 or 3 A4/Letter Pages (that I was
still in need to put together).
Yours came in handy, as I printed them in A3 paper (Tabloid) and it's great.

Ah, you are right. My 7000 notes break out PDA (post deflection
acceleration) and cathode acceleration voltages separately but my 465,
22xx, and 24xx notes do not. It is time to add a couple more columns.
The schematic and detailed theory section both say -1470 volts for the
cathode voltage and about 7kV for the PDA voltage. The theory section
says the cathode voltage is altered slightly between normal, variable
persistence, and fast storage modes. This will also affect the PDA
voltage.
Now that you mention, I feel silly for having a "theory" when careful
reading
of the schematics should have been enough! It's right there in the
schematics!
And indeed, the storage mode in effect change voltage divider of the
inverter's
regulator feedback.
By my rough calculations, if "Normal mode" the cathode voltage is -1470 V,
in "Variable persistence" mode it should "rise" to about -1450, and in
"Fast" mode,
it should rise just a bit more to "-1445".

When repairing these types of circuits, I prefer to replace all of the
high voltage resistors, capacitors, or diodes at the same time if they
are of a common type. It is just easier and safer this way.
I agree 100% with you... would this kind of HV components easier to
source or find in Brazil... But since it isn't, I sometime need to resort to
more conservative methods,
As I write this, I have dismantled most of what's around the cathode supply,
which includes the Focus divider, for proper clean up (which was due anyway)
and I will take the opportunity to test all critical components under
controlled situation.
I will connect each diode and capacitor to High Voltage (that I will borrow
from my Kenwood
scope) and look for leakages, while heating them up with a heat gun...
If the problem is in those components, I will find.

You are not the first person in Brazil to mention having problems
finding parts.
Yep... It gives us some headaches...
If at least it would be easy to import in Brazil... it wouldn't be a
problem...
But our customs are very picky and gives to the shippers all sort of
problems so, a lot of them even refuse to ship to Brazil.

So it helped but did not fixed the problem.
But what is the leakage at higher operating temperature? Whatever
part is causing the problem is only doing so after it warms up.
That's the difficult part... even with the original diodes,
which is the only thing I replaced so far, the problem takes quite
some time to happen with the outer case removed.
I measured the voltage drop at R1504, and it's ~1.4V
@ Intensity MIN (140uA), and ~2.0V @ Intensity MAX (200uA).
And I meausred the voltage dropt at R1522, and it's basically 0V
@ Intensity MIN (0 uA) and 0.5V @ Intensity MAX (50uA)
In both cases, I`m having the Intensity control limited (by R1406 trimpot)
to the point
where the CRT presents its 1st peak of intensity (double peaking... you
know)
But all those measurements were taken while it's working normally.
I didn't have chance to measure it after the HV collapses.

Usually it is the voltage multiplier capacitors which have problems
and their value is not critical. The value for the capacitors which
filter the cathode supply affect the regulator's frequency
compensation which is partially controlled by the network built around
R1473; I think you can get away with increasing them to 10nF but I am
not positive about this.
Since I already replaced the diodes and the capacitors, I will follow
your wisdom and replace the 3 HV multiplier caps.
Those are not so difficult to find here.

I have not quite decided what to make of this but I have some ideas.
And they're welcome!

If the diodes were the problem, then I would have expected changing
them to have completely fixed it instead of making it a little better.
I agree... I think that they're just helping to keep the whole thing cooler
and therefore, farther from heating up to the point of collapsing.
After doing a good HV test on them (as I mentioned above), I will put them
back... If they are not the culprits, it's nice to keep the scope original.

What I might try is looking for some other part which is getting hot
causing enough leakage to pull the high voltage supply voltages down.
If you do not have an infrared thermometer to scan with, a cotton swab
dipping in rubbing alcohol could be used to cool suspected parts after
the HV supply starts to collapse to see if it has any effect. Q1486
and Q1484 would be especially good targets for this test.
Hmmm... That is a good idea... I`m only afraid that a bare cotton swab
will not cool down Q1486 so easily!

Freeze spray is what would normally be used for this but I would be a
little leery about using it around high voltages where condensation
might cause problems.
I would too (be leery)... Freeze spray is ruled out for now!

Thanks again for your time!



 

On Fri, 30 Sep 2016 15:35:26 -0300, you wrote:

2016-09-27 23:20 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

I will connect each diode and capacitor to High Voltage (that I will borrow
from my Kenwood
scope) and look for leakages, while heating them up with a heat gun...
If the problem is in those components, I will find.
That could work. I usually end up hacking together a quick high
voltage supply as needed. Periodically I consider designing a
variable high voltage low current bench supply for doing high voltage
leakage tests up to 3 kV or so.

So it helped but did not fixed the problem.
But what is the leakage at higher operating temperature? Whatever
part is causing the problem is only doing so after it warms up.
That's the difficult part... even with the original diodes,
which is the only thing I replaced so far, the problem takes quite
some time to happen with the outer case removed.
I am still puzzled as to why changing the diodes improved the
situation without fixing it. I have difficulty believing that the
diodes are leaking just enough to cause such a marginal problem.

If the diodes were the problem, then I would have expected changing
them to have completely fixed it instead of making it a little better.
I agree... I think that they're just helping to keep the whole thing cooler
and therefore, farther from heating up to the point of collapsing.
After doing a good HV test on them (as I mentioned above), I will put them
back... If they are not the culprits, it's nice to keep the scope original.
I usually do not care about keeping the stock condition if I can make
some improvements.

What I might try is looking for some other part which is getting hot
causing enough leakage to pull the high voltage supply voltages down.
If you do not have an infrared thermometer to scan with, a cotton swab
dipping in rubbing alcohol could be used to cool suspected parts after
the HV supply starts to collapse to see if it has any effect. Q1486
and Q1484 would be especially good targets for this test.
Hmmm... That is a good idea... I`m only afraid that a bare cotton swab
will not cool down Q1486 so easily!
So use a few of them at the same time. The heat of vaporization for
most liquids is really high which is why evaporative cooling works so
well. Water is especially good but common rubbing alcohol has quite a
bit of water in it.

Fabio Trevisan
 

Hi Dave, thanks again for your comments...
I have made some progress and I think I found the culprit for the HV
inverter collapsing under heating.
See my comments next to yours below,
Rgrds,
Fabio

2016-10-02 5:07 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

That could work. I usually end up hacking together a quick high
voltage supply as needed. Periodically I consider designing a
variable high voltage low current bench supply for doing high voltage
leakage tests up to 3 kV or so.
I removed and measured all critical components for leakage under approx.
3kV, heating them up with the heat gun.
I controlled the heating between 70 and 90 deg celsius (which is already
way hotter than they should get).
The 6.8nF 5kV ceramic capacitors measure around 1uA cold and less than 5uA
hot
The 12nF 4kV film capacitors measure < 1uA cold, and < 2uA hot
The 1nF 10kV ceramic capacitors (HV doubler) measure under 1uA cold or hot.
Maybe 3kV is still little for
these 10kV beasts.

The HV transformer... between the HV winding (Cathode & PDA supply) and the
600V bias winding...
Cold, it already measures 10~15uA under 3kV (A WARNING SIGN)
Hot...enter the Drum Roll... 400uA!!! and my test HV supply sags to 1500V!
It didn't take all the heating above to present the increased leakage...
Leakage skyrocketed after about 40 deg celsius.

So... I think I (we) have a culprit!!!
At 400uA, this leakage alone is taking up 200% of the current actually
drawn by the HV circuit under normal operation.

P.S. I didn't have to resort to my Kenwood's HV supply...
I found a broken mosquito zapper (HV racket) hanging around (the racket was
bent and shorting).
It delivers about 3200V from 2 x AA batteries and it's safe (not connected
to mains and low power)

So I think now I`ll have to order one transformer from Sphere.ca.

I am still puzzled as to why changing the diodes improved the
situation without fixing it. I have difficulty believing that the
diodes are leaking just enough to cause such a marginal problem.
Looking at the orders of magnitude of the diode leakages that I measured
(less than 0.1uA),
the order of magnitude of the capacitor leakages and now at the HV
transformer leakages...
I`m also not sure if the replacement of the diodes really made a
difference, or if it was just
a mistaken conclusion... Probably just a "wrong detour".
Maybe the HV transformer leakage is erratic and better in some days and
worse in others.

I usually do not care about keeping the stock condition if I can make
some improvements.
Well... at this point I want first to fix it, whatever it takes...
So, I agree with you.
But given the unreliable source of the components here, I can never tell if
it's really what's labeled so,
ff the original diodes are not definitely bad (even if my superficial tests
tell me that the new diodes seem
better), I rather prefer to stick with the originals (because they may be
more reliable / better quality).

So use a few of them at the same time. The heat of vaporization for
most liquids is really high which is why evaporative cooling works so
well. Water is especially good but common rubbing alcohol has quite a
bit of water in it.
I`ll keep the tip, but I think there's no need anymore to go down that road!
The HV transformer looks pretty guilty at this point.

 

On Tue, 4 Oct 2016 12:33:48 -0300, you wrote:

The HV transformer... between the HV winding (Cathode & PDA supply) and the
600V bias winding...
Cold, it already measures 10~15uA under 3kV (A WARNING SIGN)
Hot...enter the Drum Roll... 400uA!!! and my test HV supply sags to 1500V!
It didn't take all the heating above to present the increased leakage...
Leakage skyrocketed after about 40 deg celsius.

So... I think I (we) have a culprit!!!
At 400uA, this leakage alone is taking up 200% of the current actually
drawn by the HV circuit under normal operation.
That is a bummer. I try to rule out everything else which is what you
did before getting to the more difficult to test and replace
transformer.

Usually the transformers fail in a more definitive way.

P.S. I didn't have to resort to my Kenwood's HV supply...
I found a broken mosquito zapper (HV racket) hanging around (the racket was
bent and shorting).
It delivers about 3200V from 2 x AA batteries and it's safe (not connected
to mains and low power)
That is a great idea.

So I think now I`ll have to order one transformer from Sphere.ca.
I see that Sphere has NOS (new old stock) 120-0909-01 transformers in
stock. I checked QService and they do not have any.

The $65 price from Sphere for the transformer is greater than some
past 464 and 466 sales on Ebay.

Fabio Trevisan
 

Hello Dave,
Just to let you know that I just ordered that HV transformer from Sphere.
It's shipping to my in-law's in Toronto, and she will be coming down to
Brazil in 2 weeks time.
The transformer costed 40% of what I paid for the whole 464... but heck...
I couldn't give up at this point.
I want to have a Tek scope and this is the first time I had chance to buy
one at
an affordable price.
So be it... it ended up not being a bargain, but it will still be cheap for
Brazilian standards.
I only hope that the CRT is not so worn out and that it will last enough
for my hobbyist use.

Talking about the CRT... I`ll reply to your answer on my other thread, on
this subject.
I want to explore what are my chances of rejuvenating this CRT.

Rgrds,

Fabio


2016-10-05 1:16 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:



On Tue, 4 Oct 2016 12:33:48 -0300, you wrote:

The HV transformer... between the HV winding (Cathode & PDA supply) and
the
600V bias winding...
Cold, it already measures 10~15uA under 3kV (A WARNING SIGN)
Hot...enter the Drum Roll... 400uA!!! and my test HV supply sags to 1500V!
It didn't take all the heating above to present the increased leakage...
Leakage skyrocketed after about 40 deg celsius.

So... I think I (we) have a culprit!!!
At 400uA, this leakage alone is taking up 200% of the current actually
drawn by the HV circuit under normal operation.
That is a bummer. I try to rule out everything else which is what you
did before getting to the more difficult to test and replace
transformer.

Usually the transformers fail in a more definitive way.

P.S. I didn't have to resort to my Kenwood's HV supply...
I found a broken mosquito zapper (HV racket) hanging around (the racket
was
bent and shorting).
It delivers about 3200V from 2 x AA batteries and it's safe (not connected
to mains and low power)
That is a great idea.

So I think now I`ll have to order one transformer from Sphere.ca.
I see that Sphere has NOS (new old stock) 120-0909-01 transformers in
stock. I checked QService and they do not have any.

The $65 price from Sphere for the transformer is greater than some
past 464 and 466 sales on Ebay.

------------------------------
Posted by: David <@DWH>

 

Since everything else in your 464 works well enough, I guess it is
worth the price and wait given how difficult it is to find good test
equipment in Brazil.

I used a 464 long ago and was not impressed with the CRT brightness or
sharpness compared to the two 7834s I have now but others have
reported better performance and maybe the one I used had a lot of
hours on the CRT or need to be calibrated.

On Wed, 5 Oct 2016 16:29:27 -0300, you wrote:

Hello Dave,
Just to let you know that I just ordered that HV transformer from Sphere.
It's shipping to my in-law's in Toronto, and she will be coming down to
Brazil in 2 weeks time.
The transformer costed 40% of what I paid for the whole 464... but heck...
I couldn't give up at this point.
I want to have a Tek scope and this is the first time I had chance to buy
one at
an affordable price.
So be it... it ended up not being a bargain, but it will still be cheap for
Brazilian standards.
I only hope that the CRT is not so worn out and that it will last enough
for my hobbyist use.

Talking about the CRT... I`ll reply to your answer on my other thread, on
this subject.
I want to explore what are my chances of rejuvenating this CRT.

Rgrds,

Fabio

Fabio Trevisan
 

Hi David,

Here's some update on the H.V. transformer issue of my Tek 464.

Yesterday I finally installed the new H.V. transformer I ordered from
Sphere.bc.ca.
It was delivered 3 weeks ago to my brother in law in Toronto and last week
he came down to Brazil and brought it to me.
It came very well packed and in perfect conditions...
Since I had dismantled almost the whole H.V. circuitry, to test all the
critical components "off-board" as I did and which led me to discover that
the H.V transformer was defective, it took me a while to put everything
back together, specially because the components in this area are laid-down
in a messy mixture of P.C.B., air-mount, piggy-back and ceramic-strip
techniques.

It amazes me how Tek managed to mass produce this line of 'scopes with this
kind of assembly technique.

Transformer installed and HV circuitry assembled... Turned-on the scope and
pop!... H.V. circuit's FUSE BLOWN!
First time I did it with an under rated 1A fuse for safety, instead of the
recommended 1.5A, and instead of the 4A fuse that was installed there
before!
Then, I put a 2A fuse... and it popped! just as instantly as the 1A.

Back to square one...
Checked and rechecked all the H.V. components assembly and looked for
possible soldering shorts that could have sneaked under some component...
Nothing found!!!
While keeping the fuse removed, checked all the regulated voltages... All
were OK.

Then disconnected the H.V.'s drive transistor (Q1486) and measured it...
SHORTED!!!

This circuit was still working when I took it apart to test the
components... And the 1A fuse blown so instantly... I don't believe that
such a short overload could have blown that massive 2N3055. The Unreg +15V
supply isn't even able to source the 15A that this beast is supposed to
withstand... And it is (was) an original TEK part!!!

I don't understand!!!

Understanding or not... Q1486 was blown... and it would be hard to source a
reliable (not fake) 2N3055 at short notice...
I managed to get an MJ15015 from a reliable supplier that I know... but I
was about the fact the the original Tek is listed as being a "SELECTED"
2N3055 device.
Installed the MJ15015, with the fuse replaced by a 20 Ohm resistor and
turned it on while measuring the drop on the resistor...
After seeing a healthy 4V drop (the 200mA draw that I once measured of this
cricuit)... I relaxed and could look at the screen...
A BEAUTIFUL TRACE WAS THERE!!! showing - as expected - a picked up 50KHz
interference, due to the H.V. cages are still not installed.
Replaced the 20 Ohm resistor by the 1A fuse and turned it back on... and NO
FUSE POPPING this time...

Still... things were not perfect. Retrace was visible... but I could make
it disappear readjusting the CRT bias (all in all, I could have touched it
while I disassembled the components all around)... But worse than the
visible retrace... the trace size (horizontal and vertical) was expanding
as I increased the "Intensity" control...
Shoot!.. Poor H.V. Regulation (or none...) I was already afraid that
something else in the circuit could have blown!!!

I went on to measure Cathode supply and...Ops!!! -1780V!!! (310V more
negative than it should)... Got suspicious that feedback loop could be
opened or that the feedback transistors could be bad...
Transistors turned out to be all good! and feedback summing point, that
should read nominally +0.4V was reading -5.2V (an indication that the
feedback divider was doing its job but even though, circuit was still not
being able to bring the output "down").

After analyzing the circuit to evaluate if it wouldn't blow anything,
decided to remove Q1484 (pulling it out of its socket) to see if the
inverter would still work (open-loop) and how much voltage it would
generate by itself (with only the baseline bias provided by R1483 of 5.6K).
It turned out that even with Q1484 removed, Cathode voltage was still at
-1780V!... meaning the the inverter was generating all that voltage without
any additional current from the feedback circuitry...Explaining why it
wasn't regulating whatsoever.
Probably, the MJ15015 has a much higher beta than the original drive
transistor...(which makes me think now that maybe Tek selected it for LOW
gain, and not HIGH).

Decided to take an intuitive shortcut and changed value of R1483, from 5.6k
to 10k... and measured cathode voltage (still in open-loop, with Q1484
removed)...
Ah ha!!... much better now! Cathode voltage "dropped" to -1100V... (about
300V lower than required)...
Inserted Q1484 back and.... Voilà!!!... Cathode voltage went to -1478V...

For everything that matters, it seems to be working fine! Cathode voltage
regulates well for all CRT loads (low to high intensity and using Store
modes or not).
I don't have a Variac to allow me to check regulation for mains voltage
variation, but I did try to further increase R1483 to 18k, and H.V. still
regulates so, I think that the intuitively chosen 10k, is placing the
circuit right in the middle of its regulation range.

What do you think? Should I leave it like that, or try something else (such
as getting another transistor or dumping some of its gain stealing some of
its base current to a drain resistor...maybe to a negative rail)?

BRgrds,

Fabio


2016-10-06 1:44 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:



Since everything else in your 464 works well enough, I guess it is
worth the price and wait given how difficult it is to find good test
equipment in Brazil.

I used a 464 long ago and was not impressed with the CRT brightness or
sharpness compared to the two 7834s I have now but others have
reported better performance and maybe the one I used had a lot of
hours on the CRT or need to be calibrated.

John Griessen
 

On 11/03/2016 01:13 PM, Fabio Trevisan fabio.tr3visan@... [TekScopes] wrote:

What do you think? Should I leave it like that, or try something else (such
as getting another transistor or dumping some of its gain stealing some of
its base current to a drain resistor...maybe to a negative rail)?
Your steps sound OK. I would just check that any resistors you used are able to take the current
through them, (wattage), and check as much as you can spend time on of the cal procedures, then start
using it.

 

On Thu, 3 Nov 2016 16:13:33 -0200, you wrote:

...

Since I had dismantled almost the whole H.V. circuitry, to test all the
critical components "off-board" as I did and which led me to discover that
the H.V transformer was defective, it took me a while to put everything
back together, specially because the components in this area are laid-down
in a messy mixture of P.C.B., air-mount, piggy-back and ceramic-strip
techniques.

It amazes me how Tek managed to mass produce this line of 'scopes with this
kind of assembly technique.
Air is a pretty good high voltage insulator.

...

This circuit was still working when I took it apart to test the
components... And the 1A fuse blown so instantly... I don't believe that
such a short overload could have blown that massive 2N3055. The Unreg +15V
supply isn't even able to source the 15A that this beast is supposed to
withstand... And it is (was) an original TEK part!!!

I don't understand!!!

Understanding or not... Q1486 was blown... and it would be hard to source a
reliable (not fake) 2N3055 at short notice...
I managed to get an MJ15015 from a reliable supplier that I know... but I
was about the fact the the original Tek is listed as being a "SELECTED"
2N3055 device.
We have discussed it in the past and not reached any conclusion about
what Tektronix selected for on these transistors; I suspect it was
either current gain or Ft (current gain-bandwidth product). Tektronix
used at least three different 2N3055 variations and sometimes even
used all three in the same oscilloscope like the 76x3 series.

The major issue appears to be that if the transistor Ft is too high,
then the high voltage inverter will suffer from spurious oscillation
at a much higher frequency. A modern direct replacement which is
readily available is the 2N3771G (40V 30A 150W) or 2N3772G (60V 20A
150W) which both have an Ft of 200 kH which is very close to the 300
kHz Ft of the original 2N3055 that Tektronix used and they are tougher
also. The later 2N3055s had a 800 kHz Ft and modern ones are 1.5 MHz
which can definitely be a problem in this circuit.

...

Still... things were not perfect. Retrace was visible... but I could make
it disappear readjusting the CRT bias (all in all, I could have touched it
while I disassembled the components all around)... But worse than the
visible retrace... the trace size (horizontal and vertical) was expanding
as I increased the "Intensity" control...
Shoot!.. Poor H.V. Regulation (or none...) I was already afraid that
something else in the circuit could have blown!!!
Having the horizontal and vertical deflection change indicates that
the cathode voltage is not being regulated. Lower cathode voltages
means lower electron velocity which gives the deflection plates more
time to deflect the beam yielding greater deflection.


* Cathode voltage was too high and out of regulation
* Changed R1483 to lower it indicating too much drive to 1486.
* Gain too high?


What do you think? Should I leave it like that, or try something else (such
as getting another transistor or dumping some of its gain stealing some of
its base current to a drain resistor...maybe to a negative rail)?

BRgrds,

Fabio
I would like to know why the circuit is designed the way it is. Why
didn't Tektronix make it so that it would not be as sensitive to
transistor characteristics? It would be nice to have a general
solution so modern high Ft 2N3055s could be used without issues.

Increasing the value of R1483 is a fine solution if the problem is
just too much current gain but can you measure the transformer voltage
using an oscilloscope to see if it is oscillating properly? Do you
have a high voltage oscilloscope probe? We know from a previous
discussion that spurious oscillation is possible.

I wonder why R1483 was needed at all. The AC impedance at the emitter
of Q1484 is low because of C1483 so R1483 only provides a minimum
operating current. I would have expected R1483 to go to ground and
Q1484 to provide all of the base current but maybe R1483 has something
to do with the startup characteristics like preventing the output
voltage from overshooting which could damage something.

Fabio Trevisan
 

Hi there,

John.. thanks for the reminder. The new resistor is of the same power
rating as the original. In fact, although we're talking about the H.V.
inverter and a 2N3055 transistors, which is a power device, this resistor
itself is a small one, a quarter or half a watt if much.

David, see my comments next to your replies... thanks again... very helpful.

Rgrds,

Fabio

2016-11-04 4:14 GMT-02:00 David @DWH [TekScopes] <
TekScopes@...>:

Air is a pretty good high voltage insulator.
i can't argue to that. As a matter of fact I had quite some trouble getting
rid of black dust and contamination from the H.V. board of my Kenwood... To
make matters worse, phenolic Urgh! board...
Putting that into perspective, air mount seems to be a good idea after all.
What amazes me in the Tek is the quality and thickness of the fiberglass
board!

We have discussed it in the past and not reached any conclusion about
what Tektronix selected for on these transistors; I suspect it was
either current gain or Ft (current gain-bandwidth product). Tektronix
used at least three different 2N3055 variations and sometimes even
used all three in the same oscilloscope like the 76x3 series.
The major issue appears to be that if the transistor Ft is too high,
then the high voltage inverter will suffer from spurious oscillation
at a much higher frequency. A modern direct replacement which is
readily available is the 2N3771G (40V 30A 150W) or 2N3772G (60V 20A
150W) which both have an Ft of 200 kH which is very close to the 300
kHz Ft of the original 2N3055 that Tektronix used and they are tougher
also. The later 2N3055s had a 800 kHz Ft and modern ones are 1.5 MHz
which can definitely be a problem in this circuit.
Wow... I didn't know the fT for the older 2N3055 to be so low... Never
mention knowing
that they improved it over the years keeping the same part identifier...
That's confusing!

I almost bought a 2N3773 instead of the MJ15015 but, as usual, I was
looking at a part
that could match or exceed the 2N3055 absolute maximum values... It
wouldn't cross
my mind replacing it by the 3771, which is 40V (and not 60)... although I
admit that in this
circuit, 40V is enough (I actually measured the Vce on the oscilloscope and
it goes from
-10V to +29V)

Indeed, I noticed that the picked-up interference that I see on the screen
(while having the
H.V. cages opened, the ~45Khz signal is clearly visible without any probe),
is less clean
than it used to be... it may be due to spurious high frequency
oscillations...
I will try to look into it with more detail... If I find evidence of higher
harmonics, I will try to
get a 2N3772 (which is 60V)...

Having the horizontal and vertical deflection change indicates that
the cathode voltage is not being regulated. Lower cathode voltages
means lower electron velocity which gives the deflection plates more
time to deflect the beam yielding greater deflection.
Ack to that! Vertical sensitivity was low by some 50% while the cathode was
at about -1800V.
After I got regulation back, it came down to the correct levels.
I immediately thought of regulation when I noticed that it was "breathing"
as I
changed Intensity control, but my first thoughts (and fears) was that
something
could have blown in the H.V. feedback divider (the large custom film
resistor).
I was relieved when I noticed that feedback summing point was negative.

* Cathode voltage was too high and out of regulation
* Changed R1483 to lower it indicating too much drive to 1486.
* Gain too high?
Well, I reached to that conclusion from the theory of operation in the S.M.
in where it says that the feedback controls how much bias is fed into Q1486
bringing it closer or farther from the conduction point and therefore
making it
conduct for more or less time and, therefore, putting more or or less energy
into the transformer which translates into higher or lower voltage.

I would like to know why the circuit is designed the way it is. Why
didn't Tektronix make it so that it would not be as sensitive to
transistor characteristics? It would be nice to have a general
solution so modern high Ft 2N3055s could be used without issues.
I confess that transformer dependent oscillator theory has never
been "intuitive" to me... I understand the concepts involved, but not enough
that I could design one from the paper up...and be sure that practice would
meet theory at the fiberglass.
I can think of building one empirically, but that doesn't count to know or
understand
why folks at Tek did it that way and not otherwise.

In their defense I can say that my Kenwood's H.V. oscillator seems quite
similar. An NPN
driving transistor with emitter connected directly (no emitter resistor) to
-12V,
collector pulling the transformer down, base connected to the feedback
winding
of the transformer which is biased, from the other side, by the feedback
circuitry.
Only difference is that it has a 100R base stopper resistor, probably
inserted there
to tame H.F. spurious oscillation.

At the point that I am right now, I am in position of doing some
experimentation...
I don't pretend to ever reach mathematical conclusions but I can look at
the circuit
as it stands now and, if it's presenting unwanted H.F. artifacts, I can
play around
the transistor and search for a reliable and consistent way of taming it.
One that wouldn't compromise the phase margins (or the lack thereof) at the
nominal
frequency, which is mandatory to assure oscillation.
That part, to make it less dependent on transistor's fT.
About the dependency on the transistor's beta, I think that a common emitter
amplifier without any emitter degeneration as it is, and without any DC
negative feedback
to its base, indeed leaves a lot to desire...
At some expense of gain (which is not plentiful in the 2N3055 but much
better in
modern devices), I think an emitter degeneration resistor could improve the
circuit's susceptibility to change of device's beta (not just from device
to device,
but also from temperature change).

As soon as I get done with the CRT rejuvenation, which is my next chapter
at this
point, I can dedicate some time to research on those improvements.

Increasing the value of R1483 is a fine solution if the problem is
just too much current gain but can you measure the transformer voltage
using an oscilloscope to see if it is oscillating properly? Do you
have a high voltage oscilloscope probe? We know from a previous
discussion that spurious oscillation is possible.
I don't have an H.V. oscilloscope probe. I have a home-made H.V. probe made
of a string of about 30 x 3.3Mohms resistors, all insulated by plastic
spacers,
inserted on a 16mm dia acrylic knitting needle. I tried... but it's not
good for AC.

But, as I mentioned earlier, the vertical pre-amplifier picks-up the
oscillator frequency
pretty well (with the H.V. cages removed) so...Can I call it an H.V. scope
probe?

And I can measure the signal at the H.V. transformer's primary... which
must be
good enough.
To my understanding whatever H.F. artifacts that may be arising, must be
present
on the primary side, if we are to fix it from the primary side or, in other
words, if it can't
be probed on the primary, it can't be fixed from the primary.

I have pictures of how those primary waves were before I changed the
transformer
and transistor... I can look at them now and see if there are differences.

I'll get back to you with the results.

I wonder why R1483 was needed at all. The AC impedance at the emitter
of Q1484 is low because of C1483 so R1483 only provides a minimum
operating current. I would have expected R1483 to go to ground and
Q1484 to provide all of the base current but maybe R1483 has something
to do with the startup characteristics like preventing the output
voltage from overshooting which could damage something.
I don't think it's a matter of being REALLY needed... I think it's more
like a practical circuit's approach.
They probably departed from a working circuit (an oscillator without
voltage level feedback)
and added what was necessary so that it self regulated its output voltage.
As it stands, the control circuit is not responsible for making it work,
but just responsible
for adding a variable (controlled) amount of bias, enough to achieve just
the desired line
and load regulation.
And you can be right as well regarding the startup... oscillators are more
or less
easy to understand while under "regime"... but how they startup is often
guesswork
(especially before Spice simulation... but even though, circuits that
doesn't oscillate in Spice
do oscillate in real world and vice-versa).

We shall see... Or not!