Date   

Both 485's done

Brendan
 

I have finally finished both of my 485's. All functions are working. One is totally recapped the other just filter caps and power board. https://groups.io/g/TekScopes/photo/37195/11?p=Name,,,20,1,0,0


Re: Tek 422 ac/dc power supply issues

 

To me, that square wave on the test point raises concerns. Maybe there is something wrong in the snubber network. Something shorted there can definitely give those transistors the blues.


Re: Nuvistors

 

Now that I think about it I bet some of these triodes could be replaced with depletion mode N channel FETs. In fact the ones I've seen in the front ends of scopes are common plate so all they do is provide near infinite input impedance, no voltage gain involved. The info I happen to have handy for the 422 has them running with only 20 volts on the plate. I know FETs can handle that.


Re: Hello from new old member

Fabio Trevisan
 

George,
I wasn't around in the group 5~10 years ago so, I`m not sure if I`m entitled to welcome you back... But feel welcomed anyway.
I am from Brazil and here is very difficult to find or get Tek stuff of any kind.
Usually, I wouldn't stick my head out to the possibility of buying something from North America because the shipping becomes prohibitive.
But I see that you live not so far from relatives (my in-laws live in Mississauga) and they come down every once in a while so that may be an opportunity.
Please let me know what you're interested in "downsizing"... I may be able to relieve you from some excess (if any such thing can ever be an excess).
I have a recently acquired 7623A mainframe, with a 7A26 and a 7B53A... It's begging for a 3rd plugin.
Rgrds,
Fabio


Hello from new old member

GeorgeP
 

Hello from Lions Head Ontario Canada.
I was on the yahoo board much more often five ten years ago when I was near Georgetown ON.
I got a lot of help and ideas (Thank You!) and was also able to help out a few times.
I am new at this TekScopes Home!
BTW I am downsizing. I have way too much stuff!
George Plhak
ffwdm and george.plhak on EBay


Re: 453 blows HV fuse

Richard Knoppow
 

Some progress. First to answer your questions; D940 appears to be good.
I do not have either a ring tester or a bulb tester. What I use is a metered Variac and put a Triplett 630A across the fuse holder to measure the current. Its set on the 12A range and I can see clearly when the current begins to rise with line voltage. I tried disconnecting the CRT at its socket and also disconnected the HV lead to the helix at the HV compartment. Made no difference but thank you for the suggestion.
So far, here is what I've found. When the line voltage begins to get to normal something in the HV compartment begins to arc. I found its a lead from the HV transformer to the filter for the "helix". Since there was HV I decided to see if I was getting any sort of trace. At around 90V line voltage (I have to note this again) I could get a trace with the beam finder. The trace was distorted and that makes me think I've been chasing my tail on the HV. I now think the problem is in one of the lower voltage supplies. I have errands to run today so will get to this tomorrow. It would be great if I had a second decent scope but I don't so will have to trace this down via a voltmeter. In any case I think the presence of a trace of sorts indicates the HV is OK and the CRT is OK. I now need to study the other power supplies to find the problem. I can run the scope at full line voltage with the HV fuse removed. The distorted trace should give me some clue as to what has failed. My guess is a filter cap somewhere since the thing overheated.
Any suggestions would be welcome. In any case I now have some confidence that I can fix the thing.

On 4/27/2018 1:57 AM, b c via Groups.Io wrote:
I had my 2465 burn HV fuses. However it ended up being that I couldn't
find any failed transistors, coil was good via a ring tester, so it had
to be the CRT - which I happened to have cracked but didn't know it at
the time. At least for me, the event that stopped blowing fuses is
when I disconnected the CRT socket.
Is D940 still good?
Do you have a ring tester to check the transformer?
Try running with the back CRT socket disconnected or perhaps one of the
second anode rectifier tubes pulled (V952/V962), does it still blow?
You may want to use a light bulb in the meantime to diagnose this
problem, fuses get expensive fast.
On Thu, 2018-04-26 at 16:42 -0700, Richard Knoppow wrote:
     I am getting back to a project I've had on the back burner
for a long time.
     About a year ago I left my 453 running in the shop on a very
hot day. When I came back a couple of hours later there was no
trace. I discovered the fan had stopped. More investigation found
that the HV fuse had blown. I don't remember if the line fuse
also blew but don't think so.
     I thought the fan had frozen up but found the rotor turned
easily and when powered up ran fine. Still does.
     Now the problem is that the HV fuse F-937 blows. I checked
Q-930, Q-923, Q-913, and Q-914 using the old fashioned method of
an ohm meter. All seem to be good. I also made resistance
measurements on T-930 and also voltage checks on it to see if
there was a short. There does not appear to be.
    I am at a loss. If I disconnect the collector of Q-930 the
fuse does not blow. If I disconnect the base of Q-930 it does not
blow. I also have measured the current though the fuse to see
what differences the connections make and to avoid putting many
fuses in. I am using a GR metered Variac as an AC source.
     The voltage at the base of Q-930 with it disconnected is +11
volts, the handbook says it should be -4.4 volts. I looked at the
+75V supply at R-925 and it measured right on the nose. The other
side of the 13K resistor should be -4.4 volts. Being disconnected
from the base of Q-930 probably affects this.
     I've made some other checks, resistor values etc. but can't
find anything. Probably I must obtain replacement transistors for
all of these guys and try substituting rather than just
resistance checks. It would be helpful if someone knows what
kinds to get these days.
    If anyone has had a similar problem please let me know what
you found. Its very frustrating. Also, this is my one and only
decent scope so I have none other to trouble shoot with.
    Measurements were made using a Triplett 630A and a
Hewlett-Packard bench DVM.
--
Richard Knoppow
dickburk@...
WB6KBL


Re: Nuvistors

Dale H. Cook
 

For some time I have been compiling an index to receiving tube manuals. It is currently complete through 1955, and can be downloaded or read in PDF format from:

http://plymouthcolony.net/starcity/radios/files/TubeData.pdf

A list of the manuals included, with links to copies available online, is at:

http://plymouthcolony.net/starcity/radios/pages/tubeindex.html

Special note - do not use the download link at the bottom of the page, as I have not yet upgraded captcha on my site. Use the first link above.

Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcity/radios/index.html


Re: 465B won't trigger

Fabio Trevisan
 

Hi Lorn,

You're right... 2N5245 is indeed recommended as a replacement to 2N4416 and, as Chuck mentioned, the 2N4416's 4th pin can be cut-out. Just don't cut the wrong one.
I`m looking right now at a data-sheet page from NewJersey Semiconductor products:
http://pdf1.alldatasheet.com/datasheet-pdf/view/459118/NJSEMI/2N5245.html
Only difference that I see that really screams to the eye is the Gate leakage current, it's 1nA on the 2n2545, against 0.1nA on the 2n4416... but either one would not cause problems on the trigger input buffer (worst case 1nA on 1Mohm produces only 1mV error).
I think you can be happy with either one, whichever you can find.
When I found mine with problem, and before I got a pair of original Tek parts from a fellow from this group (from whom I bought a few other parts I was needing back then), I also found a handful of 2n4416 and selected the pair that would show the smallest DC error.
The good thing about this input circuit is that you can use the circuit itself to match the transistors... Just swap them through all combinations and select the pair that produces the smallest DC voltage (having the input selected to External, DC, and having the BNC input shorted to ground).

Rgrds,

Fabio

On Fri, Apr 27, 2018 at 07:23 am, blackholenulled wrote:


Fabio:
Yeah, I was hoping for "been there, done that", so you've been a big help!

So this is interesting what you're saying about the cross reference. The
transistors I pulled out are 3 pin TO-92 case devices. The data sheets I'm
finding for 2N4416 show it as a four pin device. I wonder if there is a three
pin variant?

I need to compare the data sheets but there are indications from other sources
that 2N4416 and 2N5245 are electrically equivalent.

So how I arrived at 2N5245:

https://www.wbparts.com/rfq/5961-00-436-3317.html

Which lists as a cross reference 2N5245. It came from military specs, so I
figure if it's good enough for the military, it's good enough for my purposes.

This discussion indicates 2N5245 and 2N4416 are equivalent:
https://www.vintage-radio.net/forum/showthread.php?t=67673

I figure if I can't purchase a matched pair I can buy a handful and do some
matching using this article:
http://www.firstwatt.com/pdf/art_matching.pdf

So the Tektronix cross reference you linked is a great resource. I'm just
puzzled as to why Tektronix would list a four pin device rather than three
pin, unless it turns out there is a three pin variant of 2N4416 that used to
be available. (well, quick search on the internet indicates there are TO-92
case variants, but more common is TO-72 four pin for the 2N4416).

What I need to do is remove the heat sink from the transistor pair and see
what the markings are.

Lorn
KK4KRI

On Thu, Apr 26, 2018 at 01:31 pm, Fabio Trevisan wrote:


Hello Lorn,
Great news indeed!... So your problem is narrowed down to exactly the same
cause of mine: One, or both, of the input FET Transistors bad.
And I did exactly what you did, meanwhile I couldn't source another pair...
I
cannibalized the B sweep trigger.
In fact, there's very little impact to it, because 90% of the times, we use
the B Sweep in the "STARTS AFTER DELAY" mode... and this mode doesn't use
the
triggering input buffers or the trigger amplifier whatsoever...

As for the source of the transistors you wrote:
Any ideas anyone?
Manufacturer part number: SF50031, which comes back to 2N5245 N channel
JFET, which is obsolete. It's late and I haven't tried to cross reference
that
yet.

Not quite right!
I don't know from where you got the 2N5245, but according to
Tektronix_Xref_sm.pdf available at Archive.org
(https://archive.org/details/tektronix_Tektronix_Xref_sm), part number
151-0042-00 is found at page 7-7.
This part number is a characterized and matched pair of 2N4416 (which is
probably also obsolete, but not that hard to find).
From the "stock" 2N4416 whose IDss can vary from 5 to 15 mA, those ones are
selected to a narrower IDss range of 10 to 15mA, and the pair need to be
matched within 0.5mA between each other.

Even if you cannot get a matched pair, as long as they're not too different,
you can zero them out by tweaking the value of R7325 or R7327 (or both).
This will compensate for the different Vgs that will develop across each one
of the **not matched** xsistors' source resistors, in order to get the
output
voltage as close to 0V as possible,
The fact they're not matched will have the down side their IDss will also
drift differently with temperature so, tweaking the resistors solves the
matching problem at one given temperature but as the temperature changes,
they
will drift apart from each other...
But since this is **only** triggering (I can see Howard Vollum revolving in
his grave now), it's not that critical!. And you can always put the
non-matched pair on the B sweep trigger, which is rarely used.

Good luck with your 465B, This is my dream 465 scope, with Alternate Delayed
time base instead of the least useful "MIX" mode that's present on the
regular
465s.
I have a 464 with DM44 multimeter (it has a weak CRT) and just recently got
a
7623A with 7B53A timebase, so I'm probably married to the "MIX" mode for the
foreseeable future.
I'll probably sell my 464 and watch out for a good 465b, or even a 468
(which
is a 465b + digital storage).

Rgrds,

Fabio


Re: 547 in New Hampshire

Roy Morgan
 

Chuck and other Simple Green interested folks,

The FAA may well have put out a notice, but I have not seen it (a pointer to it would be appreciated), BUT:

The Army did put a notice in it’s aircraft maintenance magazine, quite some years ago now (quite possibly before the more suitable Simple Green formulas were introduced).

The Army instructions were (summarized):

- DO NOT use Simple Green on aircraft
- DO use Army-approved cleaners and procedures.

The article did not go into detail about what the stuff does to aircraft, but was clear that it should not be used.
(I can later send a copy to interested folks.)

Further, Bill Carns, much respected and long-experienced leader in the Collins community cautions about using harsh cleaners on radios. It seems to me that basing decisions about cleaners on 3 to 5 years absence of “problems” may be a bit short sighted. The Tek 545B that I expect to bring back to operation has been in moderate to poor storage conditions for a decade or so, and was made a lot of decades ago. I expect that I”ll be depending on it as my main scope for another decade or two. (Do 90-year-olds still work on radios? I hope to.)

In the past, I have used 50-50 409 and household ammonia as a magical cleaner for radios. That may well have been unwise!

I had a bit of hope to get the 547 from New Hampshire, but was not surprised the first responder took it. So — I am modestly in the market for another one. I am in central NY and can travel some distance to avoid shipping. (I lost a 547 and a whole lot of other treasures in a disaster.)

Roy Morgan
k1lky68@...

On Apr 23, 2018, at 5:51 PM, Chuck Harris <cfharris@...> wrote:

FAA put out a notice against the use of Simple Green for
cleaning aluminum airplanes a number of years ago. The
problem is it wicks into joints, and under rivets, where
it sits and corrodes.


Re: 465B won't trigger

Brian Bloom
 

Lorn,

I have some matched pair 151-1042-00 OEM Tek parts if you need them. Message me off list - I tried to message you but for some reason it's not allowing me to do so right now.

I'm on the east coast USA (WV), so it shouldn't take long before they're in your hands if you decide you want them.

Brian


Re: Intermittent no trace / sweep.

Roy Morgan
 

Vaclav,

If your ACE hardware store does not have that setscrew, I may have one I can send you. I *think* I just saw some among the huge mess left behind here after a disaster.

Let me know, and I’ll have a look to see what I have.

Roy Morgan
k1lky68@...

On Apr 27, 2018, at 8:46 AM, vaclav_sal via Groups.Io <vaclav_sal=yahoo.com@groups.io> wrote:
...

PS I am on my way to the Ace hardware store - getting hex set screw 4x40 .125 - from shop manual.


Re: 7603 Z-Axis board transistor Q1152 running a little too hot?

Al Holt
 

On Fri, Apr 27, 2018 at 09:52 am, walter shawlee wrote:

I find that Z-axis transistor is always too hot, and have routinely added a "tophat" style heatsink that radiates from the top surface, and usually avoids the adjacent parts. late
cases have a vent hole pattern right over the part to aid in cooling, but the heatsink is still important; be careful if elevated that the heatsink (at collector potential) DOES NOT
touch the cover (which is grounded).
Yes, thanks Walter! I'll get some of those, too! Thanks for sharing your history with these things. I think adding vent holes to the side panel is a good idea; now that I have a real drill press I think I can do that with fairly good expectations. :-)

--Al


Re: 7603 Z-Axis board transistor Q1152 running a little too hot?

Craig Sawyers <c.sawyers@...>
 

Tek designs were good, but as the gear ages, it's valuable to do all we can to help extend the part
lifetimes.
Indeed. Component lifetime is strongly dependent on temperature via the Arrhenius equation. That is
basically a diode-like equation with an "activation energy" on the top line, and kT on the bottom.
There is a multiplier of the exponent similar to Is in the diode equation.

So there are two factors which determine what happens with a particular component, but you can see it
writ large with electrolytic capacitors whose life is strongly dependent on the operating temperature.

But in general cool running of all parts (or as cool as feasible) is a smart thing.

Craig


Re: 7603 Z-Axis board transistor Q1152 running a little too hot?

 

I find that Z-axis transistor is always too hot, and have routinely added a "tophat" style heatsink that
radiates from the top surface, and usually avoids the adjacent parts. late cases have a vent hole pattern
right over the part to aid in cooling, but the heatsink is still important.
be careful if elevated that the heatsink (at collector potential) DOES NOT touch the cover (which is grounded).

Tek designs were good, but as the gear ages, it's valuable to do all we can to help extend the part lifetimes.

all the best,
walter


--
Walter Shawlee 2, President
Sphere Research Corporation
3394 Sunnyside Rd., West Kelowna, BC
V1Z 2V4 CANADA Phone: (250) 769-1834
@walter2
WS2: We're all in one boat, no matter how it looks to you.
Love is all you need. (John Lennon)
But, that doesn't mean other things don't come in handy. (WS2)


Re: New file uploaded to TekScopes@groups.io

bobh@joba.com
 

When you and Tony were discussing the inputs to U1275 it reminded me of the Tek Made parts book and later the Tek Xref book.

Bob.

On 4/27/2018 5:23 AM, JJ wrote:
Bob, thanks for the 155-0067-02 spec sheet and the Xref resources. They're
very informative. Wish I had them 3 weeks ago!

Best,
John

On Fri, Apr 27, 2018 at 1:06 AM, Robert Hay <bobh@...> wrote:

Look in the Tek Cross Reference book page 12-9 has some info on the
152-0502-00 and lesser information on the 152-0397-00. The text is
searchable. It may give you a generic replacement.

http://w140.com/Tektronix_Xref_sm.pdf

I thought the information on U1275 might help understand what it does in
the circuit. TekWiki has several other valuable references on the same
page.

Bob.


On 4/26/2018 6:48 PM, Yiu On Tony C via Groups.Io wrote:

Hi Robert :
It is a very useful information for me to understand the PSU of 7904 !
On the other hand , I am looking for the data sheets for the fast
recovery rectifier 152-0502-00 ( MBD5300 ) and 152-0397-00 ( MR880 ). It
was because my PSU of 7904 CR1311 and 1312 using replacements having
different shut down result at same loading conditions, it is low loading
only !
Thanks !
RegardTony CheungAPR 27 2018

From: "TekScopes@groups.io Notification" <
TekScopes+notification@groups.io>
To: "TekScopes@groups.io" <TekScopes@groups.io>
Sent: Friday, April 27, 2018 4:12 AM
Subject: [TekScopes] New file uploaded to TekScopes@groups.io
Hello,This email message is a notification to let you know that a
file has been uploaded to the Files area of the TekScopes@groups.io
group.File: 7904 - U1275 IC Information 155-0067-02.pdfUploaded By:Robert
Hay Description:
Internals info for U1275 in 7904 mainframe, PS control chip 155-0067-02
from the Tek Made handbook.You can access this file at the URL:
https://groups.io/g/TekScopes/files/7904%20-%20U1275%20IC%20
Information%20155-0067-02.pdfCheers,
The Groups.io Team






Re: 465B won't trigger

Renée
 

in this instance the 4th pin usually connects to the case for shielding, as it is not a dual gate device.
(depending on the environment (other uses- noisy places)  adding the shielding may be necessary, but not this time.)
Renée

On 2018-04-27 07:23 AM, blackholenulled wrote:
Fabio:
Yeah, I was hoping for "been there, done that", so you've been a big help!

So this is interesting what you're saying about the cross reference. The transistors I pulled out are 3 pin TO-92 case devices. The data sheets I'm finding for 2N4416 show it as a four pin device. I wonder if there is a three pin variant?

I need to compare the data sheets but there are indications from other sources that 2N4416 and 2N5245 are electrically equivalent.

So how I arrived at 2N5245:

https://www.wbparts.com/rfq/5961-00-436-3317.html

Which lists as a cross reference 2N5245. It came from military specs, so I figure if it's good enough for the military, it's good enough for my purposes.

This discussion indicates 2N5245 and 2N4416 are equivalent:
https://www.vintage-radio.net/forum/showthread.php?t=67673

I figure if I can't purchase a matched pair I can buy a handful and do some matching using this article:
http://www.firstwatt.com/pdf/art_matching.pdf

So the Tektronix cross reference you linked is a great resource. I'm just puzzled as to why Tektronix would list a four pin device rather than three pin, unless it turns out there is a three pin variant of 2N4416 that used to be available. (well, quick search on the internet indicates there are TO-92 case variants, but more common is TO-72 four pin for the 2N4416).

What I need to do is remove the heat sink from the transistor pair and see what the markings are.

Lorn
KK4KRI

On Thu, Apr 26, 2018 at 01:31 pm, Fabio Trevisan wrote:

Hello Lorn,
Great news indeed!... So your problem is narrowed down to exactly the same
cause of mine: One, or both, of the input FET Transistors bad.
And I did exactly what you did, meanwhile I couldn't source another pair... I
cannibalized the B sweep trigger.
In fact, there's very little impact to it, because 90% of the times, we use
the B Sweep in the "STARTS AFTER DELAY" mode... and this mode doesn't use the
triggering input buffers or the trigger amplifier whatsoever...

As for the source of the transistors you wrote:
Any ideas anyone?
Manufacturer part number: SF50031, which comes back to 2N5245 N channel
JFET, which is obsolete. It's late and I haven't tried to cross reference that
yet.

Not quite right!
I don't know from where you got the 2N5245, but according to
Tektronix_Xref_sm.pdf available at Archive.org
(https://archive.org/details/tektronix_Tektronix_Xref_sm), part number
151-0042-00 is found at page 7-7.
This part number is a characterized and matched pair of 2N4416 (which is
probably also obsolete, but not that hard to find).
From the "stock" 2N4416 whose IDss can vary from 5 to 15 mA, those ones are
selected to a narrower IDss range of 10 to 15mA, and the pair need to be
matched within 0.5mA between each other.

Even if you cannot get a matched pair, as long as they're not too different,
you can zero them out by tweaking the value of R7325 or R7327 (or both).
This will compensate for the different Vgs that will develop across each one
of the **not matched** xsistors' source resistors, in order to get the output
voltage as close to 0V as possible,
The fact they're not matched will have the down side their IDss will also
drift differently with temperature so, tweaking the resistors solves the
matching problem at one given temperature but as the temperature changes, they
will drift apart from each other...
But since this is **only** triggering (I can see Howard Vollum revolving in
his grave now), it's not that critical!. And you can always put the
non-matched pair on the B sweep trigger, which is rarely used.

Good luck with your 465B, This is my dream 465 scope, with Alternate Delayed
time base instead of the least useful "MIX" mode that's present on the regular
465s.
I have a 464 with DM44 multimeter (it has a weak CRT) and just recently got a
7623A with 7B53A timebase, so I'm probably married to the "MIX" mode for the
foreseeable future.
I'll probably sell my 464 and watch out for a good 465b, or even a 468 (which
is a 465b + digital storage).

Rgrds,

Fabio


Re: 7904 Mainframe damaged

Velik Kazakov
 

I will check the resistors in sensing sircuit just to be shure for it when find a time for this. But I think that the problem is with some caps in the main frame. It make the same start with other one ps that work well in my first 7904. Now power it w/o plugins. After this install the plugins and power well. Will wait some time to see if the caps going better befor to start to seek bad caps.

Thank you for the info,
Velik kazakov

On Fri, Apr 27, 2018 at 05:07 am, Chuck Harris wrote:


My ingrained memory of the brand new 7904's, is the snap
of the toggle switch, a tic-tic-tic, and then the scope
came up.

My 7904 has been doing the tic-tic-tic, and then come
up routine, on the first power up of the day, for as
long as I have had it.

If yours ticks a little longer than 3-5 times, it may be
that the resistors in the overload sensing circuitry have
drifted a little high in value, or the capacitors are a
little leaky. If it is leaky capacitors, they should improve
with time.

-Chuck Harris



Velik Kazakov via Groups.Io wrote:
This week I play with one "new" 7904. It come with fired tantalum cap in
Control Illum power supply. After exchanging the cap and checking of the ps
and main frame all looks ok. But when put new ps in my old working 7904 it
work, but all two PS installed in the new main frame power after few seconds
(sometimes 10+ seconds). All checking in the main frame looks good. Maybe this
main unit is not powered for long time and now have a caps inside that need
more time/energy to energize. It start only w/o plugins and control illum must
be off. After it power one time. I shut it off and install plugins and power
on. Now all is ok. Maybe you can try to power main unit for 5-10 seconds
waiting in tic mode. Maybe have the same problem. If need can make a video to
show the situation with this new 7904. Old one I repair and recap in 08/2017
and it work well (here is teh repair and cleanif of it
https://www.eevblog.com/forum/repair/tektronix-7904-repair-and-restoration/).


Re: Tek 422 ac/dc power supply issues

Renée
 

ahhhh....now I understand .....D1224 and 5 failed before you disconnected C1224....that makes sense...I could not understand how the failure occurred after C1224 was disconnected......never mind.......

yes excessive loading from leaky caps...guess it is time to replace or put the caps through really rigorous testing. i am thinking replacement easier with a known entity.

Yes upon my return I will do as suggested on the C, I will also go back a few spots as I think something is going on there....I may figure the load presented by the BE junction Q1174/84 use a R for load,  then feed/replace the self generating +/- 12V supplies and look real hard at the pulse circuit. I am thinking I have an issue in there since I really do not have a leading edge. the flip/flop...eehh Binary appears to be working as is the steering due to nice square wave presented to the base (and beyond) of the choppers (with the ringing)

Re your  110V supply and V increase failure- I am thinking coincidence for that cap to go bad as no matter what the input V the DC/DC converter should be self regulating. It is a big controlled loop. reading the CAL procedure for the supply suggests such. however I see increased heat generation on the choppers due to the fact the input V is uncontrolled...I  guess hence the heat sinks for the dissipation.

Hope all goes well with the other tasks at hand.....
Renée

On 2018-04-26 11:19 PM, Jeff Urban wrote:
"I am not sure I understand. C1224 in on the output side of T1201 and is the feed for the Voltage doubler input cap generating -110V ."

Yes. When I went to a higher voltage it became quite a load. First D 1224 shorted. A leaky C 1224 can cause that. With the diode shorted I got nothing at the negative side, I changed both and then C 1225 popped. I think maybe both caps were leaky. I didn't monitor all the output voltages but it was working and the trace was pretty much where it was before so the voltages didn't climb all that much.

Yes I meant D 1224, for some reason I stuck the CR in there having seen it alot.

Sure, wall AC is not directly applied but that power supply does not directly regulate the secondaries, therefore it does not regulate perfectly. Why else would more input voltage prompt the failure of the -110 volt circuit ?

I don't believe the waveform you are getting at the junction of those diodes is right, and I mean not formed right. The best thing to do is probably to go directly to the collector(s) of the chopper(s). that way you really know the duty cycle. the waveform sat the diodes is not an accurate picture of what is going on at the collector, therefore you can't be sure of the on time. At the collector you get the actual on time, right at the baseline. you can also measure the voltage across the chopper (E-C) while it is on. An overload will cause that to be higher usually. And the duration increases with reduced input voltage or increased load. If your waveforms were actually at the collector the on time would be extremely excessive. But that's not what you're looking at.

My mind is turning to mush. I just got through printing about 50 pages in triplicate of excerpts form a medical record that is about 1,300 pages long, and numbering each page with ID of which file it came from and had to clear a paper jam and all that cool shit. Now I know why they pay office workers. I also worked a bit on some correspondence with a lawyer in Pittsburgh about some mineral rights I don't want to get screwed on. I AM capable of representing myself but don't ever let anyone tell you it's easy.

Tomorrow I got half a day with the cardiologist, I am probably fine but the insurance is paying so since I got a chest pain recently and get the 'funny" feeling sometimes I got referred and have a nuclear stress test. I'll probably be ready for a nap after that, but then I will be probably back on the Tek. I have yet to take the voltage up since fixing the - 110 volt supply. I really thought the transformer was arcing internally. Of course that is probably not all that common because there is not that much voltage there. I need to see if I still get the strange "noisy" part of the waveform. If not, that cures that much but not the heat. If it is gone then good, the thing is better but just not all the way yet.

So when you get back to it, put the scope probe right on the collector(s). Let's see THAT waveform.



Re: 7603 Z-Axis board transistor Q1152 running a little too hot?

Al Holt
 

Fabio,
Thank you for your valuable and informative suggestions! The 20k resistors shown are 1W metal film...but you're right they could and should be beefier. I'll work on that. In the meantime I'll run through those checks you mentioned.

I hope this is not too far off the original subject, is there a 'simplified' set of calibration instructions? OK well, more sort of hobbyist oriented that doesn't require special plug-ins or calibration fixtures. ATM, my interest is calibrating the focus and intensity aspects of the 'scope. I haven't started thinking about the plug-ins I have, yet. So far the basic 'scope is working.

Cheers!

--Al


Re: 7603 Z-Axis board transistor Q1152 running a little too hot?

Fabio Trevisan
 

Hello Al,
Ops... As I was re-reading your original post, I realize that I advised you to do measurements you already reported done from the beginning.
You already tested the 3k9 resistors and measured the voltage at the Q1152 base at 103V... so this already determines that the bias point is OK.
If there's no excessive current coming out / in to / from the load (through the 120 ohm resistor)... then I think there's not much to worry.
Maybe only replace the resistor for one of bigger wattage, which will make it run cooler (although the dissipated power will remain the same and therefore, the air temperature in the area won't probably change).
Rgrds,
Fabio

On Fri, Apr 27, 2018 at 08:24 am, Fabio Trevisan wrote:

Hello Al,
Looking at your picture, the degree of "browning" that you're referring to is
lighter than I've seen on some hot spots in scopes of this era (and age) so,
there's nothing really worrying in there.
On the actual dissipated power... I made some calculations on quiescent point,
and each pair of transistors; Q1132 with Q1134, and Q1152 with Q1154
dissipates about 0.65W (per pair), and that's just the quiescent power, not
mentioning the added power due to loading, from the circuitry downstream.
T0-92 transistors are usually rated as 0.6W, and TO-39 transistors at 1W
MAXIMUM (i.e. if you would ensure you're keeping the case at 25 degrees, which
is not the case).
If only quiescently they're (each pair) already dissipating 0.65W, (and again,
not accounting for additional power due to current coming from or going into
the load)... I think they really run quite hot!
Not scorching hot, but hot enough to develop some browning of the PCB
throughout 40 years of use.
Besides, the 20K resistor have almost 100V across it, and it alone dissipates
0.5W, which is enough to brown the PCB and heat the surrounding transistors as
well.

For your peace of mind's sake, first of all, substitute that 20K resistor you
replaced for one bigger than the one it's there (use at least a 1W, better 2W
resistor if you want it to run cool). and measure the voltages on the circuit
to make sure they meet the calculations and there's no component drifted.
The 3K9 resistors at the emitter of the top transistors should have about 25V
accross them.
Lift one of their leads and measure their resistance, to be sure they're still
at 3k9).
If the resistors are within tolerance, and the voltage is about 25V, then you
know the collector current of the output transistor pair is correct.
Measure the voltage at the collector of Q1136 and Q1156... they both should
measure about 4V (relative to ground)... this will ensure the bottom part of
the bias resistor string is not drifted and that the bottom transistor is
operating at the intended bias point.
Finally, measure the voltage across the 120ohm resistors at the output... It
will give you an idea of how much current is being drawn by the load circuitry
downstream.
By no means this current can be more than the quiescent bias current going
through the output transistors, which is about 6mA.
My wild guess is that it shouldn't be any greater than half of it, or about
3mA, because otherwise this would seriously upset the quiescent point of a
Class A amplifier.

Please do those checks and, if it measures right, than just let it be like the
Tek engineers designed it... It lasted 40 years... how bad can such a project
be?

Rgrds,

Fabio