Date   

Re: 468 sweep offset issue

 

On 25 Feb 2016 14:41:56 -0800, you wrote:

Also, a quick question regarding bandwidth problems:

My 465 only has 55Mhz bandwidth on either channel... I noticed this when I decided to calibrate it. What would be a likely culprit? Other than that issue, my 465 is in fantastic condition and runs great. I haven't changed a single part on it since I got it last year. I also did not touch any of the LF/HF comp adjustments in the vertical section!!

My noob mind started thinking that it could be some of the RC's in the signal path, but I haven't yet learned enough about circuit functionality, HF/VHF, etc yet. I'm still studying "Electronics for Engineers" and refreshing my math skills....

I forgot to mention - the procedure for checking bandwidth is the same between the 465 and 468. This part of the cal procedure is on page 4-44.

In order to determine the bandwidth capability, I reduced the output frequency on the 191 leveled sinewave gen until the 465 display was 3.5 vertical divisions in amplitude. That point was at 55Mhz. It should not have reduced to less than 3.5 vertical divisions until beyond 100Mhz.

The output on my 191 is good. I checked it with a freq counter that was checked against a double OCXO. ( STP2145A )
How do you know that the leveled output on the 191 is really level?

I usually do a rise/fall time test. I mentioned a good source to try
in my last post.


Re: 468 sweep offset issue

 

On 25 Feb 2016 14:23:27 -0800, you wrote:

I sometimes trim the values of various parts or match transistors to
fix problems like these.

There is only one Sweep Start adjustment for both sweeps and based on
the design shown on schematic 8, there could be 100s of millivolts of
difference between the start points of the sweep generators do to the
matching of CR953/CR954 and CR980/CR984 shown on schematic
8. Differential amplifier/comparators made up of Q958/Q960 and
Q987A/Q987B control the sweep start voltages. Oddly enough, the A
sweep uses a dual transistor which will have better matching while the
B sweep uses two separate transistors; I wonder why.
With an estimated CRT deflection of 1 volt per division at the sweep
output, a difference of 1 minor division or more is possible and your
first photograph shows an offset between the sweeps which is within
the design tolerances.
So if I want to see if it's possible to reduce or eliminate the misalignment, I should focus on the voltage levels in the sweep start comparator and try to match them? I have a pile of 1n4251 diodes, so I could certainly make a matched set of 4.
I took a close look at the schematic and that is where I would start
as I think it would be the easiest way to improve the sweep start
alignment.

The diodes only need to be matched for forward voltage drop in pairs;
you do not need 4 matched diodes. The 2 discrete transistors in the B
sweep comparator should also be matched for Vbe. Ignore Q987 in the A
sweep comparator for now since as a dual transistor, it should be well
matched anyway.

Use a 5 milliamp test current for matching. See below.

Also, it's apparent that the four 36ohm resistors in that circuit have been replaced. I checked what I could in circuit, and none that I was able to measure were out of tolerance. Then again, spec tolerance being 5% on many of these resistors could easily cause a voltage difference of tens or hundreds of mV.
Operating current for the comparators is about 10 milliamps total so
the voltage drop across each resistor is 5 mA * 36 Ohms = 180
millivolts. Worst case tolerances would make that 18 millivolts of
difference which is pretty small compared to the typical mismatch in
the diodes and B sweep transistors. It would not hurt to replace them
with 1% metal film resistors. The values are not critical.

However, being that it is 'within the design tolerances', I may just leave it and move on for the time being. I want to get this 7854 operational! Additionally, I have messed with the vertical HF comp adjustments - and having no calibrated fast rise generator leaves me lacking the ability to at least get it back to where it was. It was originally quite noisy on many vertical settings - especially 5mV. ( I wonder what Tektronix up in Pittsburgh would charge me to calibrate my 106...)
Since the changes I suggest above are pretty easy to do, I would try
them.

We have diagnosed other 465B type oscilloscopes here which had as much
offset between the sweeps.
If you want to trim the offset out, then I can make suggestion for how
to do it but it will require modifying the circuit slightly.
I'll look up that topic and have a read...

I may take you up on that once I am able to finish the calibration to where I can actually use this scope. After all I've done to it, it will be my best 400 series unit.
The modification I was considering is adding a trim to the A sweep
start voltage at the base of Q987A but I bet matching the diodes and B
sweep transistors will completely solve the problem.

Also, a quick question regarding bandwidth problems:
My 465 only has 55Mhz bandwidth on either channel... I noticed this when I decided to calibrate it. What would be a likely culprit? Other than that issue, my 465 is in fantastic condition and runs great. I haven't changed a single part on it since I got it last year. I also did not touch any of the LF/HF comp adjustments in the vertical section!!

My noob mind started thinking that it could be some of the RC's in the signal path, but I haven't yet learned enough about circuit functionality, HF/VHF, etc yet. I'm still studying "Electronics for Engineers" and refreshing my math skills....
Didn't we discuss the low bandwidth on your 465 here in the past?
Maybe I am thinking of a different one.

How did you check the bandwidth? Is the signal source fast or
accurate enough for a 100 MHz or 3.5 nanosecond test?

One clever signal source you can use for a bandwidth test is the
horizontal (pin 13) or vertical (pin 14) sync outputs on an analog VGA
connector. They are designed to drive 75 ohms but in my experience a
directly connected standard x10 passive probe with a suitable
bandwidth will work fine.


Re: 468 sweep offset issue

 

Just a thought. Did you have the (any) cable terminated to 50 ohms at the scope input? How about the level from the generator, is it flat?

----- Original Message -----
From: analogaddict013@... [TekScopes]
To: TekScopes@...
Sent: Thursday, February 25, 2016 5:41 PM
Subject: [TekScopes] Re: 468 sweep offset issue




Also, a quick question regarding bandwidth problems:
My 465 only has 55Mhz bandwidth on either channel... I noticed this when I decided to calibrate it. What would be a likely culprit? Other than that issue, my 465 is in fantastic condition and runs great. I haven't changed a single part on it since I got it last year. I also did not touch any of the LF/HF comp adjustments in the vertical section!!

My noob mind started thinking that it could be some of the RC's in the signal path, but I haven't yet learned enough about circuit functionality, HF/VHF, etc yet. I'm still studying "Electronics for Engineers" and refreshing my math skills....


I forgot to mention - the procedure for checking bandwidth is the same between the 465 and 468. This part of the cal procedure is on page 4-44.

In order to determine the bandwidth capability, I reduced the output frequency on the 191 leveled sinewave gen until the 465 display was 3.5 vertical divisions in amplitude. That point was at 55Mhz. It should not have reduced to less than 3.5 vertical divisions until beyond 100Mhz.

The output on my 191 is good. I checked it with a freq counter that was checked against a double OCXO. ( STP2145A )



[Non-text portions of this message have been removed]





[Non-text portions of this message have been removed]


SC503 storage mode

Malcolm Hunter
 

Hi,

I just bought an SC503, which seems to work mostly fine as an analogue
oscilloscope, but when I use the storage mode, the display initially goes
dark but then fills with colour. Any trace that goes across it leaves
virtually no mark on the screen. Pressing erase does trigger the trace and
it briefly leaves an imprint, but this is quickly obliterated. Is this a
sign of a faulty CRT or is there a fault or just a need for adjustment?

Also, the trace goes extremely dim with the timebase set to 5ms and above.
I'm hoping this is adjustable.

I bought it as "good working order", so I'd like to know what to go back to
the seller with. This is becoming a bit of a theme :(

It's a later than B039999 S/N, so I'm waiting for a service manual from
Artek.

Thanks,
Malcolm


Re: 2465 GPIB

Michael Louie
 

Did you find any answer to your question about the 2465 GPIB Option yet?

I think that the extra parts for GPIB Option for the 2445, 2465, 2445A, and 2465A are mostly the same.


The PROMs on the A5 control board may not be the same.


And, I don't know how the GPIB Option works as I have never used one before. But looking at my 2nd 2445 (which has the Factory GPIB Option), it looks like the Front panel has extra cables on it that connect to the Buffer Board. Looks like the Buffer Board and GPIB boards are the same, but I would have to check the Service Manual to confirm.


It would make sense to me that the front panel would have extra cable attached because I read that the GPIB Board can report the switch setting over the GPIB port, and it would need to know how the switches are set on the Front Panel. If this is true, the 2445 and 2465 front panels will not work with a 2465A with GPIB, because they lack the extra cables that are fit onto the Front Panels of the A models.


Interesting enough is the fact that GPIB parts sets offered on eBay for sale have different assortments of parts included. They usually include the GPIB board, and the Buffer board, but do not always include the cables, or the LED display panel. Sometimes the rear plastic panel is included, and sometime it isn't. The plastic rear panel needs to be matched with the same type of fan that it came from, as the back/rear of the plastic panels (and the back steel panel) are different, depending on whether it was used with a Siemens or boxer fan. The GPIB kits that I have seen never include the front panels.


What parts are you looking for?


Re: 7834 horizontal jittery readout

Gordon <tekscopes@...>
 

On 25/02/2016 22:02, David @DWH [TekScopes] wrote:
I think that could be normal depending on what is being drawn at the
time. Does the photograph show the oscilloscope monitoring its own
readout?
Thanks and no, that's a 7633.

What effect does changing the readout mode to gated have? The slide
switch for this is on the front panel to the immediate right of the
readout intensity control.
It doesn't affect the jitter as I've noticed.


How does the readout behave when the sweep is very slow like
100ms/div?
I'll check again tomorrow but I don't think it makes any difference.

It is normal for there to be some interaction between the horizontal
sweep and the readout which causes the readout to shift around
somewhat. The same goes for the vertical interaction.
Yes, I've seen that but this is more. It's very noticeable. I'll take a pic of the jitter tomorrow - if it shows up well enough on a pic.

I'm not 100% sure it isn't affecting the trace as well. It's enough to make the readout a bit difficult to read but rather hard to tell on a trace. I'll try a triangle waveform tomorrow, it might show up better if the trace is affected as well.

Cheers

Gordon


Re: 468 sweep offset issue

Brian Bloom
 

Also, a quick question regarding bandwidth problems:
>My 465 only has 55Mhz bandwidth on either channel... I noticed this when I decided to calibrate it. What >would be a likely culprit? Other than that issue, my 465 is in fantastic condition and runs great. I haven't >changed a single part on it since I got it last year. I also did not touch any of the LF/HF comp adjustments in >the vertical section!!

>My noob mind started thinking that it could be some of the RC's in the signal path, but I haven't yet learned >enough about circuit functionality, HF/VHF, etc yet. I'm still studying "Electronics for Engineers" and >refreshing my math skills....


>I forgot to mention - the procedure for checking bandwidth is the same
>between the 465 and 468. This part of the cal procedure is on page 4-44.
>In order to determine the bandwidth capability, I reduced the output
>frequency on the 191 leveled sinewave gen until the 465 display was 3.5
>vertical divisions in amplitude. That point was at 55Mhz. It should not have
>reduced to less than 3.5 vertical divisions until beyond 100Mhz.
>The output on my 191 is good. I checked it with a freq counter that was
>checked against a double OCXO. ( STP2145A )


I found the topic "465B Degraded Bandwidth" through my search for the 465B topic relating to the sweep start misalignment. :-)

I suspect this topic will have the resolution to my bandwidth problem and am going to read through it for ideas.


Re: Need Help Fixing A Couple of 2445 Scopes

Michael Louie
 

I spent some more time looking on the Web for a Service Manual for the @$$% GPIB Option, and all I coiuld find was the Service Manual for 24X5A Options, so I went ahead and dowloaded,it figuring that I could compare it to what my later 2445 contained, and maybe it would be the same.

My later 31XXX 2445 must be a late one, because the rear panel has the boxer fan of the 24X5A and 24X5B.


The parts drawing in the manual I downloaded looks exactly like the insides of my 2445. The only cable that I have not figured out for sure is clearly pictured on the drawing of the 24X5A as cable #25. And it shown on the picture where it is plugged into the A20 Buffer Board (which I referred to as the motherboard/expansion board) in the original post.


But, only the end of the 7 conductor cable that plus into the Buffer board is shown. What I STILL NEED TO KNOW is where the other end of the 7 conductor cable plugs in.


After some reflection, I decided to try powering the scope, even though I still have the one cable that is not plugged it, to see what would happened.


It turns on now, and I can see a very faint (barely visible) very tall horizontal trace. I have no control over it the trace. Pushing the trace Finder button does nothing. I have scale illumination and control of it. The display is blinking at a slow rate. I think the readout lines are showing, but they are tiny/shunken/unreadable no vertical or horizontal expansion at all.


From the looks of it, although the power supplies looked to be correct when I used a Fluke DMM to look at them before I found all of the unplugged cables. not that the cables are plugged in, looks like the power supply needs some work.


I was debating buying a Power Supply Minimum Load off an eBay Seller (looks like the guy that makes them is a Contributor on this site), and he told me that from my description of the problems on the 1st 2445 it sounded like the power supply for that one was working was working. Since it looks like both of my power supplies need work now, I suspect I'll be buying one of his Power Supply Tools very soon.


Re: 468 sweep offset issue

Brian Bloom
 

Hey there,
do you have a name we can call you by?

Yes - My name is Brian. I have not yet figured out how to make Yahoo display anything other than my Yahoo I.D..... sorry.



Interestingly it looks like the "B sweep start" trim sets the start voltage
for both sweeps, so if they're significantly off, there'll have to be a bad
component someplace. So, perhaps start by measuring the quiescent voltage
of the A/B sweeps - they ought to be at the same level.
Looking closer, according to figure 67, the A sweep ramp has ~11V range
(this is for - what - 12-15divs of sweep normally?), so a division off
smells like a diode drop :).

A bad diode had been suggested as the culprit over on EEVblog at some point, but I've checked every last one of them. I've also replaced all of the unblanking diodes out of frustration when I thought it was an unblanking issue.. I suppose one positive aspect of doing so is that it made the trace a little bit more crisp! That's all it did, though.


In summary, check the quiescent DC levels of the sweeps, they should be at
the same voltage. If not, look at the start voltage diff amp for the sweep
that's off.
If the sweeps are at the same quiescent voltage, you probably need to look
at the B sweep start comparator.
The other problem commonly seen in these sweep generators is leaky or bad
FET. If you measure the voltage over R866, you should measure precisely
zero volts, as there should be no current (to mention) going through the
gate of Q867. The symptoms to a bad FET are usually more obvious, as in the
sweep hangs, goes backwards or is very non-linear, but it's easy to check
for this.


None of the transistors in the sweep generator circuits are bad. I've tried swapping them out with new ones. Doing so had no effect.


Re: 468 sweep offset issue

Brian Bloom
 

>According to the schematic for the horizontal amp, when the scope is set up in XY mode (as per the voltage measurement conditions) both plates should measure close to 52V. My 110V rail is 2V high, so it should measure 54V, but either way they should both be the same with the dot centered. Left plate measures 56.7V and Right plate measures 54.3V.
It is normal for the CRT to have some vertical and horizontal offset
to produce a centered beam. To compensate for this, the CRT
amplifiers may have a centering adjustments. x1 horizontal centering
on the 468 is adjusted with R551 shown on schematic 10 and x10
horizontal centering is adjusted with R578 shown on schematic 10.
There are separate vertical centering adjustments for each input which
do the same thing.

>The horizontal position control is off-center to the right side of the display by approx 2 divisions.
>Also, when I center the trace in a normal timebase setting and then switch it into XY, the dot is positioned that same 2 division to right offset.







This should be corrected by the procedure on page 4-64 if the
horizontal position control is already centered. Tektronix appears to
have left that out.

I have gone through the entire calibration procedure for the non-storage section with this scope. Aside from not being able to properly perform the steps which require a 106 or PG506 for fast rise, it's completely calibrated. All of the "problems" left that I've noted are after following the cal procedure exactly. (These procedures are lengthy!! Without having past experience performing them, it takes me 2 evenings to get a single cal done on the non storage section!!)


I sometimes trim the values of various parts or match transistors to
fix problems like these.
There is only one Sweep Start adjustment for both sweeps and based on
the design shown on schematic 8, there could be 100s of millivolts of
difference between the start points of the sweep generators do to the
matching of CR953/CR954 and CR980/CR984 shown on schematic
8. Differential amplifier/comparators made up of Q958/Q960 and
Q987A/Q987B control the sweep start voltages. Oddly enough, the A
sweep uses a dual transistor which will have better matching while the
B sweep uses two separate transistors; I wonder why.
With an estimated CRT deflection of 1 volt per division at the sweep
output, a difference of 1 minor division or more is possible and your
first photograph shows an offset between the sweeps which is within
the design tolerances.


So if I want to see if it's possible to reduce or eliminate the misalignment, I should focus on the voltage levels in the sweep start comparator and try to match them? I have a pile of 1n4251 diodes, so I could certainly make a matched set of 4.
Also, it's apparent that the four 36ohm resistors in that circuit have been replaced. I checked what I could in circuit, and none that I was able to measure were out of tolerance. Then again, spec tolerance being 5% on many of these resistors could easily cause a voltage difference of tens or hundreds of mV.

However, being that it is 'within the design tolerances', I may just leave it and move on for the time being. I want to get this 7854 operational! Additionally, I have messed with the vertical HF comp adjustments - and having no calibrated fast rise generator leaves me lacking the ability to at least get it back to where it was. It was originally quite noisy on many vertical settings - especially 5mV. ( I wonder what Tektronix up in Pittsburgh would charge me to calibrate my 106...)




We have diagnosed other 465B type oscilloscopes here which had as much
offset between the sweeps.
If you want to trim the offset out, then I can make suggestion for how
to do it but it will require modifying the circuit slightly.


I'll look up that topic and have a read...

I may take you up on that once I am able to finish the calibration to where I can actually use this scope. After all I've done to it, it will be my best 400 series unit.


Also, a quick question regarding bandwidth problems:
My 465 only has 55Mhz bandwidth on either channel... I noticed this when I decided to calibrate it. What would be a likely culprit? Other than that issue, my 465 is in fantastic condition and runs great. I haven't changed a single part on it since I got it last year. I also did not touch any of the LF/HF comp adjustments in the vertical section!!

My noob mind started thinking that it could be some of the RC's in the signal path, but I haven't yet learned enough about circuit functionality, HF/VHF, etc yet. I'm still studying "Electronics for Engineers" and refreshing my math skills....


I forgot to mention - the procedure for checking bandwidth is the same
between the 465 and 468. This part of the cal procedure is on page 4-44.
In order to determine the bandwidth capability, I reduced the output
frequency on the 191 leveled sinewave gen until the 465 display was 3.5
vertical divisions in amplitude. That point was at 55Mhz. It should not have
reduced to less than 3.5 vertical divisions until beyond 100Mhz.
The output on my 191 is good. I checked it with a freq counter that was
checked against a double OCXO. ( STP2145A )


Re: 468 sweep offset issue

Brian Bloom
 

Sorry, the formatting changed from when I had typed it to when I had posted it, so I deleted them in order to correct the issue.....

It appears that David has posted here in the process of me doing so.


Re: 7834 horizontal jittery readout

 

I think that could be normal depending on what is being drawn at the
time. Does the photograph show the oscilloscope monitoring its own
readout?

What effect does changing the readout mode to gated have? The slide
switch for this is on the front panel to the immediate right of the
readout intensity control.

How does the readout behave when the sweep is very slow like
100ms/div?

It is normal for there to be some interaction between the horizontal
sweep and the readout which causes the readout to shift around
somewhat. The same goes for the vertical interaction.

On Thu, 25 Feb 2016 20:56:48 +0000, you wrote:

On 25/02/2016 20:54, Gordon @GordonW [TekScopes] wrote:
Checking TP12 and 13 on the readout board (x and y out) reveals a rather
odd waveformhttp://www.mgcsoft.com/tek/7834_tp12_readout_bd.jpg.
That's 2V not 200mV, it's a *10 probe.

Gordon


Re: 7834 horizontal jittery readout

Gordon <tekscopes@...>
 

On 25/02/2016 20:54, Gordon @GordonW [TekScopes] wrote:
Checking TP12 and 13 on the readout board (x and y out) reveals a rather
odd waveformhttp://www.mgcsoft.com/tek/7834_tp12_readout_bd.jpg.
That's 2V not 200mV, it's a *10 probe.

Gordon


7834 horizontal jittery readout

Gordon <tekscopes@...>
 

Hi All. Just trying to sort out a jittery readout on my 7834. The ramp outputs of the horizontal board look very stable and the trace itself doesn't appear to be jittery, at least nothing like the readout. In inches I'd say it's jittering about 0.040" maybe a bit less but only horizontally. The supplies don't appear to have significant ripple and putting an extra 4.7u on the 130V doesn't make any difference.

Checking TP12 and 13 on the readout board (x and y out) reveals a rather odd waveform http://www.mgcsoft.com/tek/7834_tp12_readout_bd.jpg. The waveforms in the manual aren't really clear enough. Could someone confirm whether or not the waveform is something like normal please.

Cheers

Gordon


Last few 11K plugs in left at Sphere

 

My thanks to all that responded about the 11K stuff we are clearing out. Shane at Avatar Metrology ran all the plug ins for everybody in his 11302A to be sure they were good, so there shouldn't be any surprises on arrival.

All the cal, blank and comparator plug ins and many of the others are gone, here's what's left:

1 ea. 11A34 (quad) US$50 (found another one!)
3 ea. 11A32 (dual) US$50 each
3 ea. 11A71 (fast 50 ohm single) US$70 each

I also have some bad (sorry, no idea what the issues were, just tagged bad, some missing covers) 11K plug ins, they are 11A32 and 11A52, US$25 each. still look good, and no visible internal damage. looks like we can ship 2 plug-ins expresspost to the US for about C$58, or by regular post for C$39.

I also have a lot of 11401 internal boards left (known good) if any help to somebody out there. There's also a bad 11302 frame that will go at stuff day, US$150, has a deflection problem, no other details. just failed about a year ago, was set aside, may be a psu problem. Too big and heavy to ship, so pick up at stuff day (april 9th) ONLY.

let me know if you can use any of these, YES, I will hold any stuff for pick up in April at stuff day if settled now.

Lots more Tek stuff to go on stuff day, I will try to advise on as much as I can here on the list before stuff day, but we are just in the middle of getting everything restored inside the building after having the floor replaced, (took 8 months), so not a lot of time to do that.

all the best,
walter & susan (walter2 -at- sphere.bc.ca)
sphere research corp.
http://www.sphere.bc.ca


Re: TDS 380 calibration and vertical scale problem

Gilles Drolet
 

Hi David
Remove as much colder as you can without scrapping the board and Just reflow the holes with chipquick and the part will come off the board.
Good luck
Gilles

On Thursday, February 25, 2016 10:31 AM, "david.wilson92@... [TekScopes]" <TekScopes@...> wrote:



  It looks like I am in a real bind desoldering ch2 hybrid from the mainboard. The joints are through hole and some of the joints I was able to get the solder off at the top but at the bottom there is suborn solder that just refuses the come out. I tried desoldering several times but not having much luck. Since I don't want to break anything I thought about putting it back back the way it is and just use as is. A partially working scope is better than a broken one.

I could use a soldering iron to flow solder back in the through hole and try to desolder once again. The board is about 4 mm thick approximately and so are the through hole joints.

[Non-text portions of this message have been removed]

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[Non-text portions of this message have been removed]


Re: 468 sweep offset issue

 

On Thu, 25 Feb 2016 16:10:21 +0000, you wrote:

Hey there,

do you have a name we can call you by?

I took a look at the sweep generator schematics, and in your shoes I'd
start looking at the quiescent DC levels of the sweep generators. R970
trims the B sweep start voltage, and Q958/Q960 are the differential
amplifier that set the quiescent (start) voltage of the B sweep. You should
see the same voltage on the base (&emitter) of both transistors, and you
should be able to trim it with the trimmer. The schematic I got from
koo4b's site is pretty poor, but it looks like the base of Q958 is
annotated with +13.0V or +13.6V.
Somewhere in there (I didn't find it right quick) there's also a comparator
between the A sweep ramp and the delay pot's output voltage.
It's worth checking the quiescent state of the A sweep as well, as if
that's off, you may be looking at a stretched sweep that's out of range of
the delay pot at the start end.

... time passes ...

Interestingly it looks like the "B sweep start" trim sets the start voltage
for both sweeps, so if they're significantly off, there'll have to be a bad
component someplace. So, perhaps start by measuring the quiescent voltage
of the A/B sweeps - they ought to be at the same level.
Looking closer, according to figure 67, the A sweep ramp has ~11V range
(this is for - what - 12-15divs of sweep normally?), so a division off
smells like a diode drop :).
There is only one adjustment for the A and B sweep start voltage. The
comparator circuit they use for each one could have 10s to 100s of
millivolts of offset so potentially the start voltage could be offset
by that much. With a sensitivity of 1 sweep volt per division, a
sweep offset of more than a minor division (200 millivolts) is
possible.

The A and B sweep stop voltages are fixed at the same level although
independently referenced to the +5 volt supply and will have even more
offset. The stop points are not important as long as they are greater
than 10 divisions.

In summary, check the quiescent DC levels of the sweeps, they should be at
the same voltage. If not, look at the start voltage diff amp for the sweep
that's off.
If the sweeps are at the same quiescent voltage, you probably need to look
at the B sweep start comparator.

The other problem commonly seen in these sweep generators is leaky or bad
FET. If you measure the voltage over R866, you should measure precisely
zero volts, as there should be no current (to mention) going through the
gate of Q867. The symptoms to a bad FET are usually more obvious, as in the
sweep hangs, goes backwards or is very non-linear, but it's easy to check
for this.
The sweep start voltage are independent of any offset in the sweep
integrators unless they are so broken that the start voltage cannot be
attained.


Re: 468 sweep offset issue

 

On 24 Feb 2016 18:18:13 -0800, you wrote:

There is only one apparent difference between the scopes other than the A/B sweep start misalignment.

On the "good" 468, B sweep ends 1 minor division before A sweep.

On the "bad" 468, A and B sweep end at exactly the same point.
The sweep lengths are not tightly controlled although it looks like
Tektronix went to some effort to make sure they were stable. They
only have to be "long enough" to cover more than 10 divisions.

The sweep stop voltage levels are fixed and the only adjustment which
alters them is a change in the regulated +5 volt supply. The
difference between the A and B sweep stop voltage could be 100s of
millivolts amounting to a couple of minor divisions.


Re: 468 sweep offset issue

Siggi
 

Hey there,

do you have a name we can call you by?

I took a look at the sweep generator schematics, and in your shoes I'd
start looking at the quiescent DC levels of the sweep generators. R970
trims the B sweep start voltage, and Q958/Q960 are the differential
amplifier that set the quiescent (start) voltage of the B sweep. You should
see the same voltage on the base (&emitter) of both transistors, and you
should be able to trim it with the trimmer. The schematic I got from
koo4b's site is pretty poor, but it looks like the base of Q958 is
annotated with +13.0V or +13.6V.
Somewhere in there (I didn't find it right quick) there's also a comparator
between the A sweep ramp and the delay pot's output voltage.
It's worth checking the quiescent state of the A sweep as well, as if
that's off, you may be looking at a stretched sweep that's out of range of
the delay pot at the start end.

... time passes ...

Interestingly it looks like the "B sweep start" trim sets the start voltage
for both sweeps, so if they're significantly off, there'll have to be a bad
component someplace. So, perhaps start by measuring the quiescent voltage
of the A/B sweeps - they ought to be at the same level.
Looking closer, according to figure 67, the A sweep ramp has ~11V range
(this is for - what - 12-15divs of sweep normally?), so a division off
smells like a diode drop :).

In summary, check the quiescent DC levels of the sweeps, they should be at
the same voltage. If not, look at the start voltage diff amp for the sweep
that's off.
If the sweeps are at the same quiescent voltage, you probably need to look
at the B sweep start comparator.

The other problem commonly seen in these sweep generators is leaky or bad
FET. If you measure the voltage over R866, you should measure precisely
zero volts, as there should be no current (to mention) going through the
gate of Q867. The symptoms to a bad FET are usually more obvious, as in the
sweep hangs, goes backwards or is very non-linear, but it's easy to check
for this.

Siggi

On Thu, 25 Feb 2016 at 00:33 analogaddict013@... [TekScopes] <
TekScopes@...> wrote:



I didn't get to do much today, but I checked the waveforms at the
disconnect amplifiers for A and B.

The waveforms sync up, but the A disconnect amplifier (at emitter) is .5V
high ( -2V instead of -2.5V) and the B disconnect amp ( at emitter) is .3V
high ( at -2.6V instead of -3V).

I'll do more tomorrow - hopefullly I will have the time to check the
entire sweep logic section and find the out of tolerance resistor(s). It
appears that is what I should be looking for... (am I correct?)



[Non-text portions of this message have been removed]



Re: 468 sweep offset issue

 

On 24 Feb 2016 15:53:50 -0800, you wrote:

I am not sure what you mean by voltage offset. With no sweep, there
is always a voltage offset because the blanked beam is located at the
left side of the CRT.
Is the bow on the right 1/4 of the CRT visible in your photos?
It is only slightly visible in the close up photo. It is most noticeable when I have the scope set up per the calibration procedure for Y axis and Geometry adjustment. I will upload a photo of the display symptoms shortly.
This may be just a characteristic of the CRT. Tektronix sometimes
specified a tolerance for geometric distortion inside and outside of
the center area. Toward the edges, you can usually find some geometry
error if you look closely enough.

According to the schematic for the horizontal amp, when the scope is set up in XY mode (as per the voltage measurement conditions) both plates should measure close to 52V. My 110V rail is 2V high, so it should measure 54V, but either way they should both be the same with the dot centered. Left plate measures 56.7V and Right plate measures 54.3V.
It is normal for the CRT to have some vertical and horizontal offset
to produce a centered beam. To compensate for this, the CRT
amplifiers may have a centering adjustments. x1 horizontal centering
on the 468 is adjusted with R551 shown on schematic 10 and x10
horizontal centering is adjusted with R578 shown on schematic 10.
There are separate vertical centering adjustments for each input which
do the same thing.

The horizontal position control is off-center to the right side of the display by approx 2 divisions.
Also, when I center the trace in a normal timebase setting and then switch it into XY, the dot is positioned that same 2 division to right offset.
This should be corrected by the procedure on page 4-64 if the
horizontal position control is already centered. Tektronix appears to
have left that out.

Are you referring to the start point of the A and B sweeps not being
exactly aligned?
The alignment depends on circuit tolerances and there may be no
adjustment for it. At high sweep speeds, it is normal for the start
of the sweep to creep to the right somewhat.
I can probably suggest where it can be trimmed out if we determine
that there is no actual fault.
Yes, I am referring to the A and B sweeps not being aligned. My other 468 doesn't have this problem at any timebase settings. (That one needs more work than the one being discussed.)

What really bothers me about it, and also has me slightly confused, is how to properly adjust the sweep start and sweep cal settings when taking this problem into account.

As per the horizontal cal procedure (it's the first step in this section):
-Set timebase A to 1ms and B to 5us
-Set delay time to 1.00
-Input 1ms time markers
-Adjust so the pulse displayed on B sweep starts at the beginning of the sweep & start of 2nd time marker from A sweep display**

The pulse on B sweep is not displayed if I set it to where I believe it should be - which is coinciding with the start of A sweep (which is the first time marker)
If I don't have it set exactly there, won't B sweep end up being out of tolerance?

I suppose I'll have to go through the waveforms again now that I have a digital scope and see if everything syncs up with the sync pulse, A and/or B sweep start, and blanking/unblanking. When I had checked them with my 465, everything appeared to be correct. Some waveforms just had minor differences in their voltage levels when compared against the manual spec, but seemed OK when compared against the other 468.
Their calibration procedure is pretty complicated and Tektronix does
not say what the goal is unfortunately. I suspect some of the steps
could be done in a different order for better results.

I definitely want to fix this and get them aligned. I guess I'm a bit OCD...
I sometimes trim the values of various parts or match transistors to
fix problems like these.

There is only one Sweep Start adjustment for both sweeps and based on
the design shown on schematic 8, there could be 100s of millivolts of
difference between the start points of the sweep generators do to the
matching of CR953/CR954 and CRsomething/CRsomething shown on schematic
8. Differential amplifier/comparators made up of Q958/Q960 and
Q987A/Q987B control the sweep start voltages. Oddly enough, the A
sweep uses a dual transistor which will have better matching while the
B sweep uses two separate transistors; I wonder why.

With an estimated CRT deflection of 1 volt per division at the sweep
output, a difference of 1 minor division or more is possible and your
first photograph shows an offset between the sweeps which is within
the design tolerances.

We have diagnosed other 465B type oscilloscopes here which had as much
offset between the sweeps.

If you want to trim the offset out, then I can make suggestion for how
to do it but it will require modifying the circuit slightly.

I just have a couple things to do and then I can get another picture or two uploaded, then do some waveform checking.