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Tektronix 465 CH2 issues #photo-notice


Timothy
 

Can someone please point me in the right direction?

I am repairing a Tek 465 that had several problems. So far I have rebuilt the power supply with all new capacitors, a new bridge rectifier and 3 new transistors. All voltages are now as expected with low ripple. I then repaired an issue with A trigger not working.

Now I am am stumped by an issue with channel 2 vertical. I assume its a vertical issue because its missing about 50% of its amplitude and the square wave is ramping. I initially assumed it had to be part of these circuits since the problem is only on channel 2 however I can find zero issues on the Vertical Preamp board or Vertical Switching. Its originating somewhere else.

Someone has attempted repairs on the scope before me and they were ham fisted. The delay line was roughed up and had to be soldered to the vertical preamp board. I cleaned this up the best I could and Im not sure if it had anything to do with my problem, I dont see how it could affect anything.

https://groups.io/g/TekScopes/photo/253550/0?p=Created,,,20,2,0,0

I will cross post to the eevblog to get some other eyes on it as well.


John Gord
 

Timothy,
Have you tried using CH1 (or another scope) to trace a signal applied to CH2? It should get through at least to Q1833 (CH2 Trigger signal) even if CH2 is not selected.
--John Gord

On Mon, Sep 14, 2020 at 11:19 AM, Timothy wrote:


Can someone please point me in the right direction?

I am repairing a Tek 465 that had several problems. So far I have rebuilt the
power supply with all new capacitors, a new bridge rectifier and 3 new
transistors. All voltages are now as expected with low ripple. I then repaired
an issue with A trigger not working.

Now I am am stumped by an issue with channel 2 vertical. I assume its a
vertical issue because its missing about 50% of its amplitude and the square
wave is ramping. I initially assumed it had to be part of these circuits since
the problem is only on channel 2 however I can find zero issues on the
Vertical Preamp board or Vertical Switching. Its originating somewhere else.

Someone has attempted repairs on the scope before me and they were ham fisted.
The delay line was roughed up and had to be soldered to the vertical preamp
board. I cleaned this up the best I could and Im not sure if it had anything
to do with my problem, I dont see how it could affect anything.

https://groups.io/g/TekScopes/photo/253550/0?p=Created,,,20,2,0,0

I will cross post to the eevblog to get some other eyes on it as well.


Tom Lee
 

Correct -- the delay line can't be your problem, as it's common to both channels.

Your "ramping" waveform looks like a simple bandwidth reduction. It's what you would see if you passed an ideal square wave through an RC low pass filter. So you're losing bandwidth somewhere along the way.

If you don't have a second scope, use channel 1 to debug channel 2. Systematically probe along the signal path until you see the problem appear. You'll find it. It could be as simple as an oxidized switch contact (which could introduce a big R, and thus a big RC). Jiggle/push buttons (esp. the source select ones) and look for momentary returns to the correct waveshape, etc.

Good luck!

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/14/2020 11:19, Timothy via groups.io wrote:
Can someone please point me in the right direction?

I am repairing a Tek 465 that had several problems. So far I have rebuilt the power supply with all new capacitors, a new bridge rectifier and 3 new transistors. All voltages are now as expected with low ripple. I then repaired an issue with A trigger not working.

Now I am am stumped by an issue with channel 2 vertical. I assume its a vertical issue because its missing about 50% of its amplitude and the square wave is ramping. I initially assumed it had to be part of these circuits since the problem is only on channel 2 however I can find zero issues on the Vertical Preamp board or Vertical Switching. Its originating somewhere else.

Someone has attempted repairs on the scope before me and they were ham fisted. The delay line was roughed up and had to be soldered to the vertical preamp board. I cleaned this up the best I could and Im not sure if it had anything to do with my problem, I dont see how it could affect anything.

https://groups.io/g/TekScopes/photo/253550/0?p=Created,,,20,2,0,0

I will cross post to the eevblog to get some other eyes on it as well.


Tom Lee
 

Also, as a sanity check, swap the coax driving your two channels, just to make sure that the problem you're seeing is indeed originating inside the scope and not from, say, a flaky cable.

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/14/2020 11:19, Timothy via groups.io wrote:
Can someone please point me in the right direction?

I am repairing a Tek 465 that had several problems. So far I have rebuilt the power supply with all new capacitors, a new bridge rectifier and 3 new transistors. All voltages are now as expected with low ripple. I then repaired an issue with A trigger not working.

Now I am am stumped by an issue with channel 2 vertical. I assume its a vertical issue because its missing about 50% of its amplitude and the square wave is ramping. I initially assumed it had to be part of these circuits since the problem is only on channel 2 however I can find zero issues on the Vertical Preamp board or Vertical Switching. Its originating somewhere else.

Someone has attempted repairs on the scope before me and they were ham fisted. The delay line was roughed up and had to be soldered to the vertical preamp board. I cleaned this up the best I could and Im not sure if it had anything to do with my problem, I dont see how it could affect anything.

https://groups.io/g/TekScopes/photo/253550/0?p=Created,,,20,2,0,0

I will cross post to the eevblog to get some other eyes on it as well.


Timothy
 

Yes. This is a 465 (not B) so the final transistor is actually Q262. I am able to trace the entire problematic waveform with a second scope all the way from the attenuators R65 to Q262 and also to the delay line where ch1 and ch2 combine.

After testing the vertical preamp board and switches and finding nothing wrong, I was confused so I second guessed myself. I thought I must be missing something so I took a second working vertical preamp board complete with attenuators and vertical switches and swapped them and the problem was exactly the same.


Timothy
 

The problem appears very soon in the circuit in the attenuation section. The resistors all measured normal.

I have swapped cables as a sanity check and the problem did not budge. I fiddled with the switches and even used Deoxit on all the switches before switching out the boards. Unless both boards have the exact same problem on Channel 2 (what are the odds? Or Im making the some mistake twice) I think something is causing this from another part of the scope. I am going to tripple check the CH2 vertical preamp just in case.


Tom Lee
 

I’m interpreting your post to mean that the signal looks bad before you get to the input FET. That reduces the search area considerably. So the problem likely lies between the input bnc and that point in the attenuator.

A not uncommon problem is bad contacts to the fixed attenuation modules. Try changing the front panel vertical sensitivity settings to see if that makes a difference in wave shape. And try reseating the attenuator modules.

Ton


Sent from my iThing, so please forgive brevity and typos

On Sep 14, 2020, at 16:45, Timothy via groups.io <mustang_gt_y2k=yahoo.com@groups.io> wrote:

The problem appears very soon in the circuit in the attenuation section. The resistors all measured normal.

I have swapped cables as a sanity check and the problem did not budge. I fiddled with the switches and even used Deoxit on all the switches before switching out the boards. Unless both boards have the exact same problem on Channel 2 (what are the odds? Or Im making the some mistake twice) I think something is causing this from another part of the scope. I am going to tripple check the CH2 vertical preamp just in case.



Timothy
 

Yes, It starts in attenuation. After the C53. I went back to do more probing and found this saw wave in the switches at the spot the arrow is pointing to. I will see if I can figure out how its getting there. It should be a square wave.

https://groups.io/g/TekScopes/photo/253636/0?p=Created,,,20,2,0,0


Harvey White
 

Could the attenuators themselves be bad?  It's happened before.

If so, then the problems could be variable depending on the attenuation factor.

Harvey

On 9/14/2020 8:27 PM, Timothy via groups.io wrote:
Yes, It starts in attenuation. After the C53. I went back to do more probing and found this saw wave in the switches at the spot the arrow is pointing to. I will see if I can figure out how its getting there. It should be a square wave.

https://groups.io/g/TekScopes/photo/253636/0?p=Created,,,20,2,0,0



Timothy
 

I have swapped the removable attenuators around and it did not cure the problem. Also they usually dont have such a drastic affect on the trace in my experience.


Timothy
 

Anyone who suggested the problem was bad contacts (Tom etc.) was correct!

I did clean the contacts but one of the finger contacts in the CH2 AC/GND/DC switch was not making good contact. I ever so slightly bent it back and my traces are back to normal.

I guess my 2nd preamp board has the same problem, I thought it was fully working... At least I know how to fix it.

Anyways I want to offer a kind THANK YOU for those who took the time to help and sent me back to look for the problem again.


Tom Lee
 

Great job, Timothy! Glad to see that your persistence paid off.

Cheers
Tom

Sent from my iThing, so please forgive brevity and typos

On Sep 14, 2020, at 18:13, Timothy via groups.io <mustang_gt_y2k=yahoo.com@groups.io> wrote:

Anyone who suggested the problem was bad contacts (Tom etc.) was correct!

I did clean the contacts but one of the finger contacts in the CH2 AC/GND/DC switch was not making good contact. I ever so slightly bent it back and my traces are back to normal.

I guess my 2nd preamp board has the same problem, I thought it was fully working... At least I know how to fix it.

Anyways I want to offer a kind THANK YOU for those who took the time to help and sent me back to look for the problem again.