Gould OS1100 vertical amp modulation issue

[gkmaia]

I am trying to fix a Gould OS 1100 and

On the schematic attached points 50 and 51 are the outputs of mode selector that feed into the final vertical amp to crt plates.

The display only shows a trace when in chop mode.

In chop mode I see the carrying signal plus the modulation feed into the final vertical amplifier from pins 50 & 51. In CH1 or CH2 mode I only see the respective carrier signals with no modulation.

Should I expect the signal that feeds the vertical plates to have any modulation attached to the carrier signal?

[YetAnotherTechie]

Somewhere there should be a logic chip that controls the signal switching diodes, my similar scope with similar symptoms had a bad xor gate if i recall. Nothing to do with plates.

[gkmaia]

No sure If I could explain it properly. I've attached a video to it.

On the DUT I am feeding a sine and a square to each channel so I can see what each channel outputs to the vertical CRT plates.

When on CHOP I see both sine and square combined with modulation out of the vertical amplifier, which is fine, as I also get the DUT to display trace. So Chop is driving the amplifiers ok.

When on CH1 or CH2 I see a clean signal out to the CRT plates. Not that by seeing these two different signals independently means the DUT logic is working and feeding the vertical amplifier with the appropriate signal.

Trigger works in both channels. Timeband works.

But as you can see on the horizontal amp is driving the sweep in sync with vertical amplifier in CH1, 2 and Chop modes. But nothing is displayed in CH1,2 modes.

I am kind stuck with this one.

[YetAnotherTechie]

It looks like it sweeps the same in the 3 modes but only shows the very beginning of the trace when in CH1 or CH2, seems like your image is being blanked, check magnified trace, "B" time base, and their effects on "Z" modulation signal and blanking signal. Check bad contacts on alternate functions of buttons (pushed-pulled). Was the bottom channel on the HP scope DC or AC coupled? In the bad CRT the beginning of the trace shows both channels vertically offset, when you showed the vertical plate signal on the HP the chopped signal has a larger amplitude, but when you show the individual channel signals separately they appear with no offset between them.

[gkmaia]

Thanks for you tip! I havent thought about the blanking...

Ok, if I understood and looking at my schematics and what I see on the CRT I do not have a problem with my blanking circuit. It is working fine as long as it receives the right signal, which it does in CHOP mode. That means I may have a problem with what drives my blanking circuit.

If you look at the attached schematics the blanking signal is driven by bright up and chop gate which is made by 0v and 5v pulses. So my problem has to be before that somewhere on the timebase circuit?

I do also have another clue, my intensity knob does not work on CHOP as it is always very bright, but does work for the faint ch1/2 dots.

The dots that are shown on the CRT are the start of the traces... so for CH1 for example when in CHOP or CH1 mode the start of the trace has the same vertical position on the CRT this means the vertical plate is receiving the same voltage in CHOP and CH1. The same applies to CH2. So the vertical amplifier is fine. If I move to other modes the dots will ADD or ALT as they should on their expected positions.

Do you still think it is worth looking at the vertical block as what should drive the blanking circuit is the horizontal?

I am out of ideas here...

[YetAnotherTechie]

Hello,

From what i read "bright-up" is gould-speak for a free running trace, so i wouldn't look there, your trace seems ok in absolute terms of brightness. I don't know if your scope is similar to mine, but in mine the delayed trace brightens up a segment of the main trace, and that trace is then seen "zoomed in" somewhere else. You have a knob that says: "pull to locate" on the delay section, have you tried it? To me it looks like that brightening-while-delayed function is not working properly on your scope, play with it in the different modes and post results, also try applying a signal to Z-modulation input. You have the full correct schematic? the one you posted has some signal pictures similar to what you were trying to show on the video.

[gkmaia]

The full SM is attached.

Those plots you see on my schematic are my plots. Did those to follow the signal and study later.



I've added another video. Adding a 10pkpk square to Z input I got the trace on CH1 and 2 mode but as dots and as you can see:
- timeband is working fine
- vertical attenuator and am are working fine
- signal from vertical to trigger and timeband is flowing as expected
- delay expands, contracts and moves around the main wave, but I am not getting the secondary expanded wave.
- trigger works either in auto and manual.
- as trace is solid on CHOP I can discard CRT and HV circuit. 

So all main functions are somewhat working as expected, just cant get a trace and intensity is stuck on CHOP, but works fine on CH1 and CH2 modes.

As I was going mad with it. I removed and testes on a decent curve tracer all transistors and most diodes from the horizontal, power, CRT and vertical board, as well as replaced a few faulty ICs. I did mange to find a faulty transistor array on the vertical board. A faulty IC on the single sweep block and a faulty transistor at the trigger circuit which as replaced I was able to trigger again. I tested the PSU quite a few times and all voltages are good and I have no weal rails at all.

Is your a OS1100?

[gkmaia]

I know it is a big ask, but if you still have the scope would you be able to send me some scans of the following points?

Also is R309 missing on your vertical board?

[gkmaia]

Mode is now fully functional. Ch1, 2, Chop, Alt and ADD.

Problem was a shorted Zener at the blanking bistable circuit.

But still with some issues. Trace is thick and seems focus has no effect in such trace, so seems like the focus it completely out of range now.

Manual asked for a 5.6Z, but I had none so I used a 6.1Z not sure if that is the issue.

[YetAnotherTechie]

Hello,

Glad to hear you are making progress, well done!
I said i had a similar scope, not the same scope 
Mine is a clone of Goldstar O5100, but i also repaired the 30Mhz version, and they are very similar to yours, in terms of functions and design. Could you let me know the part number/schematic page of the zener you replaced?
It is possible that something new broke, but have you tried the brute force approach and short the zener that you just replaced?
If you didn't have focus problems before, you could be having jitter on the trigger, have you scoped the same signals as before, and the two banana plugs in the front (ramp/trigger)?

[gkmaia]

Trace is perfect now. It was a bad connection on the CRT control board.

I still do not have intensity control. My guess the intensity rail is either too high or to low beyond the the pot threshold.

Also the delay control does not show the magnified trace... not sure if that is exaclyt how it is supposed to work, but in all other scope that is how it works anyway...

Will investigate further.

I am nearly there... has been a tough one this old guy!

[gkmaia]

I think I may have found what is causing the intensity issue.

Rect diode 706 is leaking current when reverse biased. Basically current flowing out of the intensity circuit back in to the CRT grid.

I will put it back together and test again... fingers crossed!

[gkmaia]

That was it!!! It is fully working now.

This one was a carnage. At least 5 faulty diodes, 1 faulty transistor, 3 faulty ICs...

[YetAnotherTechie]

Very nice! congratulations! Can you make a video that shows how the delay function actually works?

[gkmaia]

Thanks!

There you go