Troubleshooting GW GOS-3310 oscilloscope

[MrElectric]

I bought a used GW GOS-3310 oscilloscope.

I did get a trace. but it seemed like some of the controls were not working as supposed. Also, when testing with a signal generator, the trace looked a bit strange. Not sure what was wrong.

I decided to follow the calibration procedure from the manual, but some of the steps were impossible, as the oscilloscope did not react as supposed when using the controls. I did record the following videos at the time:
Astigmatism adjust:
https://youtu.be/YhGh6IKJcSg

DC balance adjustment:
https://youtu.be/-EKhYkW1KEQ

Square wave testing:
https://youtu.be/eaTZdW-NnyE

After that, I decided to check the capacitors. I replaced most of the small electrolytics, as well as a few of the high voltage capacitors that looked like electrolytics with no polarity. One of the high voltage ones was leaking (fluid). I have measured all capacitors with a capacitor leakage tester, but did not find any bad ones.

After this procedure, I tried the oscilloscope again, but now it has another problem.

I only get a dot on the trace. The only controls that work as supposed are the focus and astigmatism adjustments.

The front controls make no difference, except for the X/Y-mode, which makes the dot disappear.

I found the Techtronics oscilloscope troubleshooting manual online, and it points towards the problem being in the horizontal section. Maybe the horizontal amplifier.

I have no way to check the voltages on the board, as I don't have a voltage meter for over 1000V. The lower voltage measurements seem ok though.

I'll have a look at the replaced capacitors. Maybe I'll try using the original, high voltage, capacitors to see if the old behavior returns. But as it didn't work properly before replacing them, I have no high hopes.

Any thoughts about this?

[MrElectric]

I measured a bunch of voltages, and found up to 1600 volts in some places. The problem, I found, was in the horizontal section. One of the caps was shorted, so one of the 16V zener diodes had no voltage across. I fixed that, but accidentally shorted the other 16V zener with the probe, so I have ordered a replacement.

[MrElectric]

I replaced the 16V zener diode, and the scope now works better than when I got it.

However, there are still some strange things happening.

1. The display does not get very bright. In fact, it's sometimes really dim, even when turning the intensity to max.

2. Traces move up and down depending on which V/div is selected. In some cases even outside of the screen. Quite annoying.

3. Right now, I seem unable to use the XY mode, as the trace disappears outside of the screen.

4. Some buttons does not always work as I assume they should.

So, I thought I should start checking voltages from the transformer and forwards. I'm attaching a photo of the circuit diagram.

There's 406V over R101, and 10V over R102.
Over the three 390V zener diodes DZ105, DZ06 and DZ107 there's 410V, 400V and 405V.

The green point over the zener diodes is marked -1170V on the PCB, but I measure -1217V.
The +170V point (marked green) measures 198V.
The +190V point (marked green) measures 221V.
The -HV point marked on the PCB measures -1605V (the green point above the C101 on the diagram).

Looking further to the right on the circuit diagram, there is a +9V and -9V, which measure 9.61V and -9.49V.

Should I worry about these voltages? (thinking especially about the +170 and +190V).

[MrElectric]

Just in case anyone is interested, I have scanned my manual and uploaded it here:
https://asrock.campus.ltu.se/manuals/GW_GOS3310_manual.pdf

Is there somewhere on this forum where we collect manuals? Or an external site I could upload it to?

[Chris56000]

Hi!

Please upload it to k04BB and Elektrotanya – both of these are reputable manual repositories dedicated to helping enthusiasts with errant T & M!

Chris Williams

[MrElectric]

I've uploaded to k04BB. Elektrotanya requires flash-player, which I haven't installed on any computer for close to ten years.