Goldstar OS-7020A Oscilloscope strange channel behavior

[agentrob95]

Hello all

Recently got this oscilloscope for cheap as a knockaround/backup kind of deal. Using the probe cal terminal on the front, CH1 looks to be good, and pretty close to calibrated. Ch2 however shows a very distorted waveform. I'm thinking this is a capacitor issue, as it is afterall a 25 or so year old low end Korean scope. Before I go ordering capacitors, is this the likely culprit for this behavior, or am I on the wrong track here?

[tautech]

Welcome to the forum.

As Ch1 seems to be OK IMO a PSU cap problem is not the issue.
Ch2 IMO is more likely to have been hurt by an overvoltage event and one hopes it's only in the input attenuation stages. 

Can you duplicate the poor/wrong waveform on other V/div settings as this will help point to where the problem is.
Use of probes on 1:1 settings is often the cause of this damage but in this case try both 1:1 & 10:1 probe settings to find just which attenuation divider has been hurt.

[agentrob95]

Probe attenuation is not selectable on the unit, but the waveform does stay consistent through all volts/div settings. I didn't think that PSU caps would be an issue, although I may just replace them anyway at a later time if I keep this thing around. By capacitor issue, I meant in the input section; the trace isn't perfect on DC coupling, but certainly better than on AC coupling. Would changing the AC coupling capacitor (C301 in the attenuator schematic, 0.047uF 400V metal film cap) and maybe some of the circuit's electrolytics be a good idea, or is this more likely to be damage to active components?

[rqsall]

I have a Goldstar OS-9020A, which is almost the same. On mine, the jfet source follower at the input were toast. Swap the jfet from ch1 into ch2 and check if it fixes it.

[tautech]

Probe attenuation is not selectable on the unit, but the waveform does stay consistent through all volts/div settings. I didn't think that PSU caps would be an issue, although I may just replace them anyway at a later time if I keep this thing around. By capacitor issue, I meant in the input section; the trace isn't perfect on DC coupling, but certainly better than on AC coupling. Would changing the AC coupling capacitor (C301 in the attenuator schematic, 0.047uF 400V metal film cap) and maybe some of the circuit's electrolytics be a good idea, or is this more likely to be damage to active components?

Great we're narrowing things down. 

So if V/div settings all have similar symptoms one must look further up the signal path and into the input amp stages.

So:

I have a Goldstar OS-9020A, which is almost the same. On mine, the jfet source follower at the input were toast. Swap the jfet from ch1 into ch2 and check if it fixes it.

What he said. ^^^^
Q301 and/or 302.

[agentrob95]

Replaced Q301 & Q302 (2SK304E) no change. Swapped the LF356 op-amps between channels out of curiosity, no change. Tweaking VR301 & 302 I can almost dial the trace into a good square wave, but this goes away at smaller V/DIV settings. This tells me I should probably be looking at passives around these two pots, correct?

[tautech]

It's not a good idea to replace componentry in the signal path, temporary substitution is fine as you don't want to wreck the existing calibration.

Can you post up a link for the full manual so we're all on the same page. 

[agentrob95]

http://bama.edebris.com/manuals/goldstar/os7020a

no service info, just schematics and operating instructions. This was all I could find online.

[tautech]

Replaced Q301 & Q302 (2SK304E) no change. Swapped the LF356 op-amps between channels out of curiosity, no change. Tweaking VR301 & 302 I can almost dial the trace into a good square wave, but this goes away at smaller V/DIV settings. This tells me I should probably be looking at passives around these two pots, correct?

I hope you marked them so they can be returned to the original calibration position.

I previously mentioned using different V/div settings to check for waveform distortion with differing input attenuations. Combinations of that and switching a probe between 1x and 10x will give you a range of amplitudes to check. This is where a function gen is handy, just adjust the amplitude as required.
If using this method and the problem still persists the problem will be prior to the Ch 2 input amp.
You say you have another scope.....use it to follow the cal waveform through AC coupling until you find the point in the signal path where it gets distorted.

[DimitriP]

I would have replaced or at least swaped C301 with the one from Ch1 first.
and if the problem insisted....

another scope.....use it to follow the cal waveform through AC coupling until you find the point in the signal path where it gets distorted.