Hameg 203-7, slightly deformed in first half of wave

[Kekke466]

Hello everyone

I have a really nice Hameg scope 203-7 (20Mhz) on the bench that I have cleaned up. After fixing the dreaded VAR knobs and checking some solder points and calibrating the Y-Gain, I suddenly have a slightly deformed problem in every second half of the wave.
 You can notice it the most in a saw waveform. I think that this could be calibrated, I have the service manual but don't have a clue where to begin.

Go easy on me, this is my first scope that I am restoring (and my first post).










Here is a picture that shows the problem. If a kind soul knows what I could do, please share the knowledge.


Regards,
Kenny

[Kleinstein]

The HM203-7 should still be an old school analog scope. So there is no calibration or such for corrections.

A first point would be to check how the error changes when die curve is shifted up / down and also changing the channel.

I see mainly 4 possible sources for the error:
1) A problem with the generator
2) A cross over error with the output amplifier to drive the tube (so same error for both channels)
3) The input amplifier  (could be different for the 2 channels), would likely change with continuous variable gain.
4) Coupling from the trigger signal to the displayed signal. (could change with trigger settings)

[Kekke466]

I actually just got rid of it and I don't exactly know how.
I re-adjusted the Y-gain of both channels once more and it just disappeared

This is the situation now, definetly a lot better than before. The sine wave looks as it should be!






My timebase (time/div.) is off with just a tiny bit, I guess I can't get it perfect? I have various service manuals but they don't exactly explain if it can be done.
Will I have to live with this?

[shakalnokturn]

It can be adjusted.
Allow to warm at least 20min.

Start by checking the X amplitude (x1) is right, adjustment is on the lower PCB. Correct length is in the manual ISTR it should be 10.2 div.
Then timebase calibration is done with a trim pot for slower sweeps and variable capacitor for faster ones.
They're both near the timebase switches.

Finish by switching to x10 and adjusting x10 amplitude on lower PCB if needed.

[Kekke466]

I was looking around on the PCB, it should be trimpot VR303, I guess that this is the trimmer I have to adjust?
Thanks for the help guys!

photo host

[shakalnokturn]

Yes, but only if timebase is out of calibration on the low timebase settings.
If also off on the higher ones start with the horizontal amplitude adjustment.

[Kekke466]

A total succes! Timebase correctly calibrated.