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Hameg HM203-7 oscilloscope fault


Hi there :)

I was lucky enough to be given an old Hameg HM203-7 oscilloscope by my college.
It has a fault, so I'd love to try and repair it, I've always wanted a scope to help with my other projects.

I'm not an expert when it comes to diagnostics and finding faults, so would really appreciate some pointers as to where to start looking. I am very competent when it comes to soldering though, so swapping parts out is no problem. :)

The scope switches on fine, but to get the screen to show a flat line, I have to VERY carefully and slowly tune the Y-POS knob to a specific, tiny range, at which point the line quickly comes up from the the bottom of the screen.

If I go much further than the mid-point on the Y-axis, the line quickly starts to get unstable and wobbles up and down. If I continue, the line flickers up and down the screen rapidly, and then disappears towards the middle of the Y-POS knob's range. If I keep going, the unstable line returns, and then there's another point towards the end of the knob's travel where the line is nearly stable, and finally it quickly moves off towards the top of the screen.

This behaviour is identical on both channels. If I manage to get Channel 1 visible on the lower half of the screen, and then Channel 2 visible on the upper half of the screen, switching to DUAL mode shows both lines drifting towards and away from each other in a very unstable fashion.

I have managed to locate a 203-7 service manual with some schematics here: HM203-7sch

So far, I have done a quick test of the X-Y board's -12v, +5v, +12v and +152v which all look spot-on according to my multimeter.

Does this look like a final stage Y-amplification problem based on the fact that it's happening on both channels?

Secondly, I don't have any probes for this machine and am interested in buying some. I'm not very clued-up on oscilloscopes - but I assume I can't just use any old probe. Can anyone advise me as to what I should buy? :)

Thanks in advance for any help :)


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