Hameg 203-7 Oscilloscope repair

[d0pefish]

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 trace, 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. As the trace approaches the centre of the screen, it gets wobbly and unstable before eventually shooting off.

If I can persuade the trace to come onto the screen, adjusting the FOCUS knob will move the trace up and down as well as adjust the focus as expected (although it disappears off the screen before I can see a significant difference in focussing).

However, if I turn the INTENSITY knob down to a low (dim) level, the whole thing gets more stable and nearly all of the wobbling disappears. The usable range of Y-POS increases and I can get the trace centred and move it trace up and down more easily. If I move it more than a division away from centre though, it becomes curved and then disappears.

This behaviour is identical on both channels. If I use DUAL mode to get both channels visible, changing the INTENSITY will make them drift towards and away from each other.

I can get a waveform to display if I use a low intensity setting and connect the scope's integrated 2v and 0.2v square waves. Trigger seems to work and locks onto the wave - there's no drifting on the X-axis which is promising so I believe I have a good operational timebase circuit. However, increasing the INTENSITY knob adjustment results in the amplitude of the wave quickly decreasing before the trace becomes unstable and vanishes.

I have managed to locate a set of schematics and have uploaded them 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.

What I can't test is the high voltages, as my multimeter only goes up to 1000v.

I'm thinking my high voltages are probably unstable (rippling?) and maybe a cause of this - would you agree? Something seems unbalanced or weak if I can trade INTENSITY for stability.

Given that this is a 1992 machine and it's probably had heavy usage at the college, I'm thinking at least a couple of caps are shot. Would this be a good place to start?

Thanks in advance for any help
-d

[Kiriakos GR]

Re: Hameg 203-7 Oscilloscope repair

While those old Hameg are still popular in a way in Europe, the usual reason for some one to trash such a scope is the condition of the tube it self.
Due the fact that it was in a school it does not need much of imagination that this tube it has been burn out, nor voltages or prays can restore the damage.
The usual eBay price for something that works and the tube has still some lifespan, is about 150 - 230 Euros.

If you are not willing to spent about 60-90 Euros for an another used tube, which possibly will be in a better condition, then sale it as is for spare parts.

[d0pefish]

Re: Hameg 203-7 Oscilloscope repair

Thanks for the reply.

It would be a shame if it was a faulty tube, but really? I'm not convinced it's a total write-off yet - when I can get the trace to show, it is very sharp and bright. I'm just having trouble with getting its position steady.

My college has 10 more of these oscilloscopes. There's a small chance I can try to borrow a working one and test another tube on it...

[budm]

Re: Hameg 203-7 Oscilloscope repair

If you can still get good sharp and bright display, the high voltage and the CRT is still good, if it is old and gassy and worn out filament then it will be real dim and will not be able to get it to focus.
Did you do any cleaning of the pots ans switches yet?
Diagram and user manual.

[d0pefish]

Re: Hameg 203-7 Oscilloscope repair

Thanks

I haven't cleaned any switches or pots yet, but for the pots I disconnected the major ones and measured their resistances while turning them and their resistances all went up and down uniformly and smoothly with the correct ranges.

When the intensity is low (and the picture more stable), the pots move the trace very smoothly with no jumpiness, so I don't think they are suspect.

I've been playing with X-Y mode to get the voltage measurements for when the beam is centred.

I get about 80v on the Y pins as expected according to the manuals (D3 and D4) and about 71v on the X pins (D1 and D2) which is also good.

The voltages go up and down on the corresponding pins smoothly and as you'd expect - eg, if I adjust the Y knob, D3 voltage goes down and D4 voltage goes up. They both have the same voltage limits at both ends of the knob's travel. Same for X adjust knob.

If I increase the intensity the trace will get brighter until a certain point, then it disappears, shooting off towards the top or bottom of the screen depending which edge it was closest to to start with.

If I LEAVE the intensity knob high, I'll get occasional flashes of a trace shooting up or down the screen vertically.

Even when the beam shoots off or disappears, the X-Y voltages at the back of the tube stay spot-on.

It seems that *something* can't handle either the beam deviating horizontally from the centre of the screen beyond a certain amount, or the beam being more than a certain brightness - and the two are related - less intensity, more movement. But that *something* doesn't seem to be the X-Y amplifiers as their voltages remain constant even when the beam's wandering vertically.

Is it possible that the vertical deflection plates in the CRT could have failed, or does that not tend to happen? Something isn't delivering enough vertical "steering" when a certain intensity is going through the tube, by the looks of things.

Cheers

[Kiriakos GR]

Re: Hameg 203-7 Oscilloscope repair

Basically what you are looking at is the same circuitry of the old B&W TV set.

The tube and the HV fly-back transformer they both have the same age.
The low voltages looks good but this is why the scope PSU it does not work as it should, the load is minimal.

Basically you need another working scope to test the low voltage pulses at the fly-back, if those are correct the fly-back is injured ( half burned).

If I was in your shoes I would ask help from an trained TV repair man,
mostly because he has the tools to verify what is happening by 98% accuracy.