Hameg Oscilloscope Troubleshooting Problem

[laluna]

Hiya.
Happy New Year!

I could do with some help with my Hameg HM604-2 oscilloscope. The trace has turned into a dashed line with non uniform intensity and there is a crackly buzz from the HV Z-board at the back of the tube. I suspect either the voltage multiplier cascase or the HV transformer is arcing internally. The HV TX is getting warm but maybe that is normal. Is there a way to test the 10kv voltage multiplier and the HV transformer for insulation breakdown? I managed to get the HM604 manual with the service section from ESI which was very helpful, thanks lads.

Dan.

[torbjorn]

Is the HV transformer supplied with mains frequency voltage or is it driven by a switch mode power supply?

If it is supplied with mains voltage, then an easy check would be to try supplying it via a Variac, disconnect the secondary from the cascade and turn up the voltage gradually while monitoring the primary current and listening for strange sounds from the transformer. Then, connect the cascade and repeat the test. The primary current should not be much larger than without the cascade.
In case of internal arcing, it would probably generate some disturbances that you can hear on a nearby AM radio.

[laluna]

The HV transformer is supplied with 100kHz chopped DC from a +-12 volt supply.

I have since replaced both the 10kV cascade and the HV transformer and the fault is still there. I still hope it isn't the CRT that is faulty because I assume that would make the scope a write off. I have spent £65 on parts so far and am no better off.

When cool, the scope still shows a straight uniform trace but then changes to a dotted line at the same time as the buzzing sound starts from the back end near the cathode supply board and the tube connector. It does seem to come from the 1.8 kV HV transformer but I suppose transformers can emit sounds without being faulty.

Here is a pic of the dotted trace smoothed out by the camera a bit, looks worse with the eye.


Here is the circuit of the Z Board.



Any help greatly appreciated.
Dan.

[torbjorn]

Perhaps the trouble could be in any other of the circuits supplied from the HV transformer - either the cathode voltage rectifier D616 with following components or the doublers supplied from the center tap via R636-R637?

Try disconnecting one after another of those circuits when the scope is warm (remember to discharge any charged capacitors first) and check whether the buzzing sound disappears.

Regarding the DCDC converter, i must say that is it somewhat strange that there is no snubber (R-C network) at all across the transformers primary or across the switching transistor.
Maybe it would be a good idea to check some waveforms at the primary side (with another scope) and see if anything changes when the buzzing sound comes. For instance, look at the voltage drop across the switching transistor's emitter resistor R651.

[jts1957]

Two thoughts:
Noting that scope "drawing" is 20 years old, have you checked all electrolytics?
When it's acting up, have you tried freezing components, one at a time with freeze spray?

[torbjorn]

It is no good idea to use freezing spray in such high voltage, high impedance circuits. When freezing, moisture will fall out on the surfaces causing creepage currents and possibly flashover. Of course, on the primary side of the DCDC converter, it would not be a large problem.

Although it is not uncommon with failing electrolytics, it is very seldom with an aged electrolytic that works well when cold and fails when warm. A bad electrolytic will usually perform better when warm.

[jts1957]

You are correct about HV area. I was thinking more along the lines of I.C.s, discrete semiconductors, resistors, etc.

[laluna]

The signal on the emitter of the switching transistor T610 when the buzzing sound is heard and the trace becomes dotted is shown below.



Disconnecting the cathode voltage rectifier D616 removes the fault and the emitter signal changes to the one below.



I don't understand the cathode supply circuit and what the 2kv 10nF capacitors do. Why is this supply connected to the heater also. Doesn't that mean the heater winding in the main transformer is at 1.8kV? The focus pot feeds back to the non inverting I/P of the 741 op amp, could this be connected to the instability of the circuit?

Thanks for the useful contributions everyone.
Dan.

[laluna]

I have checked every electrolytic in the scope with an ESR meter and replaced a couple of suspect ones in the X-Final circuits though I didn't think they were connected to this fault.

[laluna]

Forgot to give X,Y scales on the waveforms above. Top one with fault is X 200uS/div, Y 0.5V/div and lower one without fault is X 10uS/div, Y 0.5V/div. The oscillations that trail off to nothing on the upper pic are the 33kHz chopped DC signal.