[solved] Hameg Oscilloscope HM 512-6 no timebase

[ra-ma]

Hi! Playing around with my "new" oscilloscope Hameg HM 512 I found an issue regarding XY-Mode: It doesn't show anything, not even a dot somwhere in the middle without a signal. Only sometimes while pushing the button there's a dot in the middle for a very short (1/10 second) time.

Channel 1 and 2 work just fine. Also measuring voltages on test spots on the boards (+/- 12V, 5V, ...) hasn't shown obvious issues. I cleaned the "Hor ext" switch on the X-board with some alcohol but no change.

Does anyone have a hint for me where/what to look for? Thanks

P.S.: As manual/schematic is too large for upload here the link: https://www.sky-messtechnik.de/images/PDFs/HM512-6.pdf

UPDATE: For whatever reason the timebase has stopped working. I have only a vertical line for both channels. No change about the X-Y-mode though. 

[shakalnokturn]

"Trace Pos." trimpot on T board would not only be an excellent candidate for readjustment but also for failure.

In any case if the switch itself has been checked your problem is limited to 1/2 the T board or wiring...

[james_s]

The switch itself would be my first suspect given pushing it causes a dot to show for an instant and switches are something I've seen a lot of issues with in old test equipment.

[ra-ma]

Thanks both for your comments. I measured the switch on all contacts (also from the bottom side of the board making sure the solder joints are ok) and it's ok. Also I have been adjusting the trimpod but no change at all. What I discovered though is that sometimes, a couple of seconds after switching on "Hor ext" there's something like a "shadow"... 

[shakalnokturn]

"Trace Pos." trimpot on T board would not only be an excellent candidate for readjustment but also for failure.

Meaning: "Have you checked the trimpot hasn't gone open?"

My next candidate would be the BF199 linked to the trimpot by its emitter.

However this is guessing, with the schematic this should be a fast enough fix.

[ra-ma]

Yes, trimpod is not open. I have a general question though: measuring the +12V, -12V and +5V power rail against ground gives me an approx. 500 Ohm resistance. In my understanding that should not be the case and I have somewhere a shoirt to ground by a part.

Am I right and how should I approach this?

[shakalnokturn]

I would not consider 500 resistance measurement as a problem here. Under 5 maybe. You would be experiencing other problems if either of those voltages was too heavily loaded.

In normal sweeping mode is the trigger working as intended?

[ra-ma]

Thanks shakalnokturn. For whatever reason the timebase has stopped working. I have only a vertical line for both channels. I can move the line with the X-Pos. trigger. No change about the X-Y-mode though.

I have a second scope: The 0,2V rectangular pulse is generated but not the 5V sawtooth pulse which in my understanding is required for the timebase. Where should I start to search?

EDIT: Found one issue on the T-board: Tantalum between +12V Hor ext. and ground is shorted. Therefore I haven't had +12V on the rail when switching to Hor Ext. but only 1.0V. Now measuring the rest of the board before putting it back. Not sure if this could be also the reason for the missing timebase...

EDIT 2: XY Mode works now after exchanging the failed tantalum! Remaining Issue: Internal timebase missing.

[ra-ma]

Issue found and fixed. Transistor BF199 on the X-board was defect. Replaced and it works like a charm. Thanks to everyone giving input!

[shakalnokturn]

Congratulations on the fix. Often the hardest is getting started...

Once more the conclusions are:
Beware of tantalum drop capacitors. (Many here already know this.)
Beware of BF199's. (Not sure that many people are, I have become wary of them over the years. Same for BC327/337 in pulse applications.)