Hameg HM103 Problems & Repair

[unit998x]

So I've come across a Hameg HM103 in nice condition, but it seems to have something wrong with the vertical axis. When nothing is connected and the input grounded, there seems to be a 100Hz sawtooth on the trace, but oddly as I change the y-position the trace waveform distorts as shown in the video below. There doesn't seem to be any problem with trigger or the timebase that I can see. The Y-Scale seems to behave oddly too.



The first few seconds of video are most important, as they show the distortion of the trace as it moves through the screen. Hopefully it's an electrical fault and there isn't anything wrong with the tube.

I would be really grateful if someone has seem this kind of fault before, as I have no experience with repairing analog scopes but am fairly clued up on electronics in general.

Many thanks in advance!

[mij59]

Probably caused by bad filter caps in the power supply, very nice documentation can be found here http://www.hameg.com/downloads/man/HM103_englisch.pdf
So have a look inside, start by checking the supply voltages.

[unit998x]

Thanks mij59 for the document, I managed to get another look at the scope today, but stupidly left the multimeter at home so couldn't measure the supply rails. Nevertheless the caps looked pretty crusty so I replaced C800,801,803,804. The capacitors that filter the +-12V supply. I didn't have any 470uF 40V capacitors but did have some 1000uF 35V in the same form-factor since it was only filtering the 15V from the bridge I thought this was OK. Now the trace ripple seems to have decreased, but the scope is far from working (similar problem to before, just the trace is less rippled, y axis scale doesn't work at all). The only conclusion I can draw is there was nothing wrong with the caps, instead there is some kind of overcurrent condition on +-12V, hence the reduced ripple when I used a larger capacitor. Is there a way to test this theory?

I'll be able to get some more meaningful data tomorrow but for now I'll post an interesting trace displayed when connected to the frequency reference. (Single channel scope!)

[PA0PBZ]

y axis scale doesn't work at all

What do you mean by that? In the video you showed that the Y axis amp and vernier both work fine.
It looks to me that you just have a lot of ripple on the power supply feeding the vertical amp.

[mij59]

I do not think there is a high current draw, you would have smelled it by now.
The problem could be with the capacitors of the defection amplifiers, C802, C806 and C805.

[oldway]

Bad bridge rectifier with an open diode.

[tautech]

Bad bridge rectifier with an open diode.

If oldway is correct, and he often is, Daves recent HP 1740 repair vid had a similar fault but that one was intermittant.

[Fungus]

My first 'scope!

(And mine still works perfectly...)

PS: I lost my 'focus' knob some years ago. No biggie, but annoying. Does anybody know where to get one?

[PA0PBZ]

Bad bridge rectifier with an open diode.

Wouldn't that give a 50Hz sawtooth and not a 100Hz one?

[unit998x]

Thanks all for the replies!

Bad bridge rectifier with an open diode.

Wouldn't that give a 50Hz sawtooth and not a 100Hz one?

The 100Hz was only measured from the timebase on the scope, so it's possible if that isn't working 100% then it might not be 100Hz

Bad bridge rectifier with an open diode.

If oldway is correct, and he often is, Daves recent HP 1740 repair vid had a similar fault but that one was intermittant.

I'll be checking this first when I get back I think. I always thought that bridge rectifiers were pretty hardy and would only usually break on overload. (Although they are getting on a bit; 1983 datecode on some 74HC logic!)

I do not think there is a high current draw, you would have smelled it by now.
The problem could be with the capacitors of the defection amplifiers, C802, C806 and C805.

If the bridge rectifier isn't the fault I'll take a look at this as I don't have any 250V caps to hand. The unheatsinked 7812 wasn't getting particularly warm so I guess you're right.

y axis scale doesn't work at all

What do you mean by that? In the video you showed that the Y axis amp and vernier both work fine.
It looks to me that you just have a lot of ripple on the power supply feeding the vertical amp.

On review of the video I guess that I was fooled into thinking the y scale was bad because changing the scale didn't alter the sawtooth on the trace and that when increasing the V/div the sawtooth got slightly smaller and sometimes bigger.

I'll report back with some pictures tomorrow!

[oldway]

Bad bridge rectifier with an open diode.

I never repaired an HM103 but I wrote this because it is a very comum failure of Hameg scopes as HM605 and HM604 for example.
I always replace this bridge rectifiers by four 1N4004 diodes and it never fails again.

It is obviously a ripple problem and this should easily be diagnosed measuring power supply voltages and ripples before and after regulation.

[unit998x]

I managed to bring the scope home today and test some voltages, I don't have another scope so I measured VAC with a Fluke 75 (without true RMS) so hopefully multiplying by sqrt(2) gave the peak-peak ripple values, please let me know if this is wrong.

+180V = 184.3V (90mV pk-pk ripple)
-12V = -12.00V (200mV pk-pk ripple)
+12V = 15.6V (325mV pk-pk ripple)
+140V = 105.8V (110mV pk-pk ripple)

These results seem to confirm previous suspicions, the manual says 140V ripple is reasonable (although the DC is strangely low) but +-12V is way out (should be 6mV pk-pk) also the +12VDC is oddly high? So something is definitely wrong with the 12V rails.

I measured the drop across all directions of the bridges and seemed to be 0.49-0.56V. Is there any way to identify which bridge (if any) is faulty? Can the open circuit condition only occur whilst under load?

Since they are only low voltage secondaries on the 12V regulator side I should be able to cobble something together with some 1N4001 to replace the faulty one at some point.

Attached are some pictures of the internal construction

[oldway]

Voltage regulator 7812 of +12V rail seems to be faulty (short circuit input/output)
Reference  IC800 on schematic.

[unit998x]

Voltage regulator 7812 of +12V rail seems to be faulty (short circuit input/output)
Reference  IC800 on schematic.

Hmm, I think this might be a winner, I know it's only a crude test, but Vin-Vout resistance of +12V regulator is about 28k, but Normal Regulator is 2.3M.

I'll replace and check properly tommorow! Thanks again!

[unit998x]

Yep, all is fixed now, the 7812 was pretty broken and Vout did not regulate properly. Thanks again oldway!

[mij59]

Nice one 

Is the -12V rail still reading 200mV ripple ?

[TheEPROM9]

I have recently got hold of one of these and have found it is stuck at full brightness. I have tried both the external & internal pots & they do nothing. I replaced the optocoupler which helped a little with the focus problem however even that is not perfect probably due to the overly high brightness issues. Any one have any clue to what the cause of the issue is as I can't find anything wrong with any of the semiconductors in that circuit.