Repair vintage HAMEG 512-4 Oscilloscope, maybe refurbishing...

[lordbubsy]

Hello,

this is my first post on this forum. I’m a Commodore 64 / 65xxx single board computer enthusiast, 51 years old and living in the Netherlands.

I’d really like to repair my vintage HAMEG 512-4 Oscilloscope because it’s beautiful and serves(d) my needs well. I bought it in 2010 for about €50,- on eBay and bought some probes from China. The Y amplification didn’t work, and after some measurements I found the 30V DC power wasn’t working. I fixed it by replacing T96 with a BC537b and the scope worked until a month ago.

Suddenly again the Y amplification didn’t work anymore. Just a flat line. See first picture.

So I thought of measuring the 30V DC, and indeed it only measures 1V DC. The first thing I did was replacing T96 without any results. Then I replaced T94 and T95 without results.

Btw, I’ve got all the schematics and documentation, and printed out the schematic of the power supply. I measured some voltages and wrote them on the schematic (4th picture). I also got the measurements after the first repair (3th picture), when it was in working condition.

Then I de-soldered the bases of T91, T92 and T93, and tested between B and C, and B and E with DMM in diode testing mode. They all measured between 500 and 600 in one direction only. So they seem to be in working order.

After that I suspected IC90 L123B and ordered an LM723 and replaced it. Without any luck.

Obviously swapping out components isn’t the way to go, and since I’m not really into analogue electronics, I’d like to ask you for advice and help to repair it. If we can get it going again, maybe a few hints to refurbishing to preserve this piece of history.

Here is the data I’ve collected:

[lordbubsy]

Here's the rest of the pictures.

[Yansi]

Check the 1uF/35V cap across the 51k resistor for leakage, or replace it anyway.

[lordbubsy]

I have those here and will try it right away.

[Yansi]

From the voltages measurend written in the schematic, only three things could be wrong:

either the cap leaking current (posssible), the 51k resistor becoming small in value (almost short, very unlikely!) or the bottom NPN (T96) transistor becoming faulty - not likely, I would be suspicious about the cap at first.

It is obvious, that there is not excessive current (almost no drop across the 1R5 shunt) load from the +30V rail, and that none of the power trannies has failed. All leads to the fact the that regulating NPN transistor at the bottom (T96) gone crazy: Either incorrect setpoint (faulty voltage divider at the base terminal) or the transistor itself must be faulty - but you wrote you have changed those already.

[Cyberdragon]

Don't forget about the 10K resistor, it's what turns on T94 in the first place. It wouldn't matter about T96 if the 10K was open (or very high).

[Yansi]

If the 10K would be open, then you would not get 2.4V at the base of T94. I'd guess the 10k resistor is completely fine. T96 seems to be just commanding the power darlington for low output voltage.

And 0.6V at the base of T96 tells me the T96 is probably fine and regulating, however the voltage divider at the base is faulty, due to the likely very leaky capacitor there.

[Cyberdragon]

It's a tantalum though, I would think it would have exploded by now if it was shorted.

[lordbubsy]

Really ?! That’s all there was to it? It was the 1uF/35V cap across the 51k resistor.

IT’S WORKING! THANK YOU! I’m very happy.

I regulated the 1K potentiometer to exactly 30V DC. I notice that the power supply gets pretty hot. I think the sensible thing to do is to recap the power supply and perhaps adding a PC case fan to get rid of the heat.

[Yansi]

Well, a  tantalum? These are very prone to failing short as far as I know, especially the old types.

Glad it is working now

//EDIT: It would by no means would explode if shorted. You will just make the supply go low output voltage (~0.6V) when the 51k resistor is shorted over. Anyway, the power supply is current limited by T95 to about 400 mA, so any shorted tantalum accross the output would very likely just overheat and burn T92 if left long term.

[lordbubsy]

I tested with the  meter, it's 1k2 in both directions.

[Gyro]

Nice catch Yansi 

It's worth remembering that Tants only go into meltdown if there is enough current available to let them do it.

[lordbubsy]

I have replaced it with an electrolytic 1 uF 100V though. I'm not sure if that's a problem. But I can order the tantal's with the rest of the capacitors.

[Yansi]

I'd say no problem, leave the elytic cap there.

[Bashstreet]

I have replaced it with an electrolytic 1 uF 100V though. I'm not sure if that's a problem. But I can order the tantal's with the rest of the capacitors.

I would think there is no problem .

If your psu gets very hot adding even small fan can help a lot also if it been running hot for long you might want to have a peek to see if some caps seem getting dry etc.

[lordbubsy]

Yes, I'll see that I recap the power supply. On the Commodore 64 it's comon sense to replace the electrolytics, and this scope is even five years older.

[Bashstreet]

Yes, I'll see that I recap the power supply. On the Commodore 64 it's comon sense to replace the electrolytics, and this scope is even five years older.

btw i am also a commodore 64 fanboy. And my C-64's run with heatsinks and new caps 
I have some spares etc so if you need something i am open for trading bits or if i have something you need and i can give you can ask.

[lordbubsy]

btw i am also a commodore 64 fanboy. And my C-64's run with heatsinks and new caps 
I have some spares etc so if you need something i am open for trading bits or if i have something you need and i can give you can ask.

Thanks for your offer, that's great! I'll keep it in mind, and in the futuere we might help eachother.
I do also all kinds of stuff to / with my Commodores, but I like to do it as nondestructive as possible, i.e. no drilling or cutting etc.
C-64 is alive and kicking.

[Bashstreet]

btw i am also a commodore 64 fanboy. And my C-64's run with heatsinks and new caps 
I have some spares etc so if you need something i am open for trading bits or if i have something you need and i can give you can ask.

Thanks for your offer, that's great! I'll keep it in mind, and in the futuere we might help eachother.
I do also all kinds of stuff to / with my Commodores, but I like to do it as nondestructive as possible, i.e. no drilling or cutting etc.
C-64 is alive and kicking.

Indeed i also like to keep everything original as possible.
Maybe if we bother Dave enough he will make Commodore sub forum