Trying to repair an old hameg 203-6

[userFF]

Hi people !
I managed to get my hands on an old HM203-6 (my uncle was throwing it away). Being a student I didn't have much time to repair it during the year, but now that the summer holidays have come, I have all the time in the word !

There is a trace on the screen, so the CRT isn't dead, but it looks all wonky. There a bizzare-looking diagonal line that sweeps the screen at the same speed no matter the timebase. I managed to get my hands on the service manual ! https://drive.google.com/open?id=15mStd3EjEgfkuKCddAqQIWpmHMswMSw1 and the user's manual https://drive.google.com/open?id=1mZLlpTam8Fbq6XG5djXA1SS32ee7XwXG

So I naturally started by checking the power supplies. As far as I could test them, the AC voltages seemed fine, but the +5V and -12V were out of spec. The +5V (page 4) was near 1.5V even with no load, but changing transistor T706 and cap C700 did the trick. This supply only feeds the input amp IC101 on page 3, so I'm not too worried about it.

But for the life of me I can't manage to get the -12V supply running (page 11) ! It spits out something like -20V ! It has a weird failiure mode : first it gives a good -12V, then after a few seconds, it jumps to -20V ! so far i've changed the following components :
-T716
-T717
-C718
-R764
-R763
I also tried changing the op-amp, since it's socketed, but seeing no difference, I put the old one back in. I had also tested it using a simple follower circuit on a bread board, and it seemed just fine.   

So here I am, asking the specialists on this forum ! I really don't know where to go from here  , so any help would be really apprecietaded !

Cheers !

[userFF]

Also, sorry if I seem unresponsive, I'm on French time GMT+1

[CJay]

That's OK, I'm on UK time so we're not that far apart.

The -12V is referenced to the +12V supply so that needs to be good.

What voltage is on pin 3 of IC705 and what voltage do you have on the +12V supply when the -12V is working and when the fault shows?

[userFF]

Thanks for the quick reply CJay !

The +12V looks good to me, it's at 12.05V, it seems constant to me, even with the fault. The voltage at pin 3 of IC705 is -3.54 when the -12V supply is at -19.11V, which is what i'd expect from the voltage divider.

Thanks for the help !

[PA0PBZ]

What voltage do you measure on pin 4 and pin 6 of IC705 and the base of T717, preferably with and without the -12V error?

[userFF]

With the -12V fault, I measure -1.55V on pin 4 of IC705, -2.91 on pin 6, and -18.54V on the base of T717. I'm sorry, but the fault doesn't last long enough for me to make the measurments without it, and it takes quite a while with the CRO off for it to go away.

[PA0PBZ]

Pin 4 of IC705 is the negative supply and it should be a lot higher (or lower  ) than -1.55V. (I hope you use earth as the reference)
Check R766 (120 Ohm) and D717 (5.6V zener), so measure both sides of R766 when the fault occurs and we should have the complete picture.

[userFF]

On the side of R766 that's connected to the bridge rectifier, I get -19.38V, on the other side I get -7.90V. On the cathode of the zener I get -1.62V.
I'm a reel noob in electronics, so how did you know it was a 5.6V zener ? Does that mean that the opamp is simply not getting high enough a voltage ?

[PA0PBZ]

Re: the zener - the schematics say Z5.6 so that's a 5.6V zener
If you do the math then there's 95mA current through R766, that is too high, are you sure IC705 is ok, or maybe C714 has died?
Or R766 is not 120 Ohm anymore?

[userFF]

Sorry, I had the schematic printed on bad paper, I read 25v4 ! 
It's probably the cap, the same component was holding the +5V supply down. I'll change it and get back to you.

[userFF]

Good news ! I changed the cap, and it fixed the -12V supply ! So I tried to plug everything back in, apart from the front-end, and I got a propper trace ! The bad news, however, is that when I tried to plug the front end back on, the scope stopped triggering. It might be the front end connector that's pretty dicky, or it might be someting else.

Problem is that I won't have time for the CRO this weekend, apart from the evenings, so i'll probably get back to you wonderful people on Monday.

Cheers !