HM1005 repair: -12V rail unstable

[phil]

Hello

I got my hands on a HM1005 a few days ago. It does work, but only for about 1-2 minutes after switching it on. Hameg has a manual with schematics here: http://www.hameg.com/manuals.0.html. I cannot link directly to it, please search for "HM1005".

I checked the power rails and found out, that the -12V rail breaks down after a 1-2 minutes. It rises to ~1.7V (yes, positive 1.7V) The schematic for the -12V supply is on page D12. Looks like they use IC2002A and the +12V rail to regualte the -12V rail. The +12V rail and the rest remains stable though.

 At first I thought of a short somewhere. However, measuring the voltage over R2011 reads 130mV (R2011 is 0.39R, that translates to ~333mA) after powering up and nearly zero after the rail breaks down. A short would lead to more current here than normal operation, right?
Disconnecting any other boards didnt have much effect. It took a bit longer for the rail to break down, but nothing else.
It looks like a thermal problem *somewhere*, but I cannot find any too-warm-to-touch components in the area, T2006 and T2012 stay cool all the time. Same goes for IC2002A.

Help please? 

-phil


Update: I was able to trace the problem to the supply itsef. The scope works fine when I power the -12V rail from an external PSU.
I can reproduce the -12V break down with some power resistors as load, takes ~1 minute with a 50R load. As before, no hot components. However, the rail remains stable without any load, so it has to be a thermal issue.

[Kevin.D]

I would first check output of bridge rect ( D2003) ,if either of those bottom 2 diodes  is leaky then that would give your fault symtoms . If thats not your fault then just work along the chain from opamp to base of T2012 systematically checking your  voltages during fault conditions , from the  opamp output (output should  be hard on + ,and non inv should be higher than inv) if that is ok  then check zener drop should be 18V ,if these are what they should be then  disconect leg of T2006  to see if thats leaking and pulling down  base of T2012 .

p.s The best tool there is  when you have thermal faults is usually a can of freezer spray and narrow straw ,or a proding tool and a  bit a flexing here and there as in majority of  cases it's  usually a  partial conection somewhere that  opens with thermal expansion . 

[Mr Simpleton]

Check the zener diode D2004, and verify that OP IC2002 do operate OK, stage A may fail even if B operates ok. Assume the voltages and pin 8/4 are OK?

If I read schematics correctly, it is only the trace rotation that uses the -12V, very low current consumption. Even with failure of  VR801 the R2060 would limit current to max ~50 mA.

Check resistors R2043 and 2044.

[mij59]

I would first check output of bridge rect ( D2003) ,if either of those bottom 2 diodes  is leaky then that would give your fault symtoms . If thats not your fault then just work along the chain from opamp to base of T2012 systematically checking your  voltages during fault conditions , from the  opamp output (output should  be hard on + ,and non inv should be higher than inv) if that is ok  then check zener drop should be 18V ,if these are what they should be then  disconect leg of T2006  to see if thats leaking and pulling down  base of T2012 .

Yes your wright, start with the checking the voltage at C2006 (-18.9V ripple 3Vpp) could also be a bad cap.

[phil]

Thanks for the replys guys!

Rectifier and caps seem to be fine. Same goes for the opamp, zener and any resistors/caps in the supply circuit.
The schematic does not show all -12V connections: The X and Y amps and the HV supply need -12 as well.

I will try to move the board a bit when the fault occures and report back.

-phil

[phil]

Turns out that there are multiple cold solder joints and some broken traces. I repaird what I could without disassembling the whole scope, but I will need to do that in the future.
But there is more: Seems like the failing PSU killed the CH2 preamp. The trace is constantly deflected off screen. After common moding the intermediate amp it seems that the preamp is faulty.

The differential voltages after the preamp read 3.5V and 4.1V instead of 3.5V/3.5V for no input signal. Looks like I have to take it apart completely to get to the amps...