Fault finding problem in 80's SS unit, please help

[volki]

hi all,

i'm having a problem tracking down a fault in one of my lovely neumann w495b eq's. it's one of my favourite units, and i'm missing it in my current productions, so i'd really appreciate any help.

the schematic is at http://www.kubarth.de/diy/german/w495b_schematic.pdf

symptom: failed power stabilizing transistor (T14). the paralleling diode D4 was also damaged. replaced both, but when applying power, T14 failed again.
the initial fault obviously occurred from one day to the other, without any external impact, so mechanical shorts somewhere in the supply line after T14 can be excluded. i suspect that some part draws much more current than T14 can handle, or even shorts. tried to measure the current (from input pin 16 to emitter of T14), but got quite a spark - wasn't long enough to actually get a reading on the meter, but probably it's way too much current, anyway =oP

here's my question: is there a way to find out the faulty part(s) without an extensive procedure of unsoldering every suspectable part, applying power, checking if it works, if not, repeat, etc.? are there any parts in this circuit which are much more likely to have failed than others?

btw, according to the circuit board, the +ve input leg of the part of IC2 in the stabilizing section should measure 12v (of course, at this time, it doesn't =o\ )

thanks a bunch for all your input people!!

cheers,
volker

 

[Michael Krusch]

Are the IC's in sockets? Are you able to pull them? Then I would start with this. And maybe C47 is shortend, so unsolder one end.

 

[VacuumVoodoo]

The supply circuit is not a voltage regulator but a active ripple filter. Something is obviously drawing excessive current. The IC opamps all have built in shortcircuit protection and should be ok but i would follow Michael's advice and remove those if possible. Check the output transistors in all 3 discrete line driver amps as there's no shortcircuit protection in any of them.

 

[volki]

thanks for your thoughts. i definitely should check that cap for shorts, hope it's only that... otherwise it'll get a little annoying :roll:

 

[PRR]

Remove T14 and put a 330 ohm resistor from C to E; leave B open.

Now: What is the voltage on C47? Zero? Lift the grounded leg of C47. Still zero? Start looking around T3-T4 and T9-T12. These three stages are essentially equivalent-- does one of them look different? It may have a shorted output transistor. The emitter resistors R89 R90 R106-109 should have less than 50mV drop; if more like 300mV drop then the nearest transistor is probably shorted.

While you say it happened without abuse, study the mechanicals. Sharp PC board leads insulated from case with heavy cardboard WILL cut-through and short-out in a few long decades.

 

[volki]

boy, it did take me a while to get to look at the thing again...

PRR, thanks for your thoughts, too. those will help me next time i get a more complex fault. this time, it actually was only that (tantalum) cap michael suspected.