Vintage Synth Repair

I'd also check the electrolytic on the emitter of TR20 isn't pulling anything to ground




________________________________
 From: Peter Mörck <peter@...>
To: vintagesynthrepair@yahoogroups.com 
Sent: Friday, 19 July 2013, 18:20
Subject: Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead
 


  
Hi,
 
The -15' and +15' values are probably right as 
they're derived from a zener diode + a PN drop, so 15-0.7 and -15+0.7, roughly. 
Sounds about right.
 
From my limited knowledge I react on the voltage 
0.6V because it sounds like a PN-junction voltage drop.
So I would guess that something, coming from 
0V/ground, via a reverse PN-junction, reaches the emitter of Tr20.
 
For example, its own base-emitter junction, if the 
base of Tr20 is 0V.
Is it?
 
Also switch it off and check for a short to ground 
from Tr20 base.
 
If IC3 is socketed you could try replacing 
that. Who knows what will happen if that's fried...
 
Also check for shorts between Collector-Emitter on 
Tr22 and Tr23.
 
Not sure if all or any of that is actually 
meaningful to do, but that's what I'd do at least...
 
Maybe some electronics wizard has better 
suggestions :)
 
BR,
Peter
 
 
----- Original Message ----- 
>From: Adrian  Corston 
>To: vintagesynthrepair@yahoogroups.com 
>Sent: Friday, July 19, 2013 11:44 
AM
>Subject: [vintagesynthrepair] CS-70m  power supply issue, -15V rail is dead
>
>  
>My CS-70m (which last worked about 4-5 years ago) has a power supply  fault.  With the power supply board disconnected from all voice and  control circuitry I see the following voltages on the various test  pins:
>
>+5 = +4.95V
>-5 = -4.10V
>+10 = +8.27V
>-15 = 
  +0.616V
>+15 = +17.64V
>
>The +15' and -15' voltages (used to drive the 
  op-amp ICs in the supply that I measured on pins 4 and 8 of each of IC1, IC2 
  and IC3) test at:
>
>-15' = -14.22V
>+15' = +14.23V
>
>I have 
  checked voltages starting at the transformer secondaries and working my way in 
  and it all looks good up to the collector of the regulator transistor Tr20 
  (2SB688) which measures -26V.  On the emitter (output) it measures 
  +0.616V, as compared to Tr15 which measures +26V on the collector and +15V on 
  the emitter.  So I replaced Tr20 with a 2SA1264N, which is nearly 
  identical according to the data sheets and documented as an acceptable 
  substitute.  Unfortunately this resulted in no change, so presumably it 
  is not the cause of the fault.  All measurements were made on the 
  transistor legs directly, so it's not the wiring between Tr20 (which is 
  mounted on a electrically non-conductive heatsink) and the board at 
  fault.
>
>From that point on I'm starting to move out of my knowledge zone 
  as far as power supply circuits are concerned - obviously the fault lies in 
  some other component or components, but I'm not sure how to identify which 
  ones (because I don't know how the rest of the circuit works).  What do I 
  do next, other than replace ALL of the components that make up the -15V 
  regulator section of the supply?
>
>The relevant schematic is here (sorry 
  for the resolution and the poor splicing attempt, it's the best copy I could 
  find): CS-70m PS  schematic
>
>Thanks in advance for any help you can 
  offer!
>
>Cheers,
>A.
>