----- Original Message -----From: Adrian CorstonSent: Friday, July 19, 2013 11:44 AMSubject: [vintagesynthrepair] CS-70m power supply issue, -15V rail is deadMy 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.
Message
Re: [vintagesynthrepair] CS-70m power supply issue, -15V rail is dead
2013-07-19 by Peter Mörck
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
