Vintage Synth Repair

Also check for any shorted tantalum caps throughout the synth...
Doug 
synthparts.com


--- In vintagesynthrepair@yahoogroups.com, Nick Bell <mckenzie126@...> wrote:
>
> 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
>  
> 
> 
> Â  
> Hi,
> Â 
> 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.
> Â 
> 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.
> Â 
> For example, its own base-emitter junction, if the 
> base of Tr20 is 0V.
> Is it?
> Â 
> Also switch it off and check for a short to ground 
> from Tr20 base.
> Â 
> If IC3 is socketed you could try replacing 
> that. Who knows what will happen if that's fried...
> Â 
> Also check for shorts between Collector-Emitter on 
> Tr22 and Tr23.
> Â 
> Not sure if all or any of that is actually 
> meaningful to do, but that's what I'd do at least...
> Â 
> Maybe some electronics wizard has better 
> suggestions :)
> Â 
> BR,
> Peter
> Â 
> Â 
> ----- 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
> >
> >Â  
> >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.
> >
>