[sdiy] finding a short in a Yamaha CS-50/CS-80

[Bob Weigel]

I've used a cool trick to narrow down where the problem is coming from.  
I have an HP meter that measure milliohms.  So...if there's a cro-bar 
part somewhere you can actually measure around and find the lowest 
reading :-)  I did that on the T8 I still need to finish a while back 
and found the offending op amp very quickly. Otherwise it would have 
been a matter of going around cutting traces and patching them back 
together to isolate where the low resistance is.  But it sounds like you 
should be able to measure with diode checker between the plus and minus 
rail (not ground) and get some low reading.  Don't use the resistance 
checker (I mean you've already done that so we already have that data 
point).  Use the diode check because it will subject things to a little 
more voltage on some meters and you might find that it measures lower 
than it would measure across a static 220ohm resistor.  If so then 
you're looking at a damaged semiconductor probably. 

220 ohms, recall, with a 30V supply across it would only run 3/22 of an 
amp which isn't much.  So I think you have something that will show up 
maybe when you submit it to a diode check instead of ohmmeter. -Bob

jpitt at fhcrc.org wrote:

> I recently bought a non working CS-50.  The previous owner said it 
> was  a blowing fuses and a power supply problem...I changed some leaky 
> caps  on the PSU and have tested the power supply and it's putting out 
> the  proper voltage, but I think there is a short to ground somewhere  
> outside the PSU.  When I test the PSU voltage with the +15/+15S and  
> -15/-15S supply lines shorted (as per the cs-80 service manual), I  
> read the proper 15 and -15 volts on the PSU, but when I hook up the  
> leads from the PSU to the synth, the +15 reads 1.5V and the -15 reads  
> -0.6V.  I checked the resistance across the mounting rack rails for  
> the voice boards, which should carry +15 volts and ground, and I'm  
> only reading 220 ohms, which makes me suspect the 15 and -15 volt  
> supply is shorting on one of the 11 circuit boards in the thing.  I  
> visually inspected the boards and can't see anything obvious.
>
> Do I have to desolder every single +15 and -15 line off every board 
> to  track down the short or is there an easier way? The amount of 
> wiring  in this beast really makes me appreciate modern synths :)
>
> -jason
>
>
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