[sdiy] SCI Pro One 15+ power rail

[Barry Klein]

Use a high current low voltage bench supply in place of the standard one and advance the voltage with say 1-3A or so.  (remove the 7815).
Monitor the current as you advance the voltage to keep things within reasonable bounds.  Voltage setting will likely be only 1-3V to do this.
Borrow an infrared camera or use your finger to find the shorted component.  
Usually a shorted tantalum cap...

Barry


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of rsdio at sounds.wa.com
Sent: Tuesday, August 27, 2013 11:58 AM
To: Csaba Zvekan
Cc: Synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] SCI Pro One 15+ power rail


On Aug 26, 2013, at 07:29, Csaba Zvekan wrote:
> I'm troubleshooting a SCI Pro One and have a problem with the 15+ 
> power rail as I'm measuring the voltage regulator 7815. I get a value 
> of  -0.6V on a positive rail.
> Does that mean I'm having a short of some sort on the many OP-Amps and 
> CEM's etc that tap in that rail ? Any help or suggestion on how I 
> should continue to troubleshoot would be  greatly appreciated.

If you suspect a short on one of the ICs, then the quickest test would be to disconnect the power transformer and see if the voltage returns to +15V. If the power supply doesn't work on its own, then you know what to fix. I haven't opened my Pro One in a while, but I seem to remember a molex connector for the power. I don't remember whether the regulator is on its own board or on the main board.

I also can't remember whether any of the ICs besides the CPU are in sockets, but if they are, and the power supply is capable of +15 V on its own, then you could try removing all analog chips. While removed, and without powering anything up, test resistance from power to ground on each chip - if the chip is bad then the resistance might be rather low (under 100 k), but if it's good then the resistance should be very high (over 1 M). On that note, before removing any chips, you could test resistance between the +15 V rail and ground to see if there is a short.

Brian Willoughby
Sound Consulting

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