[sdiy] Roland SH 2 power supply issues

[Ken Stone]

I'm looking at the SH5 schematic, so I may be off the mark but ... possible
causes:
-corrosion on the trim pots for setting the correct voltages
-cooked/damaged resistors - particularly the 1.8 ohm (or whatever the low
value is)
-diode breakdown - check the protection diodes.
-did you replace the little 10uf electros?
-are you sure it is not another module loading down the PSU?
-faulty 723 chip

Ken

>Ok,
>
>My SH 2 is starting to act funny, and after poking around, I need to appeal
>to the gods here.
>
>What happens is I get brown outs on the synth, occasionally, the power LED
>will go out, and the synth will let a sound out, low pitched, loud, and it
>isn't the oscillators, it is louder, but the osc's and filter still sound,
>just softer and on top of the main loud annoying sound.  The filters affect
>the osc's sounds but not the loud sound.
>
>Since the power LED is out, I am guessing the +15V supply is what is
>dropping out.
>
>I changed the fuse and the Electrolytic capacitors in the power supply and
>no dice. it still happens.
>
>There are a bunch of polyester looking caps in the PSU, but I'd rather have
>an idea that it would actually help.
>
>This happens with the machine open so it isn't anything shorting from the
>top part of the case.
>
>It was hard getting a reading off of the power supply when this happens,
>because it doesn't happen for too long, and whenever I set the ground probe
>on the ground of the power supply, then the other probe on the V+ out of the
>PS, it stops.
>
>I did get one reading of +14V while it was happening.  At this point I'm
>hoping it isn't the transformer, or the power regulators. (the regulators
>get +20 V on the outside pins and the middle pin outputs +15, and -20V on
>the outside pins of the neg regulator outputting -15 on the middle pin.
>
>I really don't know where else to look now, so any ideas will be very
>helpful and appreciated.
>
>Thanks
>
>Ed
>
>
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_______________________________________________________________________
Ken Stone   sasami at hotkey.net.au or ken at cgs.synth.net
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
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