Mini Modular

[Chris MacDonald]

(note: cross posted reply)

> Hi there, I was wondering if you kind gentlemen/women would help me
> with some troubleshooting on my mini modular. I got it going, yay, and
> it seemed fine apart from a litttle bit of noise. Then I took it in to
> work to calibrate it with their equipment. The oscillators seemed to
> be unstable and I couldn't get an accurate reading, then i tried a
> larger transformer on the power supply and this seemed to fix that
> problem and all was well I was getting +14.90/-15.10 approx on my
> rails. I sucessfully calibrated it........... then I don't know what
> happened but the power diode starts flickering so I turn it off. 

That's not good.  Perhaps the regulators in the power supply became
overtaxed and shut down?  Or maybe one of the power traces was shorted
by something on the bench?

> Then when I turn it back on the lights come on then fade out, but the power
> LED is stable as are the others. Now when I turn it on the midi in LED
> has about 1.4 volts across it and glows faintly with no midi
> instrument connected.

This is possibly indicative of a blown 4052 chip in the MIDI/CV
converter. 4052's seem to be especially sensitive to overvoltage or
polarity reversal.  Try replacing this chip *after* solving the power
issue.  I would suggest a socket as well, JIC.

> I reverted back to the first power supply. It
> has +/-15v on the output but when I switch the synth on the voltage!
> on the rails drops to about -10/+13v.

Could be something in the supply or insufficient capacity.  Each rail
should be able to handle at least 200mA.

> I can't see anything obviously
> burnt out. The midi/cv LED does it's double flash thing when it's
> turned on, so I don't think the OS chip is stuffed. 

The PIC chip runs on 5V (which is derived via a onboard 78L05 regulator)
so I'm not surprised that it would function even if the + rail is only
delivering 13V.

> Any ideas or help
> would be greatly appreciated.

It sounds like you may be having power supply problems.  I would suggest
trying to find a good quality commercial linear power supply if at all
possible.  Then you could eliminate the supply as a potential source of
trouble and work on solving any remaining problems from there. 
Alternatively, if you have a U.S. power converter, I can send you a
"factory" MM supply.

Finally, I know it's a long trip back to the U.S., but if you can't fix
your synth even with a known-good supply, you can always send it to me
and I'll do my best to get it going for you.

Best Regards,

Chris