[sdiy] Weird Prodigy problem

[Dave Magnsuon]

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Louis van
Dompselaar
Sent: Tuesday, May 03, 2011 7:13 AM
To: sdiy diy
Subject: Re: [sdiy] Weird Prodigy problem

Dave,

Thanks for your tips.  I should perhaps have added that this is a first
revision Prodigy.  The schematic on the net is of a later revision.  The
circuit around those two transistors doesn't exist in the first revision.
In the first revision, the gate from the keyboard goes more or less straight
into one of the 4007s (through a 1M resistor) and from there with a cap into
the 556.  So it differs significantly.

Also, in this revision the keyboard isn't directly on the +12V line, but is
hanging from a 10k resistor, so it's unlikely anything in the keyboard will
cause a short.

-12V is okay by the way, so whatever is causing it is shorting between +12V
and ground.

During this morning's testing I've also noticed that it appears more random
than I first assumed.  It does still look gate related (or at least
initiated), but it might only happen after holding the key a couple of
seconds, and it might also continue after releasing the key. Sometimes also
it starts on key press but then disappears while holding the key.  So it is
a bit random.

What I haven't seen yet is voltage drop when not doing anything.  I need to
press a key for it to happen, but on further tests it doesn't appear
directly (on/off) related anymore.

Louis


Louis,

I see you mention a 556.    These things are known to "crowbar" the power
supply.   Could this be the source of the problem?

Dave Magnuson