[sdiy] Weird Prodigy problem

[Louis van Dompselaar]

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


Op 3 mei 2011, om 04:39 heeft David Brown het volgende geschreven:

> You might check to see that C24 is not shorted. If so, the keyboard trigger switch would try to connect +12 to -12 through the reversed BE junction of Q18.  I would imagine that it would blow Q18, but suppose the resistance was high enough and varied with temperature.  That might explain what you are seeing.  A long shot, I know, but it's the only single fault that I can see that would affect +12.  You might also monitor -12 volts and see what it does as well.
> 
> Dave