[sdiy] Prophet 5 power supply news...

[ASSI]

On Sonntag 11 November 2007, Gil W. wrote:
> Now, when I connect the entire synth, with no CEM and
> CA chips, I get correct -15v and about +14.90v,
> stable. But as soon as I play the keyboard, I can see
> the +15v rail voltage droping again to about +14.50v.
> Stop playing and the voltage rises back to +14.90v.

Have you ever checked how much current is drawn on each rail (don't 
forget to measure the return current on GND as well)?  There simply may 
be some more circuits that draw way more current then they should.  
It's no use poking about in a polysynth if you have no measurements 
telling you where to look.  If your P5 has seen some abuse from the 
power supply, then it is quite likely that there are even more damaged 
components than the few you've replaced so far.

> So can we assume that simply the Polysix transformer
> is not up to task ? or do you think there could be
> other issues that makes it behave like that ?

The P5 schematics call for 36VCT, 2.8A, you've stated your transformer 
is 36VCT, 3.2A.  Again, if the spec you've given is correct, then a 
beefier transformer does not help you.  If the synth draws too much 
current the regulators would get overloaded anyway.  How hot are they 
getting, are they properly heatsinked?  If they get too hot, they will 
go into thermal shutdown.  Before I'd get a custom wound transformer, 
I'd borrow an oscilloscope and look at the ripple on the supplies with 
and without load, before and after the regulators and the voltage 
waveforms out of the transformer.  That should tell you if the problem 
is before or after the regulation.


Achim.
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