Vintage Synth Repair

At 17:08 Uhr +0100 03.06.2011, Brian wrote:
> What you are saying is of course correct, unfortunately that is not the whole
> story is it?

I believe it pretty much is the whole story.

> We don't have the PCB or the transformer here so we cannot assume that there
> isn't a wire link added to match the cct diagram, or the possibility that the
> manufacturer actually used a transformer where the centre tap is internally
> connected.

http://www.pmerecords.com/Keyboards/CrumarTrilogy/CrumarTrilogy11.jpg
http://www.pmerecords.com/Keyboards/CrumarTrilogy/CrumarTrilogy14.jpg

Those pictures show the general vicinity of the power supply, including the
transformer and its connector.
I see clearly two red and two yellow wires connecting the transformer to
the power supply PCB which to me indicates two separate secondaries.

> Having said all this if the two rails are in fact isolated from the this
>could > answer why when checking continuity on the secondary a measured
>value of
> several megohms was found, if the test applied the prods to the outer two
>ends > of the windings.  The measurement would then be via the diodes of
>the
> rectifier.

Indeed. It also should make troubleshooting rather simple: The original
poster should simply unplug the transformer from the PCB and then power up.
If it still blows the main fuse: transformer is definitely at fault. If not
he should measure the AC voltage between red-red and yellow-yellow.

> Whatever we are suggesting the PCB does not match the schematic so I would be
> very wary of relying on either.  I'd prefer to rely on fundamental
>principles.

Actually the PCB matches the schematic perfectly (well, nearly perfectly).
I don't see a connection between the three terminals on the schematic,
either. This is just the "dotted outline" going through the terminals, not
a solid connection. Yes, it's not well drawn. Yes, I don't see the ground
connection on the PCB as well but there are various ways to have this one
implemented.

As far as the fundamental principles go: Your scenario would short out one
of the diodes of each rectifier, wouldn't it?

So far all schematics I've seen (admittedly not too many) with separate
bridge rectifiers use a dedicated secondary for each rectifier:

Crumar Performer
Polymoog
Oberheim OB-X
Kawai SX-210/SX-240

and probably many more.