Vintage Synth Repair

Having been in the service industry as both a service engineer, test
engineer and design engineer since 1956 I can assure you that many circuits
use commoned secondary's for two bridge rectifiers.

 

Also if the as you say the circuit and the PCB only match 'nearly perfectly'
then you cannot possibly rely on either.

 

I think this thread has gone on too long now.  If you wish to continue it
may I suggest we do it off list?

 

Regards

Brian G3OYU

 

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Malte Rogacki
Sent: 03 June 2011 6:21
To: vintagesynthrepair@yahoogroups.com
Subject: RE: [vintagesynthrepair] Re: Crumart Trilogy replacement power
transformer

 

  

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.