[sdiy] Questioning on why both of this power transformer primary windings went almost short..

[jpdesroc at oricom.ca]

No. The transformer is soldered on a PCB so no way to change the primaries
polarities.

 

De : Mike Bryant <mbryant at futurehorizons.com> 
Envoyé : 19 février 2026 21:07
À : synth-diy at synth-diy.org; jpdesroc at oricom.ca
Objet : Re: [sdiy] Questioning on why both of this power transformer primary
windings went almost short..

 

Pure guess but has somebody already replaced the transformer once, and has
JUST ONE of the primary windings reversed ?   This would mean they would
fight one another, and with larger fuses happily overheat and melt the
insulation of the windings.

 

  _____  

From: Synth-diy <synth-diy-bounces at synth-diy.org
<mailto:synth-diy-bounces at synth-diy.org> > on behalf of Jean-Pierre
Desrochers via Synth-diy <synth-diy at synth-diy.org
<mailto:synth-diy at synth-diy.org> >
Sent: 20 February 2026 01:20
To: synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>
<synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org> >
Subject: [sdiy] Questioning on why both of this power transformer primary
windings went almost short.. 

 

A customer brought me a guitar preamp that he said ‘blows its internal
fuses..’

I checked the internals and found that both of the internal 315mA primary
windings fuses were blown (F2 & F3).

The main 500mA fuse (F1) was ok.

So I connected my variac to only one of the primary windings (no fuses) to
find that
reaching around 90VAC it started to drain around 1 AMP !

I checked the other primary winding alone and it behaved exactly the same,

1AMP at around 90VAC.

These measures were done with none of the secondaries connected (free
wires).

 

So my questioning is what the heck almost shorted each of these primary
windings
exactly the same way..?

I connected external AC voltages to each of the secondaries bridge (one at
the time)
and there were no shorts on each of them. The resulting DC voltages were as
expected.

To create those shorts at each primaries it would have needed
that the 315mA fuses would have been replaced by much higher values
with some kind of shorts somewhere in the secondaries side.. ?

OR this is a manufacturing default that waited to awake ?

Anyway I ordered a new transformer replacement, but I don’t want
to fry it again because of something else that blew the first one..

 

What do you think ?

 

 



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