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

[Mike Bryant]

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> on behalf of Jean-Pierre Desrochers via Synth-diy <synth-diy at synth-diy.org>
Sent: 20 February 2026 01:20
To: synth-diy at synth-diy.org <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that the 315mA fuses would have been replaced by much higher values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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