Synth-diy-test

Sounds like a short circuit somewhere inside the transformer.  Very hard to 
find as it only takes a single shorted turn to make the off-load primary 
current go high and the transformer to quickly overheat (usually causing 
more insulation to melt and more shorted turns!)

It could have been a manufacturing defect, or it could have been wired up 
incorrectly by someone else like Mike said.  It might also have suffered 
damage due to overheating if it had previously suffered an overload/short on 
one of its secondary windings.

I would have done exactly what you did... Check all of the diodes in the 
bridge rectifiers for shorts, then apply test AC voltages to the rectifiers 
and check you get the expected DC voltages out of the power supply.  If all 
seems good then I'd just drop in a replacement transformer and bring up the 
mains supply cautiously on your variac.

-Richie,


-----Original Message----- 
From: Jean-Pierre Desrochers via Synth-diy
Sent: Friday, February 20, 2026 1:20 AM
To: synth-diy@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that the 315mA fuses would have been replaced by much higher values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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