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

[Paul Perry]

If one winding is taking too much current due to a short, I expect the
other (unshorted) winding would also draw excess current when energised,
due to coupling through the core.
You could test this hypothesis by measuring the resistance of each of the
primary windings.

paul perry melbourne australia

On Fri, Feb 20, 2026 at 2:14 PM grant musictechnologiesgroup.com <
grant at musictechnologiesgroup.com> wrote:

> I repaired a Garnet amp that blew its transformer when the generator it
> was hooked up to had been set for way too high a voltage (I don't remember
> what the customer said the voltage was, but they admitted the festival was
> smokier than originally planned). I think that was all that was wrong too.
> Aside from fuses.
>
> GB
>
> On 2/19/2026 5:20 PM, Jean-Pierre Desrochers via Synth-diy wrote:
>
> 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 that the 315mA fuses would have been replaced by much higher values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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