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

[Roman Sowa]

If not loaded transformer takes 1A on primary, and supposedly this is 
not a 5kVA transformer, there must be short between secondary windings. 
All it takes is one winding shorted. It will not affect output voltage, 
and there's no easy way to measure it at all.
If it takes 1A idle and gets hot (it shoud get hot pretty quickly at 
that current) it's definitely shorted and goes to trash.

Roman

W dniu 2026-02-20 o 02:20, Jean-Pierre Desrochers via Synth-diy pisze:
> 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 that the 315mA fuses would have been replaced by much higher values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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