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

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

2026-02-20 by jpdesroc@oricom.ca

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that the 315mA fuses would have been replaced by much higher values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 ?

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

2026-02-20 by 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 on behalf of Jean-Pierre Desrochers via Synth-diy
Sent: 20 February 2026 01:20
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 ?

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

2026-02-20 by grant musictechnologiesgroup.com

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:
Show quoted textHide quoted text

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

2026-02-20 by 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
Show quoted textHide quoted text
On Fri, Feb 20, 2026 at 2:14 PM grant musictechnologiesgroup.com <grant@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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________________________________________________________
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Re: [sdiy] Questioning on why both of this power transformer primary windings went almost short..

2026-02-20 by Richie Burnett

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,
Show quoted textHide quoted text
-----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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Re: [sdiy] Questioning on why both of this power transformer primary windings went almost short..

2026-02-20 by 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:
Show quoted textHide quoted text
> 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 ?
> 
> 
> ________________________________________________________
> This is the Synth-diy mailing list
> Submit email to: Synth-diy@synth-diy.org
> View archive at: https://synth-diy.org/pipermail/synth-diy/
> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace@synth-diy.org

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

2026-02-20 by jpdesroc@oricom.ca

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

De : Mike Bryant
Envoyé : 19 février 2026 21:07
À : synth-diy@synth-diy.org; jpdesroc@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@synth-diy.org> on behalf of Jean-Pierre Desrochers via Synth-diy <synth-diy@synth-diy.org>
Sent: 20 February 2026 01:20
To: synth-diy@synth-diy.org <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 \u2018blows its internal fuses..\u2019

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\u2019t want
to fry it again because of something else that blew the first one..

What do you think ?

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

2026-02-20 by jpdesroc@oricom.ca

There are wire links that set the correct AMERICA 120v voltages on the same PCB. These links are placed correctly.

 

De : Synth-diy <synth-diy-bounces@synth-diy.org> De la part de grant musictechnologiesgroup.com
Envoyé : 19 février 2026 22:11
À : synth-diy@synth-diy.org
Objet : Re: [sdiy] Questioning on why both of this power transformer primary windings went almost short..

 

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 ?

 

 

  <x-moz-ews://grant%40musictechnologiesgroup.com@outlook.office365.com/Inbox/1217?header=quotebody&part=1.1.2&filename=image003.jpg> 





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

2026-02-20 by Michael E Caloroso

Also make sure that the earth on the line voltage is NOT directly connected to any of the secondaries. Your schematic shows four secondary windings and none of the coil wires will be the same potential as earth, which will cause excess current flow when directly coupled.

MC
Show quoted textHide quoted text
On Fri, Feb 20, 2026 at 9:50 AM Jean-Pierre Desrochers via Synth-diy <synth-diy@synth-diy.org> wrote:

There are wire links that set the correct AMERICA 120v voltages on the same PCB. These links are placed correctly.

De : Synth-diy <synth-diy-bounces@synth-diy.org> De la part de grant musictechnologiesgroup.com
Envoyé : 19 février 2026 22:11
À : synth-diy@synth-diy.org
Objet : Re: [sdiy] Questioning on why both of this power transformer primary windings went almost short..

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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