Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: boogieWoogie on December 08, 2017, 08:21:27 am

Title: blown transformer, open primary, fuse is fine
Post by: boogieWoogie on December 08, 2017, 08:21:27 am: Hi Folks,
(https://i.imgur.com/CY3lqgc.png)

Schematic ^^

I'm repairing my bandmate's Fender Reverb tank reissue. We used it at practice, turned it off @ the end, and when he turned it on @ next practice, it wouldn't turn on.

I've removed the power transformer & found the primary of the transformer is blown open, yet the slow-blow fuse did not blow. I've taken DC ohm readings of the circuits on the secondary sides, and while the heaters circuit at rest is a bit low (~1.5 ohms vs. ~4 ohms would have I expected), I cannot find any shorts in those 2 circuits. Rectifier diodes are fine at rest. Caps seem fine at rest.

I can put a new transformer in, but I'm wondering why a primary would blow open like that & not burn up the fuse. I'd like to find any faults in the secondary side that might have caused this.

My dad suggests putting in a fast-blow fuse, installing the new transformer, and putting a meter on the test points and looking for any low voltages that don't match the schematic's expected voltages.

Any other suggestions or insights?

Thanks,
boogieWoogie
Title: Re: blown transformer, open primary, fuse is fine
Post by: shooter on December 08, 2017, 08:44:22 am: Quote
found the primary of the transformer is blown open
I have fixed many a blown circuit that was designed to save the fuse :icon_biggrin:

are you sure about your checks? primary fails are less common. with the tranny out ohm across the .1uf cap on the secondary side, you should have thousands of ohms, charging to many, many thousands.
Title: Re: blown transformer, open primary, fuse is fine
Post by: boogieWoogie on December 08, 2017, 09:07:14 am: Thanks shooter. I removed the power transformer, and the primary is indeed an open circuit. (The secondary windings are pretty close to spec: 238 ohms & 0.5 ohms for high-V & 6.3V). The resistence across the 0.u1F cap you mentioned is 5.1 Mohms, and increases slowly as the cap charges.
Title: Re: blown transformer, open primary, fuse is fine
Post by: sluckey on December 08, 2017, 09:13:35 am: Quote
I've removed the power transformer & found the primary of the transformer is blown open
Not much to say except replace it. Unfortunately, shit happens.

A couple things I would try before ordering the transformer... Check primary resistance using some needle probes to pierce the insulation of the primary leads close to the body of the transformer. If no needle probes, then nick the insulation with a utility razor knife. If still open, remove the transformer end bells and look closely for a broken or poorly soldered connection to one of the primary wires. You may have to peel back some paper layers to uncover these connections.
Title: Re: blown transformer, open primary, fuse is fine
Post by: tubeswell on December 08, 2017, 03:15:41 pm: Quote from: boogieWoogie on December 08, 2017, 08:21:27 am
..We used it at practice, turned it off @ the end, and when he turned it on @ next practice, it wouldn't turn on.

I've removed the power transformer & found the primary of the transformer is blown open, yet the slow-blow fuse did not blow. ...
Any other suggestions or insights?

Maybe one of the primary leads has become disconnected/unsoldered from the winding end?
Title: Re: blown transformer, open primary, fuse is fine
Post by: DummyLoad on December 08, 2017, 06:50:28 pm: admiral shallowater murphy's law of electrical engineering: a $100 part will blow to save a 50cent fuse...

--pete