Tube reverb with burnt power transformer

[Dan Mc]

Here is the background: weber 5G15 (kit) with no power. (schematic on their site)

I did not build this unit, but the owner wants me to repair it.

I traced the no power fault to burnt power transformer - no AC voltage from secondaries. Checked power supply components; no problems...
I replaced the PT unit ran for a while on bench seemed happy. Then the owner brings it back a week later saying it never powered on
I traced the fault to a burnt up PT! What is going on?

[R.G.]

At the risk of being somewhat obvious, there is an intermittent in there that is killing the PT. What is the fusing like on this unit? Is there only a primary fuse, no secondary fuses?

[Dan Mc]

It has a thermistor between the + end of the bridge rectifier and first filter cap.

I should modify the circuit Put a fuse in the B+ line after the rectifier(s) and before the first capacitor filter.

[52 Bill]

I have to make a couple of assumptions here as you are not telling us all of the details. When the first transformer failed it was an open primary winding correct? Did you autopsy it to see if there was a thermal breaker in the primary circuit?

Did you replace the transformer with the exact same unit from Weber? Even though the unit worked for a while on the bench, did you check to see if the new transformer was getting warm? Did the new transformer fail in the same way, open primary winding?

Unless there is some sort of manufacturing defect, I would think that there would have to be a serious overload to cause the primary winding to open on two transformers, something that you would have to notice before it cooked the tranny.

[Dan Mc]

Sorry for the lack of details. the original WRVBPT was replaced with weber part, and both times the primary side open - visibly burnt up after removing the shielding - with no thermal breakers. the mains wiring is correct as per the schematic diagram. This circuit is so simple...

oh yeah, i have another PT on order....

[52 Bill]

Visibly burnt? Both times? Is the primary fuse correct? 3 Wires AC cord?

Have you asked the owner about how he is using this? You did say that the second time it didn't even power on for the guy right? How well do you know and trust the owner?

[Dan Mc]

Visibly burnt through the yellow plastic wrapping first time, second time just browned the plastic. Primary fuse is 1a slowblow and 3 prong / wire AC wired as schematic shows.

I only know the owner through my guitar player buddy... so ... I called him and the owner says to me "I have another identical unit (he had someone else build) in use daily and it sounds great. that one - the unit in my shop now- never worked right , Hmmm...."

it left my bench working had it running for oh 20 minuets or so, the guy brings it back 2 weeks later claiming "it never turned on"
I opened it up and sure enough open primaries on PT...