Testing Power Transformer Princeton Reverb Clone

[Johnrcurry]

I am working on my Princeton Reverb Clone, built from a Weber kit. While playing the amp it went dead and I found the fuse had blown. I replaced the fuse and plugged the amp in to a light bulb limiter. The bulb glowed brightly. I removed the chassis and ran a series of tests. The transformer has the primary windings and 5 secondary windings: 1) a 5V secondary for the rectifier tube filament; 2) a 6.3V secondary for the other tube filaments; 3) a 540V secondary (which is the one in use); 4) an unused 680V secondary; and 5) a 45V secondary for the bias circuit. Voltage tests were conducted with the amp connected through the light bulb limiter. I discovered the following:

Voltage:
There is no voltage output on any of the secondary windings

Resistance between the leads of each secondary:

Primary = 2.9 ohms
6.3V Secondary = 0.4 ohms
5V Secondary = 0.4 ohms
540V Secondary =13.8 ohms
680V Secondary = 14.0 ohms

Resistance between windings:

6.3V Secondary to 540V Secondary = 32 ohms
6.3V Secondary to 680V Secondary =32 ohms
6.3V Secondary to 5V Secondary = infinite
6.3V Secondary to 45V bias tap = 9.2

Do these readings indicate a short between the windings and a bad power transformer? Are there other tests I should conduct?

Thanks

John

[The Dude]

Are those resistance readings with the secondary completely disconnected? If yes, the transformer is shorted. If not, you could be reading through commonly grounded center taps and other potential circuit shorts. I would unhook the secondary completely and see if the fuse still blows. If the fuse holds, then measure secondary voltages to test the transformer. If the fuse blows with the secondary disconnected and primary is wired correctly, the transformer is shorted.

Edit: For sake of the thread, I believe this transformer should be the same or similar to yours.

[Johnrcurry]

Thanks, the Dude. My readings were made with the secondaries disconnected, but the center taps were still connected to ground. Once I disconnected the center taps from ground and re-checked resistance among the secondaries there was infinite resistance between all secondaries. Still, there is no voltage on the outputs and the bulb in the limiter glows brightly. It certainly acts like it has a short, but I can't seem to find one.

[The Dude]

Don't discount the possibility that the short is on the primary side. Try unhooking the primary wires and connect them directly to AC mains to eliminate any potential wiring errors or shorted components. There is likely switch, fuse holder, and other things that may be at fault.

[Johnrcurry]

Thanks again, the Dude. With all leads disconnected and the transformer removed from the chassis, it still acts the same. I figure it must be shorted somewhere.

[The Dude]

Maybe shorted primary to a secondary winding? At any rate, if you get no secondary voltages, even with the transformer completely out of the chassis, it must be shorted.

[Helmholtz]

Looks like some shorted primary or secondary turns. In both cases the primary will look like a short to ACV.

As a further test you can use this method: neontest-1.pdf

Question is what caused the PT to overheat and melt the wire insulation and why didn't the fuse protect it.
Especially as the PT seems to have worked initially.

[Johnrcurry]

Thanks, Helmholtz. Yes, I built the amp in 2018 so the PT has been working until yesterday. There is no visual evidence of damage elsewhere in the circuit, but I will certainly proceed with caution and investigate further once I have a known working PT.