Crack 1.1 – PT-10 transformer overheated and leaked resin

[John Fong]

Hello,

During initial power-up, the PT-10 transformer overheated rapidly (within one minute), produced smoke, and brown/yellow resin leaked near the terminals.

Tubes lit briefly but there was no audio output.

Transformer continues to heat quickly even after disconnecting secondary loads.

Measured cold resistances (after full discharge and removal):

Primary 14–15: 3.6 Ω
7–9: 0.4
11–12: 2.4 Ω
11–13: OL
12–13: OL


I would like to confirm whether the transformer is damaged and if a replacement PT-10 can be purchased.

Thank you.
John

 

[Paul Birkeland]

Yes, that transformer is toast, but the question is why. Some quick observations...

1. You're missing the #8 nuts that hold the power transformer on.
2. Your solder joints have not had enough head applied to flow out.
3. There's a glow test on page 57 of the manual to test the heater wiring in the amplifier. What happened when you performed this test?
4. What is your incoming AC line voltage?

Blowing up a power transformer like this requires a fault in the amplifier, generally on the low voltage output side of things (green twisted pair of wires). If these wires touch each other, or one of these wires touches the chassis, then the power transformer will burn up. We see heater wires touching the chassis up by the 9 pin socket sometimes where people will strip too much jacket off the green wire going to pins 4/5 on the 9 pin socket, and that extra stripped wire can touch the chassis.

It's somewhat possible that a backwards UF4007 can also cause some damage like this, though it should blow the fuse pretty quickly. I can see 3 of the 4 diodes in your power supply, and they are oriented properly, but it could be that 4th one...

 

[Doc B]

Red wire from PT terminal 11 should go to T9, not T10. That miswire grounded the power transformer secondary, which would smoke it.

 

[Paul Birkeland]

Red wire from PT terminal 11 should go to T9, not T10. That miswire grounded the power transformer secondary, which would smoke it.

Nice catch!

 

[Doc B]

Oops meant red from PT term 11 should go to T19, not T20. Thanks for catching that, Jameson.

 

[Jameson]

So overall there seems to be a number of things to do when you are ready to get going again, including:

1. As PB mentioned you have a bunch of cold solder joints. Check out this video on soldering techniques. Turn your soldering iron up, increase your dwell time. If you're using lead-free solder now's a good time to make the switch to good ol' leaded solder.
2. Make sure the hardware is installed (PB mentioned #8 nuts missing).
3. Fix the red wire going to terminal 20 that should go to 19.
4. Take your time and go through every test, including the glow tests toward the beginning and each resistance and voltage check.

We'll send you a quote for a new power transformer and shipping.