Twin Reverb.... PT bad ?

[Doc Stitches]

Question :

See pictures attached........ Blows fuse immediately when turned on. 1973 model

with 9 wires from the PT. With all tubes pulled, still blows fuse ( 3 amp ) I just put in

new filter caps.The pictures show transformer has gotten very hot. Should I just replace

it or can I pull it out of chassis and test it ?

Thanks
Doc

[melvin]

If it blows the fuse with all secondaries disconnected, it's bad.

[Doc Stitches]

Thanks Melvin
...................no surprise........

Do you have a schematic for it that shows all 9 wires ?.... my schem. shows only 7.

thanks
Doc

[52 Bill]

To my eyes, the melted heat marks are from the original manufacture, when the transformer was baked to seal the coating.

Be aware that removing the tubes alone, does not remove the transformer from all of it's loads. You must cut or unsolder all of the secondaries to test for shorts.

Why not show us a photo of the 9 wires coming out of the transformer?

[Doc Stitches]

new pic 022756 PT

Good idea....thanks Bill

Here are some pics

[Rhodesplyr]

To my eyes, the melted heat marks are from the original manufacture, when the transformer was baked to seal the coating.

Bill,

I don't know--I've never seen damage like that from manufacturing...

[52 Bill]

Most of these transformers are dipped in lacquer and oven baked to dry. The two flat spots on the outer bell look to me to be where the transformer sat in the oven. Sometimes there are grid or grill marks left on the outside, depending upon what surface the oven rack has.

In general most Twin transformers have at least nine leads. Originally there was the center tap for the filament secondary. Your transformer has the added orange lead, which is an electrostatic shield.

The nine leads are as follows:
2-Primary winding
2-6v filaments
3-High tension secondary with center tap
1-Bias winding tap from the HT secondary
1-Electrostatic shield

[pdf64]

There's nothing in those pics that I would think as being particularly unusual.
As per above, the secondaries need to be isolated before the PT gets condemned.
At a guess, I'd suspect a shorted diode or cap in the B+ or bias supply - almost anything is way more likely than a bad PT.

[Enzo]

Yeah, I see no damage, the interior shots show no smoke or bubbled out goo, the top has some surface crap, the kind of thing I ignore daily.

And as said above, pulling tubes is not the same as disconnecting the wires.

My first suspect will be the pilot light socket. has one of the solder tabs rotated around to touch the frame? Almost looks like it in the one photo that shows it. That will blow fuses.

And the above mentioned shorted rectifier, don;t forget the bias has one too.

Once you know the basic stuff is OK, then disconnect the wires, except the primary, and see if it still blows fuses.

[Doc Stitches]

Pilot light socket is OK...I disconnected secondaries ( now pilot light stays on ) secondary readings 240, 240, 40 .....

[Enzo]

You disconnected the other secondaries and it no longer blows fuses? Good. As a side note though, if you truly had disconnected the secondaries, then the light couldn;t work. So with SOME secondaries disconnected it powers up.

Power off, check resistance with an ohm meter for resistance to ground at pin 3 of the power tubes. If there is a low resistance, the OT may be shorted. And if so, disconnecting the center tap wire from the OT primary would stop the fuse from blowing.

[Doc Stitches]

Power off, no readings on the 4 ...( 3 pins. )

[Enzo]

What does no readings mean? No resistance? No continuity?

[Doc Stitches]

With ground to 3 pin of power tube, meter reads OL....(open/infinite resistance)

Note : I forgot to mention this chassis has been damaged....face plate bent in 1/2 inch behind master vol, and PT mashed the chassis in 1/2 inch..........when amp was dropped it actually separated the cab ( 2 12s ) from the chassis along with the top of cab........the brass plate/shield along the faceplate broke 2 solder joints but I repaired them. The light solder tabs were close but showed no signs of arching....I moved them further apart.

Also what are the voltages of the 3 secondaries going to the bias board ?....I have 240, 240, and 40. ....(this is a 1973 Twin Reverb with master vol,..but no push/pull )

[Jazz P Bass]

The 240-0-240 wires go to the bridge rectifier.
This becomes your high voltage (B+)
The bias voltage comes off of two separate taps.
Most likely 48Vac.
This gets rectified (diode in reverse bias) to your negative (-52Vdc) voltage.
What is left (usable taps) is your heater wires.
2 or 3. Depends on the amp.
6Vac.
Seeing that the chassis is damaged, I would pay close attention to the ground wires that are soldered to the chassis.
(can you say 50 watt soldering iron)