Stumped by Silverface Twin Reverb...PT?

[palomarsound]

I took a friend's 135w Twin Reverb home that was having some issues that were never really sorted out from a previous tour. I can't remember the model number. The one with the pull boost on the master volume and hum balance pot on the back.

One of the power tubes had fallen out and he never got it working again, even though a tech gave it a shot and apparently cleared it for use. It had been sitting for about 5 years before I brought it home.

I noticed a bit of wiggle on the power tubes, so I pulled the chassis and re-tensioned the sockets, popped the tubes back in, and powered it up. It was working just fine, except for some scratchy pots causing some intermittent volume drops. I could also hear some weirdness with the reverb tank, so I powered down, unplugged, and took the tank out. One of the springs was completely disconnected so I grab a known functioning reverb tank.

I cleaned the pots, hooked up the reverb tank, plugged the amp in, flipped the switch, and...nothing. No light, no hum, not a thing. I double, triple checked everything. Fuse, wall outlet (119vAC), etc.

I'm suspecting an issue with the power transformer at this point.

Wall voltage is good, voltage through the fuse to the switch is good, the switch itself is functioning correctly. I'm getting no voltage at any point beyond the PT. Absolutely no part of the secondary is showing voltage, AC or DC. Either side of the primary to ground show full 119vAC. I'm getting 0v across the primary. I do see continuity across the primary. Tubes in, tubes out, no difference.

I'm confused as to how it was fired up and working fine, and then completely dead in the course of twenty minutes. I hadn't touched anything inside the chassis, except spraying the same contact cleaner I've used 100 times before to clean the pots.

I guess the next step is to disconnect the PT, but I'm wondering if anyone might have some insight as to what's going on.

[pdf64]

… I'm getting 0v across the primary. I do see continuity across the primary. …

It seems like the mains transformer is fine then.
With no voltage across the primary, you can’t expect any voltage out of it.

[Ten Over]

Either side of the primary to ground show full 119vAC. I'm getting 0v across the primary. I do see continuity across the primary.

It's hard to get those results with neutral connected to the primary. Track down the neutral connections.

Fender Twin 135 Layout.png

[Stevem]

Stop looking for voltage.

With the amp unplugged from the wall check to see if the PT primary reads open, with what you have described I bet it does.

A confirmation test after that would be to unhook the high voltage secondary side of the PT and clip lead in wall voltage and then see if any AC shows up on the primary side.

[palomarsound]

A confirmation test after that would be to unhook the high voltage secondary side of the PT and clip lead in wall voltage and then see if any AC shows up on the primary side.

Clip into the full HV or use the center tap?

Thanks for the other info.

Edit: With the amp unplugged I'm getting continuity across the primary, about 1ohm. I'll disconnect everything later and test completely isolated, but there's nothing else in the mix really so I don't see how that will change my primary reading? At least it'll be a good first step to replacing the PT if I need to.

[palomarsound]

It's hard to get those results with neutral connected to the primary. Track down the neutral connections.

Fender Twin 135 Layout.png

Thanks for the suggestion.

It's wired just like that and traces fine, just without the death cap on the ground switch. I assume that was snipped when the power cord was replaced some time ago (looks fairly recent, probably within 10 years). None of the connections look bad and test fine.

[sluckey]

Don't connect your meter to chassis ground when checking voltage on the primary side of the PT. Doing so will give you false readings that don't make sense. Instead, connect the meter black lead to the power cord white (neutral) wire. Then use the red meter lead to trace voltage from the power cord black lead through the convenience outlet, ground switch, fuse, power switch, and finally to the PT primary.

[R.G.]

If both sides of the primary show 119V and the primary tests with a low resistance, then neutral is not getting to the PT primary while line voltage is.

If it was me, I would
- unplug the line cord
- unsolder the primary leads of the PT; this prevents the low-ohms primary from messing with continuity tests
- use my ohmmeter from the prongs of the line cord back to the terminals where the PT primary connect
- trace the open neutral side back to the place where it's not connecting to the PT neutral side.

I would bet you a dozen donuts that neutral is open somewhere between the line cord and the PT wire terminals.

[palomarsound]

Don't connect your meter to chassis ground when checking voltage on the primary side of the PT. Doing so will give you false readings that don't make sense. Instead, connect the meter black lead to the power cord white (neutral) wire. Then use the red meter lead to trace voltage from the power cord black lead through the convenience outlet, ground switch, fuse, power switch, and finally to the PT primary.

Just tried this and I'm getting 0v at every point, but with the switch off I'm getting 90vAC at every point (up to the switch, of course).

[palomarsound]

If both sides of the primary show 119V and the primary tests with a low resistance, then neutral is not getting to the PT primary while line voltage is.

If it was me, I would
- unplug the line cord
- unsolder the primary leads of the PT; this prevents the low-ohms primary from messing with continuity tests
- use my ohmmeter from the prongs of the line cord back to the terminals where the PT primary connect
- trace the open neutral side back to the place where it's not connecting to the PT neutral side.

I would bet you a dozen donuts that neutral is open somewhere between the line cord and the PT wire terminals.

Thanks, this will be my next step.

[palomarsound]

If both sides of the primary show 119V and the primary tests with a low resistance, then neutral is not getting to the PT primary while line voltage is.

If it was me, I would
- unplug the line cord
- unsolder the primary leads of the PT; this prevents the low-ohms primary from messing with continuity tests
- use my ohmmeter from the prongs of the line cord back to the terminals where the PT primary connect
- trace the open neutral side back to the place where it's not connecting to the PT neutral side.

I would bet you a dozen donuts that neutral is open somewhere between the line cord and the PT wire terminals.

Thanks, this will be my next step.

Back up and running. Thanks everyone for your suggestions.

Turned out to be the line cord. I guess I was just lucking out when I tested it before.

[Stevem]