Model T Reissue (Fender) w/ hole burnt in PCB... PT ok?

[jamesmafyew]

This is a Fender-made 1999 "reissue" Model T. I know they're nothing like the originals (at all). I have the full schematic for it.

This one has a pretty big hole burned in the main PCB where the terminals are for the parallel PT primaries. CP1 and CP2 are melted and TH1 is literally gone... vaporized? Nice big hole where they used to be.

Anyway, no chance of getting any replacement components for this amp (I tried). It looks like nothing beyond the little PT primary terminal area was harmed, so if I knew everything worked, I could just splice the wires back into the parallel-primary 120V configuration and the amp would be ok (I'd replace TH1 in a point-to-point fashion).

Problem is, I still don't know exactly what happened here. When I first looked in the amp, I thought: oh, CP1 and CP2 shorted. But looking at the schematic, they're connected. All the stuff that burned up is supposed to be connected. What's more, NONE of the fuses were blown!! What shorted? The only thing I can think of is that P32, is pretty close by and has the HT secondary on it. Could that have caused it? Maybe a piece of dust (this thing is pretty filthy) flashed over between the two? I'm at a loss...

If a short from P32 to CP1/CP2 was the cause, what are the odds the PT is ok? I've been able to confirm that all the windings are still isolated from each other and the core, and none of them have burned open. The only thing I can't test for (limited equipment) is whether the windings have shorted internally. Looks like there's an overload relay inside the PT... could that have protected it?

Any ideas would be appreciated. If the PT is likely ok, I am going to try and resurrect this thing.. with the lightbulb-in-series for powerup, of course.

[Enzo]

What do you mean you can;t get parts for it? WHy not?


Probably nothing shorted. More likely the thermistor cracked its solder, and the arcing and sparking at its leads snowballed into the whole thing burning up. It turns to charcoal and shatters.

The C60 is a NTC thermistor 10 ohms cold 5 amp rated. it is an inrush limiter. You can short across it for testing the rest of the system, but I recommend replacing it. Since where it mounts burnt up, scrape away any carbonized materials - which would be conductive now - and probably the best idea would be to mount a little terminal strip nearby and run wires from the board to it.

Your mains fuse is 4 amps, so at 120VAC, it will take at least 480 watts of power being used in the amp to take it out. Your thermistor burnt up with less than that. SO the fuse held.

The thing between the VIO wire and the BLK/RED wire inside the transformer is a thermal fuse. If there is continuity between the two wires, then it is not blown. I doubt it is bad, since the burning up of that NTC likely put no stress on it. Though anything is possible.

Since a couple push-on posts have no home, you will have to recreate some wiring off board, but nothing there is very complex. A couple more terminals on that terminal strip I suggested. Or if I see things correctly, it looks like CP8,9 are just for parking unused transformer wires. You might be able to secure the wires elsewhere and press those posts into service here.

In any case, any copper traces you need that wander into the burnt area should be cut away. We want no current flowing through that area now.

ANd it doesn;t take much equipment to check transformers. RG has a nice simple tester on his Geofex web site.

[jamesmafyew]

Enzo,

Thanks, this is SUPER helpful. I was kind of hoping you would reply.

I should have been clearer about "replacement parts" --- I have talked to Fender about a replacement mainboard or replacement transformers (just in case), and it is a no-go on all counts. Since they stopped making this amp around 2000 (?), the best they could do for me was an unpopulated mainboard. I said no thanks to that, given that the damage and potential fix are fairly simple.

As far as the parts themselves, I already have a new thermistor w/ same ratings on the way, and I figured I would go ahead and replace C46 for good measure. You're right about setting up a little terminal strip, that's a good idea.

I'm guessing P31 and P32 are too close to the scorching where they are (the two red wires in the picture. Should I desolder and move them off? C46 (the disc cap) too?

And if the PT has an internal short in one of the primary windings, and I power this thing up, the worst that will happen is a blown mains fuse (or the series lightbulb lights up if I hook that up)... right?

Again, thanks.

[Enzo]

Especially if you use a light bulb limiter, power the thing up and see what happens. ANything can happen, but I will bet my lunch money the tranny is OK. I use a variac and ammeter, but same difference to a bulb.

A whole board? Well, no, Fender wouldn;t likely have that, but in the trade, we wouldn;t replace a whole board anyway. A bare board? I am surprised they have that. Boy that would be some project.

P31 might be OK, but P32 looks a little close to the burn for me, I'd move it and cut away the traces underneath there.


http://www.geofex.com Find "tech tips," and scroll down to Transformer Short Tester under tube amp tips. Make sure to pay attention to the first sentence.

[jamesmafyew]

P31 might be OK, but P32 looks a little close to the burn for me, I'd move it and cut away the traces underneath there.

Yeah I was thinking I would put P31 and P32 in where C46 is, then move C46 to a terminal strip or right on the standby switch. Or, I had also thought of going further and replacing the rectifier diodes with a diode bridge bolted to the chassis.

New Page 1 Find "tech tips," and scroll down to Transformer Short Tester under tube amp tips. Make sure to pay attention to the first sentence.

This looks like exactly what I need. That site has been a great resource for me before, but I hadn't seen that particular page.

[jamesmafyew]

Update: Fixed!

I scraped away the crust from the burnt area and moved P31 and P32 to C46's spot. I replaced C46 on a little terminal strip mounted near the standby switch.

The NTC thermistor got replaced with an equivalent 10R/5A piece and wired on a terminal strip. I patched the transformer primary leads back together with wire nuts and shrink tubing.

Everything works fine now. Thanks a lot Enzo!!


Now, anyone know where I can find a rear grille for this thing? It's that pretty standard Fender metal speaker grille stuff, that they use on low end cabs.