Fire indeed!!! Fender Blues Junior

[Sowhat]

This Tweed Fender Blues Junior came in the shop and the complaint was that fire was coming out of it. Fire indeed!!!

My guess is output tube anode short of some kind, that's about all I can think of off hand that can cause the entire trace and most of the circuitboard substrate to just evaporate. It's got a standard 2 amp mains fuse which ain't blown but, you know, I didn't check the HT fuse to see if it's the right value... sticking a higher than specified one in there can indeed cause fire if a short happens... I'll have to look at that in due course. For now it's mop up the burned soot, carve out some paths for new wires and try to stitch the circuitboard back together again. This thing is a mess... pity.

Oh yeah, order up a new matched pair of EL84/s as well.


(The circuitboard shown is the underside of the tube socket board, V4 and V5 are the EL84 power tube sockets).

[Randall]

Oh man, that's a bad one! You are going to lose a lot of pcb after you are finished grinding all the black stuff away. I am anxious to see a before/after shot. I have a Dean Markley in with the same problem, but it's tiny compared to that.

[Sowhat]

This was the last job on the last day of the week and I got as far as gutting the chassis (all boards out) ,restoring the vaporized paths, and wash all that soot away too. I scraped away all the loose carbon but I'm not going to deal with that much at all, I'm going to ignore it instead, so I ran two wires from the main board to the socket pins avoiding the carbon all together. I did a kind of sloppy job and I'm proud of it because I'm in a hurry. Anyway, Tuesday morning I can touch up my sloppiness and get into the circuit tracing and analysis stuff, hope the tubes get here by then. I will post an AFTER picture on Tuesday, I forgot to take one before I left the shop this afternoon.

[Enzo]

Probably an arc formed on the board, moreso than the tube failure.

[Sowhat]

Enzo, yeah, I guess that's a very high probability seeing that both the anode and cathode traces vaporized and they are (I mean were) real close to each other. I'll find out Tuesday.

[diydidi]

This is becoming increasingly common with these amps. Ive had a couple that self destructed. Worst case replace board, or make one.

[R.G.]

Too bad what's-his-name isn't around to read about a fault that burns out a circuit trace not blowing the mains fuse.

The most likely scenario is as Enzo said, an arc started and kept going. There's a lot of energy stored in those first couple of filter caps, and once an arc it going, it can continue to feed.

[gtrplayr1976]

I've seen some used ,and new replacement boards ,also Doug Hoffman makes replacement hand wired turret boards for these.

[drewl]

I've seen several of these where the output tube socket PC board connections become faulty.
any sign of a previous bad repair?
Or maybe a loose trace shorted to another pin.

Anyway, after removing the burnt carbon ot may be able to be repaired.

[Intripped]

I've not read through this thread at TGP, but the pictures speak by themselves: same damaged board
https://www.thegearpage.net/board/in...1234279/page-2

...could it be that the Kathode and plate traces are too close (and possibly the PCB material for some reasons gets slightly conductive)?

Also here, what seems to be an upgraded replacement board; traces are differently located.

[Sowhat]

Here's the after shots right before I reassemble the whole thing. Not real pretty but an arc can never happen again. I would not call it a design fault but a design shortcoming instead.
Push comes to shove when designing circuit boards and sometimes there is no real good solution to a routing problem. It's a shame the circuitry of the mainboard was not such that both V4 and V5 would have the same circuit pattern, but they don't. V4's pattern is real nice keeping anode and cathode far apart, V5 on the other hand squeezes those paths in too tight due to the pinouts of the two power tube mainboard connections.

Oh, yeah, Enzo, as you suggested, the power tubes check out good.

[Sowhat]

Just a quick final here, the amp works fine now, it was indeed the arcing traces on the board. The customer is very pleased that it was only a bench charge to get it fixed. Quick and dirty out the door!

[Sowhat]

Just a little follow on, A few days ago the customer brought the amp back in and said it did it again. I opened up the back and ramped it up with the variac and sure enough... FLASH! CRACKLE! But, this time it was inside one of the EL84's, the same one that had it's socket connections vaporized previously. I pulled the pair and the suspect tube showed clear signs of leaking, the getter flash was almost white. I replaced the pair with the ones I bought to initially fix this amp and now everything again works fine. I suppose this is a warranty repair so no bench charge but I did have to charge him for two new JJ EL84's apex matched tubes.

I subsequently found out that he had a "friend" switch out tubes after I fixed it initially because he wanted it to sound cruddy for his harmonica. I guess his "friend" stuck in junk tubes... not my fault at all, but I still get blamed for it.

[nevetslab]

I assume you HAD installed the new EL84 tubes you had ordered after repairing the fire damage, and got ALL of the carbonized residual out of the circuit paths. I couldn't quite tell in the photo on the foil side of the PCB. Just odd that your client's 'friend' had popped in a different set of power tubes, and the same tube socket was associated with the new fault. Coincidence? None the less, that sucks having to eat bench time for what wasn't your fault.

[nickb]

I assume you HAD installed the new EL84 tubes you had ordered after repairing the fire damage, and got ALL of the carbonized residual out of the circuit paths. I couldn't quite tell in the photo on the foil side of the PCB. Just odd that your client's 'friend' had popped in a different set of power tubes, and the same tube socket was associated with the new fault. Coincidence? None the less, that sucks having to eat bench time for what wasn't your fault.

I think the problem is down to the single sided board. The FR4/copper foil glue fails with heat. When the EL84 was replaced the plate pin track developed a hairline crack. This then arcs, evaporates copper and leaves a conductive carbonized area. The arc can then continue evaporating copper until there is no track left.

Coincidentally, I am just about to replace one of these boards as four of the pads on one tube socket base had lifted. I getting some double sided boards made which will be a lot tougher. Give me a pm if you want one.