Hot Rod Deville - a different kind of board burn.

[tdlunsfo]

On the bench is this Deville and I was a little surprised to see this when I opened it up. Thoughts on cause? The PT seems to ohm out okay.

[nosaj]

Are the connections really where they're supposed to be?
maybe someone else was in there?
Nosaj

[tdlunsfo]

Good question. I'll check that out tomorrow, but I suspect they are in the right spots.

[Chuck H]

How about an explaination of what part of the circuit caused this overheating and a schematic...

For those of of us that haven't worked on a hundred on these amps. I mean, How can we know this is is unusual?

[glebert]

Which generation HRDv? There are different board designators in the variations.

edit: I guess maybe the schematic lists the board connectors as CPx and the board labels are just Px. The hot spot looks like it was P1 to P3 (CP1 to CP3 on schematic) which is really strange in that those just have the inrush thermistor between them. I've seen other Fenders (solid state, ironically) get burning at the thermistor solder pads but not at the connectors like that.

Since it is the primary side of the transformer it isn't "high" voltage, will be interested to see if the board can be cleaned up enough to not have arcing due to carbon deposits.It should also be possible to move that thermistor completely off board if necessary.

Was there a tube failure or anything that would cause a high current condition?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[tdlunsfo]

glebert I appreciate you posting the
schematic and your description of the connectors. I figured out the cause and the ceramic sockets strike again. The tubes were pulled at the store and I just hadn't looked inside the amp to inspect them yet. The broken 6L6 keyways were a dead giveaway. BTW, it had the correct 3A fuse installed.

[Chuck H]

Wow! Both tubes? And then someone must have just guessed plugging the tubes back in and flipped the switch to see what would happen? Just wow.

When you say "ceramic sockets strike again" do you mean to say that the ceramic sockets tend to break off keyways? I don't do repairs and I'm not aware of this.

[Delta362]

Gotta love the top socket with the broken index key not in the correct orientation. I just had an amp come in that the customer installed new tubes and said it blew the fuse right away. Turns out he forced one of the power tubes into the ceramic socket with the index key shifted.

[Helmholtz]

What's wrong with those ceramic sockets?
Keyhole not wide enough?

[Mick Bailey]

I've had two that were burnt up like this in the area of the thermistor and transformer connections. Neither could be cleaned up enough to be confident that it wouldn't happen again. We're on double the primary voltage over here. With both amps the mains connectors were very loose, so this could have been the initial cause of the problem. Perhaps it was continually arcing and developing a flyback voltage.

With one I cut out all of the damaged board and wired in a piece of FR4 on which I'd built up the missing circuitry. On another I left the hole but relocated the connections and thermistor to tag strip. With other serious burnout damage I've etched a new section of board and grafted this in, but that was at a time where I was etching boards regularly and was better equipped for this.

[Tom Phillips]

... I figured out the cause and the ceramic sockets strike again. The tubes were pulled at the store and I just hadn't looked inside the amp to inspect them yet. The broken 6L6 keyways were a dead giveaway...

I think that a contributing factor is the bear claw tube retainers. Many users don't understand how to properly release the tension when removing a tube and they end up rocking the tube to excessive angles which ends up breaking the key pin. The later versions of the bear claws are much stiffer than the old blackface fender versions and it takes a lot of finger force to hold them open while removing the tube with one's other hand. This is especially troublesome when removing a tube while the chassis is still mounted in the cabinet because of the awkward hand positions required with the upside down chassis.

[xtian]

I'll take this opportunity to plug my buddy's replacement PCBs--turn your HRD/DeVille/Blues into an ODS: https://www.luigiretro.com

[glebert]

I think that a contributing factor is the bear claw tube retainers. Many users don't understand how to properly release the tension when removing a tube and they end up rocking the tube to excessive angles which ends up breaking the key pin. The later versions of the bear claws are much stiffer than the old blackface fender versions and it takes a lot of finger force to hold them open while removing the tube with one's other hand. This is especially troublesome when removing a tube while the chassis is still mounted in the cabinet because of the awkward hand positions required with the upside down chassis.

I agree in general about the bear claw retainers, but in this case the picture shows someone forced tubes in misaligned before the key pin broke off. Now, did the too stiff retainers make it difficult for them to feel the correct alignment? Perhaps.

[Chuck H]

My thinking, after Delta362 pointed out the misalignment, was that maybe the hole for the keyway is too large on some ceramic sockets making it harder to determine correct alignment.?. I can't imagine forcing that. I've plugged in a lot of tubes but I don't have any amps with ceramic octal sockets. I'm sure I've worked on a couple but I'm not a doofus.

[Tom Phillips]

...the picture shows someone forced tubes in misaligned before the key pin broke off...

Oh! I missed that. Now I see it in the photos. I think this is just one of the many bad design and/or quality control issues with current manufacturers. It turns out that I recently noticed that it was possible to insert a tube in one of those sockets in the misalignment position and it doesn't take very much excessive force to do it.