Mesa Boogie V5a resistor burning up

[chrispy32]

Hi Folks,

I bought a second hand Nomad 55. It was a little crackly and microphonic. So I replaced V1 and that fixed the microphonics. I took out all the tubes and gave the sockets a clean with contact cleaner and cleaned some sections of the PCB with contact cleaner and a soft rag. When I put the amp back together it let out some smoke. I feel I may have disturbed something in the amp. I've replaced the 4.75k ohm resistor that keeps burning up. When I removed it, it only read ~2.2k. See PCB pic below.

I've verified the schematics I found on the web - please see attached with notes. I've also attached a video using a FLIR thermal camera on my phone when all the tubes were still installed. At the 42 second mark I turn on the HV supply/standby to on. You can hear the amp making some bad sounds and also see the 4k7 resistor start to heat up immediately. Even after replacing it with a new 4k7 and removing ALL the pre-amp and power tubes, it still burns up. All I can think of is that may be a shorted coupling cap or some tracking on the PCB.

https://www.youtube.com/watch?v=OoVwcKdJjDA

Can anyone please assist? Thanks in advance.

Cheers, Chris.

[Enzo]

I see nothing in the circuit that would burn up that resistor.

Experience tell me, that when an amp does not have a certain problem, then you work on it, and now it has this new problem, it is due to something you did while inside. Schematic may not help. I'd be looking for pinched wires, bits of stray solder, etc.

[Leo_Gnardo]

I see nothing in the circuit that would burn up that resistor.

Experience tell me, that when an amp does not have a certain problem, then you work on it, and now it has this new problem, it is due to something you did while inside. Schematic may not help. I'd be looking for pinched wires, bits of stray solder, etc.

What Enzo said ^^^. BUT do tell us what smart phone has that FLIR feature on it, way kool! Imma have to get me one of those.

[The Dude]

As far as I know, you have to have a thermal capture device or thermal camera. The app just displays, and saves the image. I don't think you can turn a cell phone into a thermal camera without additional hardware.

As to the original question: What type of contact cleaner did you use on the board? Is it possible that some residue is conducting? Or, maybe it soaked into the board and made it a conductor? I might try a hair dryer to make sure there isn't any left over residue causing conduction where it shouldn't be.

[Helmholtz]

What DCV do you measure across that resistor?

[g1]

As far as I can tell, that resistor is part of the NFB loop. The mod you removed across the OT primary looks like a stability mod (not sure about the symbol you drew, but 432J sounds like a cap).
So you may be dealing with an oscillation issue, which can also cause your crackling. Maybe someone tried to cure it with that mod. Was the resistor burning when that mod was in place?

[Helmholtz]

So you may be dealing with an oscillation issue, which can also cause your crackling.

As it also happens with all tubes out, it must be a DCV problem.

[Mick Bailey]

The resistor/cap combination across the primary is a corrective filter (sometimes referred to as a conjunctive filter). It's possibly been added to smooth out the upper-mids and top-end. The RCA manual gives a full description and some Dr Z amps use it.

My thought is that many Mesa amps have HT traces running very close to low-voltage ones and something has maybe bridged across somewhere.