SOLVED Nobelium DI Preamp Issue: MOSFET Overheating

[sjbass]

So I picked up this gem a few months ago. Used it for a couple of shows and was blown away by how incredible it sounds!
Then, halfway during a hour-long set, it crapped out on me. I plugged it in and all I could get was a 60 cycle flicker from the LEDs.
Tried different power supplies, all of them with the same result.
Well, earlier today I opened it up and tested it again. At first it was working, and I was like 'yeah'! But then, the MOSFET got really hot - smoking hot. And then, nada. Back to the flickering LEDs.

I'll assume the MOSFET (M1: 550mOHM@6A) is toast. Is there something else though that could cause the MOSFET to be cooking???

 

[Feral Feline]

POST PICS

 

[sjbass]

 

[sjbass]

That MOSFET in M1 is where the smoke is - actually melting its solder joints.

 

[PunchySunshine]

@vigilante398​

 

[Brett]

While you're in there, you might want to look at fixing this:


It's dangerously close to causing a short, at least based on what I can see. While you're at it, you might as well check all other off board wiring to make sure they don't have whiskers either.

 

[PJS]

I am not an expert on this particular circuit. The MOSFET is part of the HT for the tubes. Getting too hot suggests a short. I would be removing the tubes and testing all the components around the MOSFET for shorts. If that all seems fine I would be replacing the MOSFET and seeing in the voltages to the tube sockets are correct (Still with no tubes in). The build docs also say that if the power is turned on without the NE555 then the MOSFET will get very hot and potentially damage itself. Given NE555s are cheap it might be worth just replacing it to be safe. If that is all working then try with the tubes in - although I would be careful for a bit because if a tube is faulty then you may get problems.

I have also mostly seen this kind of circuit with the MOSFET on a small heatsink. I don't think lack of heatsink is your problem here as you seem to be getting more significant issues than a heasink would solve, but I think I would be tempted to get a heatsink, non-conductive heatsink pad and a mounting kit for it so it runs a bit cooler once you have it running again.

 

[sjbass]

UPDATE: While on the test bench, we removed the unit from the chassis. All was going good until a couple of stress tests showed the 555 timer fail and immediately heated up the MOSFET. So it's off to the electronics store for a new 555 (and a socket bed, since there isn't one here in the pic), and a new MOSFET. Wish us luck....

 

[vigilante398]

A bad or missing NE555 is the most common cause of a MOSFET overheating. I would also mention you'll want to make sure the FET isn't bending backwards and shorting against the pins of the transformer behind it.

 

[sjbass]

Welp, we've got a new 555 Timer (w/ a socket bed), and a new MOSFET and she's back to her ol' sweet toobey goodness! Thanks for all the help, and I'll see y'all on the road!