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Hikari1 23rd June 2013 07:42 AM

F5 mosfet thermistors on fire
Well I decided to replace my 200ohm gain resistors on my f5 with 2x200, basically go back to stock.

I pull the boards, add another set of 200ohm resistors on top of the existing, put in new sil-pads, set the pots to 0 and fire it up. Seems good, 0V bias then one of the thermistors on a mosfet catches on fire. Burns up completely.

No resistors burned, Mosfets aren't grounded on the heatsink. What happened? Thermistor accidently touching something it shouldn't? Where to start?

Zen Mod 23rd June 2013 12:06 PM

check them all , replace what's needed, case solved

with so low number of parts , there is no use of playin' Poarot , if you already don't remember what was the cause

I know for F5s caught in smoke , when - due to overtemp (sissy htsnks) mosfet goes oped G-D , and that's causing adjacent NTC go puff

in your case ........ dunno

tinitus 23rd June 2013 01:37 PM


Originally Posted by Hikari1 (
.... set the pots to 0 and fire it up.

hopefully not turned in the wrong direction

personally I never tweak or experiment with amps that work fine
when people ask, did your try this or that cap, etc etc
I say, no why should I, it works fine as is

Hikari1 23rd June 2013 07:25 PM

Yes, I should have left well enough alone.

So, what's the odds that the mosfets and jfets are dead? Bias was 0V, I double checked.

I am going to order new thermistors and pads.

dimkasta 23rd June 2013 10:36 PM

And build a lamp checker :)

george a 23rd June 2013 10:58 PM

pardon my ignorance- but in putting your gain resistors "back to stock" - is your method of adding in 200ohm resistors - is it in parallel and thus ending up with 100 ohms? which is the "gain" resistor in your case ? -what impedence are you seeking?

Art M 23rd June 2013 11:29 PM


Originally Posted by dimkasta (
And build a lamp checker :)

Biasing up Class A with a "lamp checker" ? Are we checking for dim bulbs ?

Hikari1 24th June 2013 12:56 AM

1 Attachment(s)
Well, it was an amp that was up and running good. Guess I could have used a bulb tester.

I had 200ohm resistors on R5-R8 which I had read will increase gain by 6db. Since I am using a preamp now I decided I didn't need the extra gain so I added another set of 200ohm resistors in parrallel to the existing 200ohm to make 100ohm, the stock value.

Since I have to put in new mosfets most likely I'll just put in the proper 100 ohm resistors. Although I don't think that was the problem.

All I have on hand for mosfets is IRFs, I had fairchilds in the amp.

So, if a mosfet grounds with the heatsink is it likely the mosfet is fried? That's my best guess as to what happened.

Zen Mod 24th June 2013 02:55 AM

just pull them out of circ , and check (with usual mosfet test setup)

no need to dismantle them , just desolder one leg of S and G resistors

of course - check their isolation from htsnk , prior to everything else

Hikari1 24th June 2013 03:35 AM

Good info, thanks Zen.

Anything else I should consider? So, what exactly happens when the mosfet is not isolated? It's dumps V+ to ground? Is there potential for jfet/transistor damage when this happens as well? Nothing was hot after tis happened (except the thermistors) and all resistors look good.

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