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Old 23rd June 2013, 07:42 AM   #1
Hikari1 is offline Hikari1  United States
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Default 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?
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Old 23rd June 2013, 12:06 PM   #2
Zen Mod is offline Zen Mod  Serbia
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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
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Old 23rd June 2013, 01:37 PM   #3
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Hikari1 View Post
.... 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
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Old 23rd June 2013, 07:25 PM   #4
Hikari1 is offline Hikari1  United States
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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.
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Old 23rd June 2013, 10:36 PM   #5
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And build a lamp checker
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Old 23rd June 2013, 10:58 PM   #6
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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?
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Old 23rd June 2013, 11:29 PM   #7
Art M is offline Art M  United States
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Originally Posted by dimkasta View Post
And build a lamp checker
Biasing up Class A with a "lamp checker" ? Are we checking for dim bulbs ?
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Old 24th June 2013, 12:56 AM   #8
Hikari1 is offline Hikari1  United States
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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.
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File Type: pdf P-F5-1V20-schematic-revision1.pdf (353.9 KB, 67 views)
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Old 24th June 2013, 02:55 AM   #9
Zen Mod is offline Zen Mod  Serbia
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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
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Old 24th June 2013, 03:35 AM   #10
Hikari1 is offline Hikari1  United States
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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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