Red Plates, Fender Deluxe Reverb Reissue

[mosfet]

Plates were glowing red. No bias supply voltage. I tested R69, and it was bad (metal film, fusible, flameproof resistor, 22 Ohms). The bias filter cap read around 95 mF, which seems close enough / good. Could the bias filter cap short once the amp is powered up? If the bias filter cap is good, why did R69 blow? Where can I get the appropriate replacement fusible resistor? Any input would be appreciated.

Peter

[Enzo]

Certainly C36 can be bad. It runs with 50v across it, your meter uses maybe a volt. And don;t forget C29.

R69 opened because excess current rqan through it. COuld be from a shorted cap, yes. Also could be from a shorted rectifier. Did you check CR1? A shorted CR1 puts AC instead of DC on that cap, and it won't be happy with that.

Since R69 burnt, I would replace CR1 anyway, since all that failure current went through it too.

Nothing special about R69. it is just a common ordinary flame proof resistor - a metal fim resistor.

And what if the cap is OK, but the circuit is grounded somehow? Measure resistance to ground across the cap, and over by the bias adjust components, just to see if something is shorting to ground. SOmething like a loose nut that came off a screw and lodged under a circuit board could happen.

[mosfet]

Dear Enzo:

Dear Enzo, I will replace CR1, C36, C29, and R69 tomorrow. Nothing seems to be grounded, except the positive legs of C26 and C29, which I believe are supposed to be grounded. I especially thank you for the pointer about the resistor - you have saved me a lot trouble looking for a 'fusible resistor'. Best wishes.

[mosfet]

resistor

Dear Enzo, I tried getting 22 Ohm 1/4 watt flameproof metal film resistors at all of my usual supply places in Toronto, but no luck. I will try again at some other suppliers today. I am wondering, will metal oxide flameproof do? Thanks.

[mosfet]

Red Plates Were Caused By...

I believe I determined the reason the resistor blew: While I was pulling the bias pot out of the way, the white wire (wiper) pulled out of the solder joint cleanly! Talk about cold soldered. Shame on Fender for their shoddy soldering! I re-soldered all three bias pot terminals with plenty of flux, replaced the 2 caps, rectifier, and the resistor. Voila! Thanks again, Enzo, for the excellent troubleshooting tips. Three thumbs up!

[Enzo]

And yet people are always screaming for companies to go back to hand wired amps. I have to repair FAR more problems with the hand wired parts in brand new Fenders than I ever do on their circuit boards.