sparked bias supply, dead.

[chipaudette]

Hi All,

I've got a 65 Deluxe Reverb Re-issue (DRRI). This evening I was probing some voltages on my amp. Being careless, my probes accidentally jumped my bias supply to ground (C- to ground). There was a spark. Now the bias supply is only -16V or so instead of -50. And, there's a huge AC component (another 20Vrms) riding on top. As expected, the amp's sound pulses badly and doesn't really work. Sad.

So, yeah, I broke my amp. Now I'd like to fix it. I'm assuming that I blew a component. I'm starting by looking at near-by components. I'd really appreciate a little help. The schematic can be found at:

http://www.fender.com/support/amp_schematics/pdfs/65_Deluxe_Reverb_Schematic.pdf

I started by looking just at the components in the bias supply (bottom of the schematic). I see only a few components related to C- (the bias supply). Using my DMM to measure component values in-circuit, I find that both C36 and CR1 are OK. R69 (the 22 Ohm resistor), on the other hand, is reading a couple megaohms. When I flip the DMM probes the other way, R69 reads 170kOhms. Odd. Either way, though, the value doesn't seem right at all for a 22 Ohm resistor..

Did I smoke that resistor, or is my reading just an artifact of measuring in-circuit?

Also, if I want to replace the part, the schematic shows it as "Fp22Ohm". What does the "Fp" stand for? The resistor doesn't look like any of the other resistors on the board. Can I replace it with a plain old metal film resistor? Is a 1/4W resistor OK?

Thanks,

Chip

[Paul P]

Looks like your R69 resistor is shot. The different measurements must be
due to the diode, when you measure across the resistor you're also measuring
across the diode by way of the rest of the circuit loop.

Fp most likely means "flame-proof". Probably not essential but it may have
prevented your amp going up in flames...

Paul P

[tubeswell]

If you're getting AC hum, and not much DC, then my guess is the bias supply diode is shot.

[Don Moose]

While you're in there, though, you very likely gave the bias filter cap a nasty overvoltage - you might as well replace that, too. Remember that the bias is a negative voltage, so the caps (+) terminal goes to GND.

Hope this helps!

[MarkusBass]

FP means "Flame Proof". There are several resistors marked FP on this schematics - in the power supply section and screen resistors.
More info: http://newsgroups.derkeiler.com/Arch.../msg00454.html

Marek

[Enzo]

That 22 ohms is open. There are some residual voltages in the circuit, so the ohm meter reads something one way and something different the other because of it. Diodes are cheap, I'd replace it even if it seems OK.

[chipaudette]

Hi All,

Thanks for your suggestions.

I dropped a new 22 Ohm resistor in there and left all the other components untouched (mainly because I didn't have any large caps on hand). When I measured the busted 22 Ohm resistor out of the circuit, it measured 2+ Megaohm. It was definitely fried.

So, with new resistor in place, I fired up the amp to see if it worked.

With no guitar plugged in, it was quiet (no hum or oscillating sound). Good. When I plugged in a guitar and started to play, it was just fine. Thank goodness.

I brought out the DMM again and measured the bias rail. It measured about -50V. AC ripple was down around 10mV or so. I think that this means my caps are fine.

Thanks again for your help. For the moment, it looks like the problem is solved. If only they were all this easy.

Chip