Fender HR Deville Had a Drink.

[Pryde]

Schematic:
http://www.blueguitar.org/new/schem/...od_deville.pdf

I have a fender HR deville in that had a beer spilled on it/in it at a rowdy gig. He kept playing for a minute then it crackled/popped, died This happened 3 weeks ago so the beer has since dried up .

The amp powers up, isolated to preamp section with no voltages getting to V1 screens/grids. Traced back to bad resistor R74. Had 480vDC on the Z side and nothing on the Y side. Swapped in a new 4.7k 1W, fired up the amp and the new resistor started smoking. Shut it off and here I am.

What might be causing excess current to pull through the resistor here? Not sure what to check next. thanks for the help.

[Enzo]

R74 burns up, so something on the downstream side of it - the Y end - is shunting current to ground. C35 could be shorted of course, but look at wherever Y goes.

As far as I can see, the Y node only serves the two PI plate loads. Since those are 100k and 82k, nothing in the PI tube could burn put the R74. So if that filter cap is not bad (disconnect one end to see), then look for anything shorting that traces to ground on the Y node. Look closely for ssigns of an arc on the circuit board. Fluids on the board can cause an arc to strike, and once one exists, it will continue on.

[Pryde]

R74 burns up, so something on the downstream side of it - the Y end - is shunting current to ground. C35 could be shorted of course, but look at wherever Y goes.

As far as I can see, the Y node only serves the two PI plate loads. Since those are 100k and 82k, nothing in the PI tube could burn put the R74. So if that filter cap is not bad (disconnect one end to see), then look for anything shorting that traces to ground on the Y node. Look closely for ssigns of an arc on the circuit board. Fluids on the board can cause an arc to strike, and once one exists, it will continue on.

Thank you Enzo. I will check C35. I don't see any apparent arcing on the back of the board anywhere. Perhaps I should scrub it with some naptha or denatured alcohol to get any residue off.

[Pryde]

So C35 checks out ok. I can't find any evidence of arcing anywhere on the board. I cleaned the back of the pcb thoroughly with denatured alcohol, as well as the tube board. Is it possible the PI tube is bad and pulling that current through R74?

I am not sure what else to do except replace R74 again, bolt it all back together, and hope for the best?

[Alex R]

try to eliminate things, narrow it down. Run it without preamp tubes, does r74 still get hot? And so on. A light bulb limiter would be a good idea, might stop the resistor burning. And/or lift R74 and start looking for unusually low resistance to ground somewhere in the preamp. What's the voltage drop across R75 - should be something like the schematic suggests, if so it's definitely in the PI.

[Mark Black]

Also after a drink spill I would be concerned there may be residue lurking underneath components which may only reveal itself as conductive under applied voltage.

[Enzo]

As I said, if you were to outright ground the plates of the PI, those 100k and 82k plate resistors would prevent the B+ from burning up. If C35 isn't bad, measure resistance to ground from there. ANy indication of short?

[g1]

There are many ways for C35 to "check out ok" and still fail when voltage is applied.
How did you check it?

[Chuck H]

Even if C35 "tests" good it may still fail at 440V. It's easy enough to find out. Do as Enzo said and disconnect one end of C35. This would be the right way to determine if the cap is shorting at high voltage. Your meter is not the right way to see if the cap is shorting at high voltage.

Simply putting in another R74 and hoping for the best sounds about as smart as it is. You've done nothing to correct an existing problem.

[Pryde]

There are many ways for C35 to "check out ok" and still fail when voltage is applied.
How did you check it?

I lifted one end and tested capacitance with the DMM. It measured ~20uf.

So I should install another R74, then lift one end of C35? If R74 doesn't burn up then it is likely C35 that is causing the issue? Just making sure this is how I would test this?

Sorry Enzo but I am not understanding what you are suggesting with the PI plates?

[Chuck H]

So I should install another R74, then lift one end of C35? If R74 doesn't burn up then it is likely C35 that is causing the issue? Just making sure this is how I would test this?

That's about right. If you have the cursory "light bulb current limiter" it would be a good tool to use, but not necessary if you have extra 4.7k 1W resistors

[Pryde]

That's about right. If you have the cursory "light bulb current limiter" it would be a good tool to use, but not necessary if you have extra 4.7k 1W resistors

I do have a bulb limiter as well so I will try this out

[Mark Black]

"but not necessary if you have extra 4.7k 1W resistors"

Oh - feeling evil today are we?

[Pryde]

OK I lifted one side of C35, replaced R74 and hooked up amp with bulb limiter. Turned amp off standby and bulb was "fairly" dim but immediately noticed R68 now burns up

R68 is a resistor off of the presence pot. Hmm.

[Chuck H]

What voltage do you read across R74?!? That's sort of the point of this test.

If R68 is burning up, especially with a dim current limiter, something is horribly wrong. Do you have the amp plugged into a load? It's a stretch, but with the filter cap disengaged there may be instability in the PI such that it's causing overdissipation.?.