Unusual ceramic resistor Fender Pro Reverb

[Markorock37]

Ah same voltages tubes in that circuit.

[Markorock37]

Resistor smoked in the 5 min it was on. 4.7k

[Markorock37]

The original resistor was cooked as well.

[Tom Phillips]

Resistor smoked in the 5 min it was on. 4.7k

The original resistor was cooked as well.

This indicates that too much current is flowing through the 4.7k resistor. The low voltage downstream from the resistor is further evidence. The most likely culprit is the 20 uF/500V filter cap. It is probably shorted. If not that capacitor then something else down line from the 4.7k resistor is drawing too much current. By applying the principles of Ohm's law you will be able to troubleshoot and solve these kinds of issues.

[Markorock37]

I am well aware it's drawing excess current. I find it hard to believe the replacement cap has the same symptoms as the old one. Digging further today, hopefully I will find the issue.

[Enzo]

And hard to believe the government would let you drink your water, yet it happened. The thing to do is forget any preconceived ideas. Even if the cap is new, it can still be faulty. or have a tiny solder bead where it doesn;t belong. Unsolder the potentially bad cap and see if it makes the difference. S)mething is dragging down the low end of that 4.7k resistor. There are only so many possible paths to ground. A small number of parts, and then the circuit board and wiring themselves.

[Markorock37]

Glad I have a well! But yes unfortunately for a lot of people here. Disconnected that cap and still same reading. Resistor gets hot within mere seconds. I'm not seeing DC on any coupling caps in that circuit or evidence of anything touching or shorting. All resistors are within 10%. It's really throwing me for a loop.

[mozwell]

Divide & conquer !
Pull V1, V2 and V4 which are connected to the 4k7 resistor. If The 20u cap is pulled, then the only other connections are the 100k resistors.
If the voltage goes back to normal on the 4k7 resistor, then put the valves back in one at a time & see if one of them pulls the voltage down

[Markorock37]

Valves were out on previous test.

[Enzo]

Even if a tube plate shorted directly to ground somehow, the 100k oplate resistors limit the current pretty well. SO power off and what resistance to ground from that point, the left end of 4.7k? The number MAY suggest some circuit element.

Turn off ALL room lights, and look at the amp closely while it heats up that resistor. Is there a tiny blue spark anywhere? Even reflecting out from under the part board? Evidence of an arc.

Got a variac? Dial it back say 20%, still loads down the resistor?

Groud your meter, but instead of probing component leads or solder points, put the hot probe NEAR an eyelet or solder node. Is there any stray voltage on the (supposedly) insulating part board surface?

[Markorock37]

About 830 ohms to ground. No sparks or stray voltage I have found. I don't have a variable but I have it on a light bulb limiter right now and resistor still gets hot.

[g1]

Have you quadruple checked your work, twice?
You said the unit was in for a cap job. That implies it was working fine. Resistors and caps in the doghouse were changed. Any chance something is amiss? Or touching the doghouse cover?
Sorry to bring this up, as I'm sure you've looked closely, but the amp was ok before, or did I miss something?

[Markorock37]

The amp came in not working. Customer states it worked before he wheeled it out of the room to do carpet and when he wheeled it back in it came on but no sound. That's when I initially found the voltage problem and also attributed that to old caps. I also noticed that 4.7k was toasty. I always take pics beforehand just to make sure everything goes 100%. I have already checked multiple times and wiring is correct.

[Tom Phillips]

About 830 ohms to ground...

Given that you measure resistance that low from the pre-amp side of the 4.7k Ohm resistor to ground then you will likely be able to track down the problem using your Ohm meter with the amp powered off. As Enzo said, there are only a few paths and you can isolate sections by lifting resistors to disconnect sections of the circuit.

[Markorock37]

Well this was a tricky one, but it's up and running with proper voltages and bias. The leakage was found between the board and chassis. The best I can figure is a resistor had poked a tiny hole in the bottom layer, maybe not entirely all the way through but enough to leak. I removed the screws holding the board down and read the correct resistance around 34-35k, but as soon as I put the screw back in the middle of the board it started shorting again! I put a nylon washer in between the board and chassis at each screw hole and bam was in business. The amp is working great and sounds good too. Thank you all of you that steered me in the right direction. This one was a doozy!