I love the smell of burnt resistors in the morning. (Acoustic 150)

[Boy Howdy]

Yeah, Tom. I thought of that . . . after I disconnected everything and moved it into the next room. But I will get it all hooked back up and check that.

[Tassieviking]

I would disconnect both inputs from the pre-amps into the power-amp and see if it runs quiet and the voltage across R311 returns to 35V, or 35V & 70V on either side of R311
If disconnecting the pre-amp signals returns the power-amp to what it should be then the problem is in the pre-amps, if not then the power amp is at fault.
You have to isolate the problem to the right section by divide and conquer, you might have a signal coming from a pre-amp that's causing the problem.
Isolate the problem to pre-amp, pre-amp, power supply, power-amp, it might be something silly like a bad earth on an input jack, you can't tell till you divide the whole amp into sections.
To me it sounds like you might have an external bad signal coming into the power-amp.

[Boy Howdy]

Thanks TV, I'll have a look at all that.

I did some snooping around in there and discovered a few things: R303 (470K) reads 190K; R304 (330K) reads 192K; R307 (22K) I can't get a stable reading on at all. What does that mean when the ohmmeter won't settle on a number? I've run into that before. And lastly, C301 (1uF) reads 1.6uF. Should I replace that one, or is it close enough? What is that thing anyway? It's supposed to be an electrolytic, but it doesn't look like a normal electrolytic.

Also, if I can't find a reason for the fact that the voltage is reading ten volts lower than it should be, should I consider changing the value of R311? I saw a picture of a board (the same one I'm working on) that someone had changed the value from 560 to 680. Would that bring the voltage back to spec?

Oh, and upon closer inspection R311 is starting to get a bit toasty (pale brown).

[Helmholtz]

Never try to measure resistance in a powered amp. Might even damage the meter.

With amp off, in-circuit readings are typically too low.

Lift one end of each resistor and measure again.

[Boy Howdy]

It was off. But you're right, while I've picked up a few things over the years, never assume I know anything.

Everything else is reading about where it should be, just these above are off.

[Helmholtz]

Everything else is reading about where it should be, just these above are off.

So you measured the resistors out of circuit?
I've never seen a resistor that had drifted to a lower value.

[Boy Howdy]

No, they were still on the board, but with the amp off/unplugged.

[Helmholtz]

No, they were still on the board, but with the amp off/unplugged.

No wonder some read too low.

IN circuit resistor measurements are not reliable as the surrounding circuit provides more or less parallel resistance.
Unstable readings are caused by capacitor charging.

[Boy Howdy]

I've been a little busy lately, picking up after Ian. The him-icane didn't get too bad here in Columbia SC, just a bunch of minor pine limb debris and about a million pine cones to pick up. This is just about my favorite tool in the whole wide world. https://app.skufetch.com/images.tmp/...0760_b11fc.png Great for picking up pine cones and the occasional dead bug that made the mistake of wandering in to escape the weather. By the way, Bayer makes fantastic bug killer, and the only thing I've found that actually eliminates fire ant mounds.

Per TassieV's suggestion, I disconnected the preamps from the power amp circuit board and observed no significant change in the voltages. (72 and 32) I haven't listened to it yet. I'll get to that tonight, I think.

I think I'll go ahead and order a 10 watt 560 ohm resistor, unless there's some reason you guys can relate as to why I shouldn't.

[Boy Howdy]

I reconnected the preamps and confirmed that the noise is still there and on both channels, so it's not the preamps. I think I'll just replace R311 with a 10 watt (when it gets here) and see if that fixes the problem. And then if that one starts acting up (the 5 watt replacement took a few months at least) I'll cross that bridge as it comes.

[Boy Howdy]

I just put in the 10 watter. It didn't fix the problem. I may have to get this thing to somebody who knows what they're doing. I know a bit about tube tech (I've built, modified and even repaired a few) but when it comes to SS (and lord don't I hate circuit boards), like Sgt. Schultz, "I know nothing!"

[Helmholtz]

The circuit can't know the power rating of a resistor. It just "sees" its resistance.

[Boy Howdy]

So it wouldn't be less likely to burn up, which is what has been happening?

[Helmholtz]

So it wouldn't be less likely to burn up, which is what has been happening?

It will less likely burn up, but won't cure the reason for the excessive current.
If the resistor doesn't open, something else will.

[TomCarlos]

Can you take a photo of the Power Board? I'd like to see the components up close.