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

[g1]

The resistor in question is meant to run hot. That's why it's a 5W, and gets 'brownish'. As far as I know from previous posts, it still measures fine. Did a replacement resistor burn? From the voltage readings in post #14, it is running around 3W. That's a little hot, but not going to burn unless conditions are getting much worse with signal, or some intermittent fault.
Maybe the source of the symptoms is something different this time?

[Boy Howdy]

Okay then. I'm gonna take that 10-watter out, it's way too big anyway.

I'll try to get a pic, Tom, but I haven't so far found a way to turn the board completely over without de-soldering a bunch of wires. It may be Tuesday before I can get to doing anything with the amp. We'll see.

[TomCarlos]

The board is held onto the case with some brackets and screws. You can remove those and the board will come loose. You may not be able to turn it over 180 degrees, but do what you can. We can then take a very close look at the components for burn marks and bulging caps.

One thing I did on my 150, there is a wire going from the big 1000uf cap that connects to that board. Another wire connects there and then goes to the output jack. Look on the schematic I posted - it is the Signal Out that goes to the PC board. I guess they did that for the purpose of taking everything to the PC board before going to the speaker? Anyway, that is one wire you can remove from the board and route directly to the speaker.

As suggested in other replies, there must be something happening on that board that is causing higher than normal current flow. So you might want to remove one end of each resistor and measure them. If you have a cap checker, I would check those too. Caps can become leaky and cause current draw.

[Boy Howdy]

Here's the pics. I checked some of the caps. I guess I need to check them all. I hate the thought of having to lift a leg on each of those resistors. Every time I stick anything through that circuit board the traces lift and scare the hell out of me.

[TomCarlos]

BH....

It appears the Electrolytic caps are all original. They will fail bad at some point in time - meaning :::Poof::: smoke. They could be leaking (dc current) now and what you are seeing with a very hot R311 is the sign of that potentially happening.

If you intend to keep this amp in working condition for some time to come, you will need to invest some time and money into parts.

Regarding the traces coming up from the board, perhaps your solder iron is too hot. Work with a solder sucker and learn how to remove parts without damaging the traces.

When while you are checking resistors, don't forget to look at the .5 ohm resistors on the output transistors - just for good measure.

[Boy Howdy]

I do really like this amp. It's so weird that it can both sound so solid state and yet have a such beautiful *tone*.

I looked into the possibility of replacing the big electrolytic at one point. I can't remember how much it cost to just now, but it was a lot. I don't know if I'd be willing to deal with that or not, but the others certainly, no problem.

I'll check the caps and proceed from there.

[Boy Howdy]

Well, that was an exercise in frustration. Half of the caps came up close, but not very; half were way off, and yet another half I couldn't get a read on at all. What if I just go ahead and replace everything but the transistors and those weird round resistors over to the left end of the board?

The .5 output transistor resistors all measured .6. The 1000uF cap underneath them (c311) was 1239.

C309/C310 I couldn't get a read on. C302 (470pF) came up 42pF.

Everything else wasn't radically wrong.

So, should I just go ahead and replace everything (exceptions noted above) on that board?

[TomCarlos]

The round thingies R316 and R317 are called "Negative Temperature Coefficient (NTC) Thermistors." They are temperature sensing resistors. So they change in value with temp. At room temp, they should read around 100 ohm.

UPDATE - take a look at this thread - Thermistors and Capacitors -- what the???

[Helmholtz]

Do you measure caps out of circuit?

Non-electrolytics rarely drop in value. Can you trust your C-meter with pF caps?

[TomCarlos]

Don't forget, when using your ohm meter, start with shorting the leads and look at the result. On my digital meter, is says .3 oms. So when I measure one of those emitter resistors I will probably see .8 ohms. Yours look ok at .6 ohm.

[Boy Howdy]

Do you measure caps out of circuit?

Non-electrolytics rarely drop in value. Can you trust your C-meter with pF caps?

So, caps need to be measured out of circuit too? I'm getting quite a lesson with this one! . . . and I do appreciate it.

I've only just gotten the C-meter. I'll keep an eye out for bad behavior.

[g1]

What is the C-meter you are using (make and model) ?

[Boy Howdy]

Don't forget, when using your ohm meter, start with shorting the leads and look at the result. On my digital meter, is says .3 oms. So when I measure one of those emitter resistors I will probably see .8 ohms. Yours look ok at .6 ohm.

Thanks, Tom. I'll check on that.

I think I'll just start pulling everything one leg at a time, measure, and pull it out if it's off by much - and just get rid of all the electrolytics.

BUT, I'm going to have to put it off for a while. I promised my old bandmates a jam sometime this fall, and I have several other projects that I have to attend to first. But I'll check back in when I get back to the amp.

Thanks for all the help, guys.

[Helmholtz]

So, caps need to be measured out of circuit too?

Depends on meter (measuring method) and surrounding circuit. Pull a cap and see if the reading changes.
Generally I don't trust in circuit measurements - even though I use professional grade meters.

Regarding pF caps, test your meter with good low tolerance caps of known value.

Also read the meter manual.