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

[Boy Howdy]

I posted about this amp some months ago. It's an Acoustic 150 head I picked up for $100. It had a burned resistor between the power amp and the preamp. I replaced it, but now the bad sound that caused me to search out the problem has returned. I haven't opened it up but I'm pretty sure it's the same issue.

I posed the question before but didn't get an answer: can I replace R311 (560ohm) (top center) with a 10 watt to fix the problem of an overtaxed, overheating resistor?

[mozz]

You can't just replace a burnt part, something caused it to burn. Well, you could, but the next weakest link will burn out and you will be chasing a rabbit.

[Boy Howdy]

Thanks, Mozz. I was afraid of that.

If anyone would care to have a look upstream of R311 and offer suggestions, I'd be much abliged. Here's the service manual (the only schematic I can lay my hands on quick). Scroll down to about mid document for the power supply. Thanks, guys.

https://drive.google.com/file/d/0B01...pXMbCXWknqnLIw

[glebert]

What are your DC voltages on either side of R311? I would guess that something over in the Q301, Q302 area is drawing too much current.

[TomCarlos]

I agree with Glebert....

Start with Q301 and Q302. You will most likely need to pull those from the board. Unfortunately, the leads from Q302 are short. So be careful the that one. Then check everything else on that side of R311. I hope these pictures help.

[Helmholtz]

I would start with checking C507(hard to read), the 250µ/50V filter cap.
But first let's see the voltages.

[nevetslab]

Just in case it's needed, I have some of those RCA 40410 folded-heatsink TO-5 xstrs on hand...they were used on early BGW 250 Power Amps in the Voltage Gain stage back in the mid-70's. NOS.

[Boy Howdy]

When I replaced it R311 before it was reading 70 volts upstream and 35 downstream.

[TomCarlos]

BH.... When you pulled R311, did it still read 560 ohms (or dawg on close)? That resistor will get very hot. 70 and 35 volts doesn't sound that far off.

Is the amp working? The bad sound can be due to something else.

Check the output. Do you have any voltage there? It should be "0". You have that big 1000uf cap that blocks dc from the output. There should be around 35 to 37 volts on the plus side of the cap. Give all caps a good lookover and see if any look cooked (or having goop coming out of them).

[Helmholtz]

When I replaced it R311 before it was reading 70 volts upstream and 35 downstream.

That 35V voltage drop shows normal current by Q301/Q302.

Resistor dissipation is 2.2W.
Nothing wrong with using a 10W resistor.

But first see if it's burnt again.

[Boy Howdy]

I'll open it up tomorrow and have a look. It is working. The bad sound is a fairly subtle version of what it was doing before I replaced R311, but it's the same sound. It was sounding great until just the last week or so.

[TomCarlos]

BH... Give the amp a good inspection. As suggested before, look for other parts that might physically look ill. Clean the pots too. Check for bad solder points. Please do report back.

[The Dude]

Also check C311 (capacitor coupled output). If it's leaky, intermittent, or bad, it will allow DC to pass to the speaker and the amp will draw excessive current.

[Boy Howdy]

Okay, I opened 'er up last night. I haven't checked C311 yet, but: There's no voltage on the outputs. R311 is 555 ohms, C307 is 246 uF, and the voltage is at 74 and 31.5 on either side of R311. R311 looks good, no sign of it burning up just yet.

Like I said the distortion I'm hearing is very subtle at this point. I don't think I'm imagining things, but stranger things have happened, I suppose. I'm playing it at 1 or less on the volume, just to be clear.

[TomCarlos]

Oh... one more thing... Are you testing both channels? Same results? Turn down (or off) the reverb and tremolo,