SWR W/M15 Combo hot component

[Don Moose]

I have on the bench an SWR Workingman's 15 combo, serial number EIGHT, with a date on the circuit board 12-3-94. The schematic I have is dated 12/15/2004 and I note some differences. SWR have not responded to a request for the proper schematic.

Symptom: On the schematic I do have, R23 (2k7, 1W) was burnt. I've replaced it with a 2k2 1W and 510R 1W in series, and they both get almost too warm to touch. All the transistors test 'OK' in circuit with my DVM's diode function. I get 56VDC across the new R23, and 135VAC, but no hum.

There's also 135VAC from the C1/R1 node to GND - again, no hum.

Ideas?

[Twist]

This may not be of any help but I traded for an SWR LA15 100 watt amp that seem to work fine but the back of the chassis got extremely hot. I opened it up and found the bias was set very hot. It looked like the amp had never been opened up since it was built.

[Enzo]

Comparing to a 1998 drawing, your drawing should be close enough.

Once power is on a couple seconds, Q2 is turned on and that applies power to the front end of the power amp. Q5 is the bias transistor, and i expect it to be centered on the output bus voltage, which ought to be zero. So since Q2 applies +59v, and Q5 collector sits at abou9t +1, your 56v sounds about right.

SO what does 56v across 2.7k look like to you? Looks like 1.16 watts to me. That is too much for a 1W resistor, no wonder it gets hot. I'd put a 3w or 5 w in its place.

I'll try to get a scan of the old print up here later. Fender may not even HAVE the old drawings. They are not on the service disc.

I cannot even imagine how you get 135VAC across that resistor, especially at idle.

[Don Moose]

SO what does 56v across 2.7k look like to you? Looks like 1.16 watts to me. That is too much for a 1W resistor, no wonder it gets hot. I'd put a 3w or 5 w in its place.

And with a 2k2 + 510R (each 1W), I get 1.42W across the 2k2, so I need to locate something beefier.

I cannot even imagine how you get 135VAC across that resistor, especially at idle.

Maybe I should try that series-cap trick. 56VDC may be enough to upset this meter.

Thanks!

[Don Moose]

Man, that meter doesn't like DC bias on an AC measurement.

A cap in series with the meter leads and the AC reading drops to 100mV.

OK, much happier now, just a wildly underspec'd resistor. (IMO not a great design - cheap, but wasteful of static dissipation).

Biased to dead center, too.

Cheese!

[Mars Amp Repair]

As I recall I've had this same issue with that resistor overheating.
I seem to recall that I found one version of the board where this was actually a 27K resistor & replaced it with that & all was fine.
I can't find any notes to that effect, but I remember like that.
glen