SWR Super Redhead Power Amp Board Question

[dorrisant]

I have an SWR Super Redhead here on the bench. I opened it up for a quick once over and now curiosity is getting the better of me. I noticed a carbon comp resistor that just stands out like a turd in a punch bowl. Besides looking as if it was installed with a 100 watt iron, it also looks as if it has been heat stressed. I am hoping that some of you may be familiar with these and know more about the power amp board layout.

This one has had a few small repairs done to it prior to my ownership, so I want to be sure it is correct before I put it through its paces. I would like to confirm the correct value for that component position. Does anyone know what part number it is? I'm sure I could figure it out from there. The measurement says 2k5...




This is where it is located... Bottom right of the board attached to the heat sink.



With the power amp board rolled up for better visual, at the top left of the board in this shot. Just to the right of the burnt Wima cap... I'm thinking it is R7 on the schematic.

Tony

[Enzo]

The resistor looks to be 2700 ohms, the third band - red - is discolored to brown from heat. it is right next to a transistor with the number 5416 on the side, so that looks to me like R34 or R7 and Q14, upper left schematic. The schematic only calls out one round case transistor, and that is it.

The current to burn the resistor looks to me it probably come through Q14, which is instantly suspect. Though C17 beckons me.

R7 is called out as 2W. and R34 is looking like the 1/4 watt next to the hole with the screw inside. SO I believe you - R7.

A shorted C17 will do you no favors. I am guessing it is the melted one.

[dorrisant]

Thanks Enzo,

I will order some parts and bring back the joy.

[Axtman]

I worked on an SWR Basic 350 amplifier that had the exact same issue. If I remember correctly the resistor was undersized and eventually burned thus destroying the transistor.....or maybe it was the other way around. I replaced the resistor with the same value but higher wattage and replaced the transistor with an equivalent since I believe the transistor is now obsolete.

I wonder if there was a bunch of SWR amplifiers that left the factory with the wrong resistor. SWR amps tend to use the same power boards for different models of amplifiers.

[Enzo]

I wonder if there was a bunch of SWR amplifiers that left the factory with the wrong resistor.

This almost never happens.

[Axtman]

This almost never happens.

...unless it is Fender.

I have worked on a few old Fender amplifiers where resistors were not as indicated on the schematic. The most notable wrong resistor I found was on the bias eyelet board. The amp left the factory and sounded crappy (biased too cold) for 30 plus years before I fixed it by using the correct value. Sometimes it seems like they ran out of a certain value and just used the value in the next bin. With +/- 20% it probably did not affect the amp's tone all the much (depending on where it was in the circuit).

[mikeydee77]

I have an SWR SM-400 in front of me. It looks like a similar power amp module with an ordinary carbon film 1 or 2w resistor in that place.



Let me know if you need to see anything else ...

[dorrisant]

Thank you to all...

I got some 2N5416s in and replaced the one that was on the board. Biased it according to the document posted below (SM-400). Sounds pretty good now. Nice and full. Started making my acoustic guitars buzz the frets with huge sympathy vibrations. All this and no smoke. I'll give it a real workout at jam tonight.

Odd thing that I need to revisit though... the fan never came on. I ran the amp pretty hard and had the heatsink up to about 155*F. I thought it would have kicked on at about 131*F (55*C) according to the manual. I may need to replace the thermal sensor or fan. Gotta find out which one first. I don't think I'll try to run it too hard, too long until I get the fan situation sorted though.

Tony

[Axtman]

I was wrong. <---- You won't see me post those words very often. ;-)

I had to dig around in the trash to find something that I accidentally threw out. I found the burned bias resistor and it is exactly the same size as the ones previously posted. So I guess I owe Enzo and apology. SWR IS consistent with the builds.