SWR 350 bass head not working?

[The Dude]

You are just looking for shorts, so there's no need, at this point, to unsolder anything unless you read a short at which point you'll need to remove the part and verify that the part is shorted and not something else in the circuit that is across that part.

I don't have a pic of the amp board, but generally the emitter resistors are a larger wirewound resistor.

There's no reason I can see that a light bulb limiter wouldn't work to check for shorts on any amp. When you use it, your main purpose is to see if the amp is going to fire up without blowing the fuse (or anything else).

Every other transistor, other than the outputs, is a driver of sorts. In my experience, it's not a bad idea to do a quick check of them all for shorts since you'll have the board out anyway. If you miss one short in a direct coupled amp, it can blow itself up again and take you back to square one. It's best to be thorough before you reapply power. A few extra minutes of testing can save you lots of time and money down the road.

[Jam]

Thanks for the help. I ended up pulling the output transistors out and when I tested them only one I had 556 on diode setting the rest were O . Are these the resistors?

[The Dude]

Ah, it indeed appears you have some shorted outputs. The emitter resistors are R23-R26. Make sure none of them are open.

[Jam]

I tested the emitter resistors and I get 3 to 4 ohms and I unsoldered one end of them to test them

[The Dude]

Are you sure it's not .3 and .4? The schematic says .1 ohms. I suspect it might be just a meter zero issue. If you touch the two leads of your meter together, what do you get?

[Jam]

With the meter on 2k ohms I get .000 is that the right way to check it?

[Jazz P Bass]

That reading indicates the resistor is good to go.

You are looking for the ones that are open.

Move on.

[Jam]

Ok sorry for asking so many questions. So with the resistor being good I only need to replace the output? Thanks

[DrGonz78]

Ok sorry for asking so many questions. So with the resistor being good I only need to replace the output? Thanks

Yes. You talked about Q7 and Q8 so we are assuming that those parts are not shorted when they were tested. Light bulb limiter is a must the first time starting the repaired amp. After that point you will be monitoring the amp and then setting Bias of the amp for stability on the output.

[Jam]

I replaced the outputs Q9 toQ12 and I hooked it up my light bulb limiter and getting around -54 dcv and +54 on the Q9 to Q12 I replaced. Is that in mean of what I should have with it hooked up to the light bulb limiter? What would a good next step? Thanks

[DrGonz78]

Just to chime in... I guess so depending on what wattage of light bulb you are using, it is gonna be a bit lower. The main point here is that the rails are balanced and that is good. It passes the light bulb test... meaning the bulb goes dim right? Before you pull it off the bulb go ahead and take a reading to check if there is any DC voltage on the output.

[Jazz P Bass]

I would , very carefully, measure the output transistor base voltage.
That will give you an idea wherther or not the amp is in a stable configuration.

[Jam]

Thanks agains everyone. Ok here is where Im at I have +55V on the collector on Q9 and Q10 and I have -55V on the emitter on Q11 and Q12 and thats with it hooked up to the light bulb limiter and yes when it turns the light bulb goes bright then dim. The speaker output I get +2v what should I have? Thanks

[DrGonz78]

So you have 2vDC on the ouput? Still something wrong with the output circuit and should be less than 1/2 vDC on the output. Ideally 0 volts on the output but 0-250mv DC is very common on many amp. So do not connect it to a speaker yet. Start looking for other bad parts.

[Jam]

Thanks so what is that caused by ? like transistors? Or wheres a good spot to start? Thanks