SWR 350 problem

[Axtman]

I have a SWR 350 amp that I am trying to repair. It is hooked up to a light box current limiter. When I turn the amp on with no speaker load the light is bright initially then dims to a barely visible glow. When I plug in a speaker dummy load the light starts flashing slowly. If I turn the amp on with the dummy speaker load then the light is full brightness.

Do you have any ideas? Thanks!

[Enzo]

What are you trying to fix, what is wrong with the amp?

[Axtman]

The amp has a loud buzz in the speaker output.

[Ronsonic]

DC offset on the output? Have you measured for it?

[52 Bill]

There is something about the start up bias circuit in SWR power amps that will sometimes cause light bulb limiters to show full brightness and not allow the amp to power up correctly. Just keep that in mind when you are working on this one.

If there is a loud buzz on the speaker output, I'm with Ronsonic and suggest looking for dc on the output terminals. Try it with and without the light bulb limiter, but definitely without the speaker or load resistor. If there is dc there look for shorted output transistors, etc.

[Enzo]

Limiters are great, but they are to protect the amp when it blows fuses. Is this amp bliwubg fuses? If not, get rid of the bulb.

I'm with the others, lose the speaker, and check for DC on the output. Loud hum almost always means one of two things - DC on the3 output, or loss of a filter cap.

[Axtman]

The amp blows fuses. That's why I use the light bulb limiter.

When the dummy speaker load is plugged into the amp I get positive 10 volts DC on the speaker output.

When the dummy speaker load is NOT plugged into the amp I get a fluctuating average of about 50 volts AC on the speaker output.

[Enzo]

Just so I am sure, does it still blow fuses with no load? The reason we want to work without a load is that most solid state amps can sit there with 50v on your outputs and not blow a fuse if there is nothing to load down that 50v. As soon as we put on a loadd, now that 8 ohms or whatever wants to draw current from the amp. But if it can sit there unloaded and not kill fuses, then that is OK.

So using the bulb to save fuses, you have two main power rails, I forget what they are, but the popsitive and negative should be close to the same voltage, so if one is about 50v so should the other be. How are yours?
Once you have those DC voltage readings, flip your meter to AC volts and remeasure. We shouldn;t see much AC voltage on those DC supplies. If you see tens of volts AC, then your filter cap is not working.

[Axtman]

I checked everything out again just to be sure.

The amp does not blow the 8A fuse when operating without a speaker load. Actually I put in a 2A fuse to see what would happen and it blew it. But the amp is designed for a 7A fuse.

With the 8 ohm dummy load attached the amp does NOT blow the fuse. But the dummy load gets really hot and starts to smoke. When I connect up the light box the light bulb up to full brightness when I attach an 8 ohm dummy speaker load.

The schematic shows +75VDC and -75VDC rails. I get +73.2VCD and -73.2VDC.

The speaker does not get DC but gets about 50 VAC.

[Enzo]

OK, so leave the load - of any kind - off until you get zero on the output.

Just for science, is there a large AC signal at the power amp input? Your main power seems OK, how about.

Look at the upper left of your schemastic. Is there a point coming in marked AC? Feeds through a single diode to a small cap or two? Is that diode shorted? Is there AC on those caps?

[Axtman]

The diode checks out okay. There is 20 VAC coming in and 25 VDC on the caps.

[Enzo]

Darn...

OK, you have 75v rails, look at the collectors of the driver transistors, is the 75v presnt - one per polarity.