SWR 750

[lowell]

So got one on the bench here. It's a "green board" PA. I just replaced most of the output devices as it was blowing fuses and I found a couple shorted output transistors. So the thing works now, but there is a hum, and it goes away when the master is turned down. I found that the lowV supply is not symmetrical. It's like +13.5 and -15v. Would this slight difference cause a good amount of hum? Finally, I can't find a schematic that matches what I'm seeing in the amp in the lowV supply. The 7815/7915 regulators have 2 diodes from one side to center terminal and 1 diode from center to the other outer terminal. On both of them. This ring a bell with anyone? thanks!

[Enzo]

So ignore the extra diodes. NOW does it look like it matches your amp? We are looking at the Bass750, which is not the same as the 750x.

The extra diodes are reverse voltage protection. I think you will find them reverse biased.

In any case, your volt and a half low is 10%. I expect the two 15v supplies to be pretty close to each other. There are several ways this can happen. One is the regulator is failing. ANother is that the filter cap before it is bad and so the incoming supply to the regulator is full of ripple. I see a pair of 4700uf filter caps for each of those raw supplies. COmpare to the 15v one. Do both raw supplies measure about the same, or is the 13v reg one really low too. And full of ripple?

yet another possivility is something is loading down the suply, drawing more current than it can handle. A quick check of current would be the dropping resistors before each regulator. My schematic shows them as having different resistances, so careful calculating. But measuer the voltage across each of those 2k/2.3k 5w resistors to calculate current. I expect both 15v supplies to draw similar current.

On the other hand, it may just be a droopy regulator but the hum is from something else. Never assume. There is an op amp stage after the master volume, so presumably any power supply problem might show up there too, after the control.

SO other simple things to check would be the tube, and the tube B+ supply. Does pulling the tube stop the hum? Is the B+ clean or rippled?

And I guess a good question might be: does the amp pass signal or ONLY hum.?