SWR Mo' Bass Rail Problems

[rf7]

I've got a SWR Mo' Bass on the bench which blew up, taking out both output channels and the preamp too. I replaced one of the output modules and reworked the other one, but they both have the +75V rail shorting to the output. They use the same output circuit as the SM-900 in this amp. All the semiconductors and resistors measure OK with my DMM. Capacitor C7 smoked because there was 150V across it. I've got the preamp pulled.

If I pull the AC Terminal input to the output module, the rail short goes away.

If I short R34, thereby shutting off Q14, the rail short also goes away.

What is the purpose of that AC input circuit?

I'm thinking that the collector of Q14 should sit around 2 diode drops above ground, not at the positive rail.

The power supply filter caps get their ground through the preamp PCB, It makes the amp real fun to work on until you figure that out.

Any ideas?

Thanks in advance.

[Enzo]

Ah, the MoBass. Let's take every feature we can think of and stick it in an amp.

R34, then you must have the older version. The boards are not blue, right?

Maybe it is just me, but I am uncomfortable with the term shorted used like that. To me shorted implies a connection. I myself would prefer some term like DC on output, or offset or something. But that is just an opinion.

Q14 turns the amp on. Turn it off, the amp goes off, so if the amp has massive DC problems, you turn it off and they disappear. Of course the amp doesn;t function that way, but the DC goes away.

Q14 is turned on by the charging of C?? there through R30. Once that charges up enough to overcome the -75v coming through the 68k resistor, the base of Q13 conducts and Q13 turns on Q14. This little delay helps protect your speakers, especially any with horns.

SO by killing this circuit, you are just removing the +75 from the amp circuit.
Q14 should turn on hard and thus become invisible after the very brief turn-on pause. So indeed it wants +75 on its C.

You are not blowing fuses, right? Though you probably would with a load on it. No load please. No blowing fuses means the amp is not blown up, it is being driven off to the one side. But do check for shorted outputs all around. SHorted Q9,10 would put +75 on the out. But open Q11,12 would not pull down to center the output and same result.

Many amps are this way, the input stage ground comes up from the signal cable. Disconnect the signal cable, the input ground is gone, and the amp slams over to one rail. SO if you are trying to run this with no input, it won;t work. If you removed the preamp, then you may have removed the ground connectiion to the powr amp input, and that is enough to slam the thing over to +75.

But look farther back. This is an extremely conventional amp circuit. Like any amp, the V+ and V- outputs are driven by smaller transistors. Q7 Q8 are your drivers, and their bases are held at a steady voltage space by Q4. That is your bias, that volt or two space. Then the voltage amp stage Q3 yanks that whole thing up and down to the music, and the outputs follow. Wherever the base of Q8 sits, the base of Q7 ought to be a volt or two more positive. By the way, if there is doubt, you can short dead across Q4, shorting the driver bases together. This will increase crossover distortion in the output, but the amp will be biased coldest that way for testing.

Diffy pair Q1,Q2. Q1 drives Q3 with the input signal, and Q2 samples the output and corrects distortion by modulating the Q1 current.

So fire up the amp, make sure the input cable is connected for input ground, get your unwanted +75 on the out, and start reading voltages.

WHat is on the bases of the drivers - about +75 give or take? On both or just the top? And if that is the case, what is on the collector Q3? +75? And if that is the case, what is on its base? Remember, the emitter of Q3 is bound to -75v. So the base should be checked with respect to that. if the base of Q3 is a drop or so above the emitter, then it is being turned on and the rest of the thing is following along.

Remember, finding wacko voltage in the output stage doesn;t mean it isn;t working. Q3 can easily tell the whole thing to go to +75, in fact if the amp didn;t follow Q3, it would be defective.

[rf7]

R34, then you must have the older version. The boards are not blue, right?

Correct, they are the green PCB's.

Maybe it is just me, but I am uncomfortable with the term shorted used like that. To me shorted implies a connection. I myself would prefer some term like DC on output, or offset or something.

Sorry, you're right, it is DC on the output.

Many amps are this way, the input stage ground comes up from the signal cable. Disconnect the signal cable, the input ground is gone, and the amp slams over to one rail. SO if you are trying to run this with no input, it won't work. If you removed the preamp, then you may have removed the ground connection to the power amp input, and that is enough to slam the thing over to +75.

You nailed it, right there. No wonder both modules behaved the same way.

Thanks for the explanation of how the circuit works, that really helps.

Thanks again for your help!