Mag 300 Drawing too much current? Blows fuses.

[harry]

When I tested my Mag 300 a week ago, it had V+ on the speaker jack plus a 10V ripple.

I've just plugged it in now and got a clean 250 watts rms out of it, until it blew a fuse (fuse was correct, 5A time lag). I replaced the fuse and it blew at around 200 watts rms. It should be good for 300 watts rms. It is behaving otherwise normally on the scope.

The attached schematic doesn't have any reference voltages in; I'm not used to troubleshooting solid-state amps, so I'm not sure how to approach this one.

Any suggestions would be greatly appreciated! Thanks,

Harry

[Cosworth]

Nice. Direct coupled class A push pull. If you are seeing 10 volts of ripple or DC at the speaker, that would be about 80 watts (@ 8 ohms) disipated in the speaker even before you start the music. Concievably it could still work and only give you about the 200 watts of remaining power availible or so that you say you can get. Do the speakers pull in or out when the power is turned on? If you are seeing 10 volts of DC, it is probably a leaky output transistor. If so even with no signal this leaky transistor would likely get warm or even hot without signal. If the DC is positive I would look toward the positive side output transistor or if the speaker has negitive dc with respect to ground it would be the bottom side transistor. Leakage would increase as it gets hotter so as you noticed, a reduction of power before the fuse pops again would be realized, and would make sense for this diagnosis. Sounds to me like you may need a new output transistor.

[harry]

Thanks for the reply Cosworth, it's helped clarify things; although I don't think I was entirely clear - the DC is no longer there. The amp appears to be working fine on the 'scope, apart from its blowing fuses, which confuses me greatly and makes it even harder to troubleshoot the problem.

Either way, it points to a leaky transistor on the V+ side. What you say about leakage increasing with heat makes sense and would explain why the fuse blows at around 200 watts (I'm calculating this as 27 Vrms into a 4 ohm dummy load).

I'd heard about these amps sometimes having PT primaries that short to the chassis, but this measures about 10 ohms, which is reasonable from other people's MAG300H testimonies. I thought that might explain the high current draw, but your idea explains it better, Cosworth.

[Enzo]

Problems are not always bad parts.

An open connection can cause just as much trouble.

Leaky transistor on the V+ side? How about a creaky solder joint on the V- side?

COnnect a speaker to the amp, and some gentle signal, doesn't have to be loud, ball up your fist, and whack the top of the amp hard. Any pops or crackles or other noises? The amp should NOT react to the whacking. If it does, there is some loose connection.

Am I missing something? Isn't 10v across 8 ohms more like 12 watts? Not that we want it either way.

[harry]

Interesting! Looking at it on the scope, when I punch the amp, it definitely reacts. Also, arcing is apparent on the fuse in line with mains neutral (this is not the original fuse that blew).

Thanks, Enzo, that's given me food for thought.

[harry]

So! Tapping R20 or R23 (the emitter resistors on the positive side of the output transistors) with a chopstick makes the output jump towards the negative rail then back again. These resistors are close together, so it could be either or both of these resistors. Tapping other components has no effect, apart from those in the immediate vicinity of these resistors - tapping these resistors directly seems to have the most effect.

I've re-done the solder joints but it hasn't seemed to do have any effect. These are 0R33 resistors, so hard to measure exactly, but none of them measure anything ridiculous. They are 6W wirewound.


Edit: It's R20. Put a DMM across it, set to resistance, and hit R20 and R23 with a chopstick. R23 was stable, R20's resistance jumped every time you hit it. I'll report back once I've changed it out.

[Albert Kreuzer]

Ah. The famous Enzo whack. Never fails.

[Enzo]

Hmmm, so when someone mentions a whack job, my name comes up?

[52 Bill]

Yes, but only in a nice way.

[harry]

Haha! It's an excellent technique.


So, replacing R20 makes no difference. You hit it with a chopstick and the resistance still jumps. In fact, now when you turn it on, the output jumps up to V+ and a fuse blows. With no load, this doesn't happen.

I'm a bit confused here. Hitting other components seems to have little effect, apart from hitting the 4R7 filter in parallel with the output jack - but that's physically located close to R20, and I would expect whatever happens at R20 to affect the output impedance.

[Enzo]

So if it is not the resistor itself, it can still be the connections around it, or a cracked PC trace.

[Steve Conner]

Did you check the output transistors? Maybe one of them is shorted, and the amp only works because that transistor's emitter resistor failed open too. If that open were intermittent, the symptoms would match what you reported.

This might be a bit of a long shot. When transistors go they usually short all three terminals together, which would disable the amp completely, emitter resistor or not.