Earth Module 440 Bass "Producer" Head (i.e Peavey Musician 400) Problem

[JerkyPudding]

Hi all, I'm working on a Earth Module 440 Bass Head, direct rip-off of the Peavey Musician 400. When I first powered up, it hummed very loudly, then blew the fuse. I replaced the fuse, powered up, and now it's quiet. No sound at all. I measured no DC at the speaker jack, I've been using a schematic for the Peavey Musician 400 to get into the ballpark.

What would cause the hum to stop and no longer blow fuses? I suspect a power transistor, but I'd like to hear from more experienced folk.

I appreciate the help, thanks!

[Leo_Gnardo]

What would cause the hum to stop and no longer blow fuses? I suspect a power transistor, but I'd like to hear from more experienced folk.

Right, a shorted output transistor would be a top suspect. What usually happens is that transistor serves rail voltage (plus or minus what, 60V or so?) straight to the speaker, with lots of hum riding on it. That big DC jolt isn't good for a speaker, perhaps the one you had plugged into the amp is now toast. I hope not, but you could test the speaker real fast with an ohm meter, do the battery "pop" test, or plug it into a known working amp.

[JerkyPudding]

Right, a shorted output transistor would be a top suspect. What usually happens is that transistor serves rail voltage (plus or minus what, 60V or so?) straight to the speaker, with lots of hum riding on it. That big DC jolt isn't good for a speaker, perhaps the one you had plugged into the amp is now toast. I hope not, but you could test the speaker real fast with an ohm meter, do the battery "pop" test, or plug it into a known working amp.

Yeah, it's running in the upper 40v range. That thought crossed my mind, so I tested the speaker earlier, it's OK, it also has a fuse in the cabinet. Lesson learned, regardless. I'm still rather green with the solid state amps. Would an open resistor on the likely culprit transistor have caused the hum to stop?

[52 Bill]

It could also be that you have a shorted output transistor and it burned open the emitter resistor that connects it to the circuit. Now that the resistor has acted like a fuse, the amp will power up to some extent without blowing the line fuse.

Additionally, are there any internal power supply fuses in there?

[JerkyPudding]

It could also be that you have a shorted output transistor and it burned open the emitter resistor that connects it to the circuit. Now that the resistor has acted like a fuse, the amp will power up to some extent without blowing the line fuse.

Additionally, are there any internal power supply fuses in there?

No, there is only the main fuse.

[Enzo]

Try not to focus on what made teh one problem shift into anothwer problem. Just fix what is wrong.

Work with no speaker on it until we know it is stable and not producing DC. Check power supplies, no circuit works right without proper power supply.

Any shorted transistor you find, always check any associated resistors for opens.

[JerkyPudding]

I've been carefully tracing the circuit, trying to fill in the gaps (Peavey schematic is not exact). So far, I've found the a diode on the negative leg of the output to be open, along with a 1ohm 10W resistor reading about 18ohms. I have them both marked in the attached image. I've checked every large output transistor, all are fine, no other emitter resistors are open or shorted. Could this be the extent of the failed components? Bear in mind that the amp started out with DC on the output, blew a speaker, blew a fuse, and then the DC vanished from the output after replacing the fuse. Thanks for the help on this.

[Enzo]

The diode reads open BOTh ways? That would not affect operation, though we do want a good part there.


Unusual for a wirewound resistor to go up like that. Schematic calls for 0.1 ohm, yours is 1 ohm? SHort your meter probes together, what resistance do they read? Also, corrosion and oxidation can get in the way, scrape the wire leads on that resistor so your meter can make nice fresh contact, and see if the resistor comes down to earth. (No pun intended)

NO LOAD until we know it is right.

Even if the circuit isn't identical, it is close, so the voltages on the drawing should be representative. Do you have +/-50v on the rails? Got +/-15v on the LV supplies for the ICs?

Is signal from the preamp getting to the power amp input? Is it on the op amp output pins? Is it on the bases of the drivers and outputs? Is it on the ballast resistors?

[JerkyPudding]

The diode reads open BOTh ways? That would not affect operation, though we do want a good part there.


Unusual for a wirewound resistor to go up like that. Schematic calls for 0.1 ohm, yours is 1 ohm? SHort your meter probes together, what resistance do they read? Also, corrosion and oxidation can get in the way, scrape the wire leads on that resistor so your meter can make nice fresh contact, and see if the resistor comes down to earth. (No pun intended)

NO LOAD until we know it is right.

Even if the circuit isn't identical, it is close, so the voltages on the drawing should be representative. Do you have +/-50v on the rails? Got +/-15v on the LV supplies for the ICs?

Is signal from the preamp getting to the power amp input? Is it on the op amp output pins? Is it on the bases of the drivers and outputs? Is it on the ballast resistors?

The diode is out of circuit and reading open in both directions. The resistor is indeed 1 ohm (also out of circuit), the matching parallel resistor is in range. I'm getting +/-45 on the rails, I haven't checked the preamp yet, just been focusing on the power supply and power amp. I'll put a signal generator and scope on it tomorrow, see what's going on there.

[Enzo]

45, 50, whatever is fine. I was focused on the power amp as well, but I had in mind we were feeding it a signal. Look at your drawing, aside from the 45v, there are also voltages on transistor legs and stuff, those are important too.

[JerkyPudding]

I'll get a signal on it, it had not occured to me that a signal is required to accurately measure transistor behavior. I'm learning so much about solid state! Question: what is the function of diode CR15?

[Enzo]

Doesn't it require a signal through a tube amp to check its operation?

CR11 and CR15 are protective diodes. If the speaker load generates an inductive kick, that can go higher than the power supply rails. That would effectively put reverse voltage across the transistors, and they would not like it. SO if the output ever goes beyond the two main rails, those diodes will become forward biased and thus conduct. They will shunt any such spikes into the low impedance of the power supply.

[JerkyPudding]

Doesn't it require a signal through a tube amp to check its operation?

CR11 and CR15 are protective diodes. If the speaker load generates an inductive kick, that can go higher than the power supply rails. That would effectively put reverse voltage across the transistors, and they would not like it. SO if the output ever goes beyond the two main rails, those diodes will become forward biased and thus conduct. They will shunt any such spikes into the low impedance of the power supply.

Yep, I'm an amateur, but I've been able to fix them based on the symptoms alone. Not the most efficient way, but I'd like to be better at it, That's why I'm here. I've got the replacement resistor and diode in the amp now, with everything in place, I'm reading 46v at the output again.

When I check the circuit with a signal applied, what am I looking for?

I appreciate your help, and your patience with a rookie

[Enzo]

Well, you need to get rid of 46v of DC on the output before even thinking about signal.

The amp has a stable condition at idle, with predictable DC voltages, many of which are printed on the schematic. That has nothing to do with signal. If we have a DC-stable amp, THEN we apply a signal and trace it through the circuits.

[JerkyPudding]

Ok, I measured all 8 power transistors in circuit: positive leg, they all measure a voltage of .648 or so between emitter/collector. On the negative leg, all 4 power transistors, 88v measured between emitter/collector, they seem to be reading +44v on the collector side, -44v on the emitter. What does this indicate?