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

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

    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!

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by JerkyPudding View Post
    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.

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    Quote Originally Posted by Leo_Gnardo View Post
    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?

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    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?

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    Quote Originally Posted by 52 Bill View Post
    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.

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    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    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.pv4001.png

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    Senior Member Enzo's Avatar
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    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?
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    Quote Originally Posted by Enzo View Post
    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.

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    Senior Member Enzo's Avatar
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    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.
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    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?

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    Senior Member Enzo's Avatar
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    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.
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    Quote Originally Posted by Enzo View Post
    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

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    Senior Member Enzo's Avatar
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    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.
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    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?

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    Senior Member Enzo's Avatar
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    It indicates what you already know: there is +44 on the output. The emitter is tied to the negative 45v, and the collector to the output. So if there is 45v on the output, it will be on that collector.

    I asked earlier and don't recall an answer, is ther +15 and -15 respectively on pins 8 and 4 of the IC U2? Or if you prefer, the power pins of the TL074?
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    Quote Originally Posted by Enzo View Post
    It indicates what you already know: there is +44 on the output. The emitter is tied to the negative 45v, and the collector to the output. So if there is 45v on the output, it will be on that collector.

    I asked earlier and don't recall an answer, is ther +15 and -15 respectively on pins 8 and 4 of the IC U2? Or if you prefer, the power pins of the TL074?
    There are no IC's or 15v. This is an older amp.

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    Senior Member Enzo's Avatar
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    I was looking at the drawing you provided in post #7.

    So maybe it is closer to a 400 module instead of a 400BH.

    In which case, look for a diode clamped to the heat sink or even clamped to one of the output transistors. Got one? Measure voltage at one end of it. Do you see about 45v there? If so, then all the power stuff is probaly OK.

    Will this drawing open for you?
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    The drawing opened up, thank you for that! I measured +44v at the diode mentioned.

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    Senior Member Enzo's Avatar
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    SO that means in a working amp that everything to the right of that diode will also sit around +44v.

    Oh, does that circuit look more like your board?

    Using my drawing then, look below that dual diode on the heat sink. it is within the dotted line rectangle. Below it is a 430 transistor. On your board that is more likely a TIP29 or TIP31. A TO220 of some sort (the tab top type).

    If that transistor is open, or if it is not being driven at its base, then the dual diode and the whole rest of things will snap up to V+, your +44v.

    Check it.
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    The drawing you supplied matches much closer. The heat sink mounted diode does not appear to be a double, but I did trace it to a TO220 transistor, which measures 44v at base, collector and emitter.

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    Senior Member Enzo's Avatar
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    It might not be a double, but the double one looks like a normal diode too.

    Look at the schematic. Only way that transistor could have +44 on the emitter is if the 10 ohm resistors there were open, one or both. The transistor could also be shorted - check for that - but if so it would burn out the resistor too.
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    20170517_172442_hdr.jpgpv-400.jpg
    I told you the wrong transistor, I traced the circuit more thoroughly to find a PNP tab transistor measuring 43.3v at base, -44.5 at collector, and 43.5 at emitter. Also, the heat sinked diode is measuring open (it's been isolated from the circuit via the socket connection). The positive side of the diode reads 44v, the negative reads -43.3v. Also, when I replaced the bad 1ohm 10W resistor and the diode (circled in green), 44v went back to the output (voltage at the output dropped to .003v after the fuse blew).

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    Senior Member Enzo's Avatar
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    The heat sink diode in the PV, and maybe the Earth, is a dual, so it will have twice the voltage drop, that may make it seem open. If you have +40v at the base of that transistor, the only place it can come from is that diode.

    And that points me back at the transistor I mentioned previously, the 430 just left of your circled one.
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    So I got the 44VDC off the output, turns out a bad solder joint on an emitter resistor was to blame (this amp's past has been in rough hands, many bad solder "repairs"). Now I'm reading -0.5DC on the output.

    Also, is it possible for this amp to function at all with a reversed heat sink-mounted dual diode? I checked and rechecked, and based on the marking (which looks like the typical base marking for any diode, but it is rather difficult to tell), and it seems to be reverse to what the schematic says (the schematic has been very accurate so far). The amp has been through some rough hands, so I have suspicion mixed with naivety.

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    Senior Member Enzo's Avatar
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    Probably not reversed. otherwise likely blows fuses.
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    I measured the 430 transistor at the base of the heat sink diode, readings are: base= -42.9v, collector= =-1v, emitter= -43.7. Reading -.53v at output now.

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