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Thread: Earth Module 440 Bass "Producer" Head (i.e Peavey Musician 400) No Output

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    Earth Module 440 Bass "Producer" Head (i.e Peavey Musician 400) No Output

    So this is a continuation of a previous post concerning this same amp. I initially had 44VDC on the output, changed out a bad diode and 1ohm resistor, and with the help of Enzo (many thanks), I managed to trace the issue to a bad solder joint on an output transistor's resistor, which put the 44VDC back where it belonged.

    Now I've put a signal through it, the preamp is checking out ok, slave amp out works fine, wave looks good on the scope when measured at the input point ahead of the differential transistors. Problem: no output. Scope measuring at output jack reads a very dirty, very weak signal, substantially weaker than the preamp is putting out. I've measured and compared to all the voltages in the supplied schematic, everything checks out except where noted in green.

    I measured the 430C transistor (Q6)
    and reads: Emitter- -509mV,
    Collector- -1.1V,
    Base- -42.7V

    Any thoughts? I appreciate the help!pv400moduleb.png

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    Supporting Member The Dude's Avatar
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    Are you sure you have the basing diagram right? Or maybe someone put the transistor in wrong? The emitter should be close to supply since it connects to the negative supply through two 10 ohm resistors. Or, maybe one of those resistors is open?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Quote Originally Posted by The Dude View Post
    Are you sure you have the basing diagram right? Or maybe someone put the transistor in wrong? The emitter should be close to supply since it connects to the negative supply through two 10 ohm resistors. Or, maybe one of those resistors is open?
    Sorry, typo. Emitter is -43v, I was looking at the wrong part of my notes.

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    Supporting Member The Dude's Avatar
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    Can you post the voltages on that 60142 transistor nearest your readings in green?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Quote Originally Posted by The Dude View Post
    Can you post the voltages on that 60142 transistor nearest your readings in green?
    E- -43.1v
    C- -451mV
    B- -43.3

    Also, I forgot to mention that I measured -488mV at the output, and the 60142 transistor on the positive side, tied to the 1/10W resistors in series with the 60142 in question is reading 100mV at base.

  6. #6
    Senior Member Enzo's Avatar
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    What does the scope say over left of the inductor, such as at any of the 0.33 ohm 10 watt resistors or emitters of output transistors?

    Is the amp out loaded? Get rid of any load, still the same? Check with an ohm meter to see if either output jack has shorted.

    Your green reading tells me the transistor between them is not conducting hard enough. So...

    There are eight power transistors identical, but note they are in two groups of four, and the end one of each group is wired as a driver. The between the green one is the driver for its group. SO swap it for one of the others in its group to see if it makes a change.

    meanwhile, you have signal to the diffy pair. WHy stop there? Signal at the base of Q6? At the colector of Q6?
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    What does the scope say over left of the inductor, such as at any of the 0.33 ohm 10 watt resistors or emitters of output transistors?

    Is the amp out loaded? Get rid of any load, still the same? Check with an ohm meter to see if either output jack has shorted.

    Your green reading tells me the transistor between them is not conducting hard enough. So...

    There are eight power transistors identical, but note they are in two groups of four, and the end one of each group is wired as a driver. The between the green one is the driver for its group. SO swap it for one of the others in its group to see if it makes a change.

    meanwhile, you have signal to the diffy pair. WHy stop there? Signal at the base of Q6? At the colector of Q6?
    Ok, I swapped the power transistors as you suggested, no real change, but it is reading 50mV, increases as it heats up.
    Plugged signal in at base of Q6, reads a saw wave
    Plugged in at collector of Q6, reads weak sine wave, identical to output reading.
    Plugged in before inductor, no change, same wave as output reading.
    Removed load, no change, no shorted jack.

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    Is this an indication of a bad driver Q6, or is there something going on with the diffy?

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    Senior Member Enzo's Avatar
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    Whatever is on the collector of Q6 will appear on the output. All the stuff to the right of Q6 is just current amplifier. Got signal at its base? I'd be trying a new one, sure.
    Education is what you're left with after you have forgotten what you have learned.

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    I'll get it ordered and let you know what happens when I install it. I can't thank you enough for helping me on this, it's been a real education to be sure.

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    So, I changed out Q6, and all is well. This is a surprisingly good sounding amp, very loud and clean signal! I can't thank you all enough for your help, especially Enzo, this project has been a valued education for me.
    The Dude likes this.

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    Hi...i am a new user here. As per my knowledge the emitter should be close to supply since it connects to the negative supply through two 10 ohm resistors. You need to check whether resistor is open or working properly.

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