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

[JerkyPudding]

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!

[The Dude]

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?

[JerkyPudding]

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.

[The Dude]

Can you post the voltages on that 60142 transistor nearest your readings in green?

[JerkyPudding]

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.

[Enzo]

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?

[JerkyPudding]

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.

[JerkyPudding]

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

[Enzo]

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.

[JerkyPudding]

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.

[JerkyPudding]

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.

[RickyTerzis]

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.

[FrogT]

Hi everybody, I had the same issue with it, I fixed it so let me tell you what I've done.
Before, thanks a lot for the help here.
So I had no sound and DC voltage (+6,4v) on the output. I pulled out Q13 (see schematic I annoted ) to test, but nothing changed, I pulled out Q15, Q17 and so one... I Finally found a voltage data schematic and merge it with the one at the start of this thread, and annotate the PCB to find easely the right component. I measured voltage on all points and saw around Q6(2N4888) a problem. It was leaving to much current from C to E. I've change Q6 and Q7 with 2N5401 (the most near datasheet) and got sound !! Now I've got to clean all my pots to hear clean sound.

[POrKÔPOL]

Hello, I’m looking for an image of the BCB annotated with the Peavey 400, anyone have that?

completely cleaned, redone the power cable… I changed all the chemical capacitors (very ugly) …

I have a breakdown on the 24 volt, it swings from 40 volts direct the preamp does not like too..
The power stage works, but I’m measuring - 100mv on the resting & charging HP output, I find it a bit high

It’s complicated without the component layout diagram on the PCB…
Thanks to you ..