Peavey 400BJ

[lowell]

Finally had a chance to look at this again. These are measured in reference to the chassis.

0Vac on + supply
70mvAC on - supply

So I'm thinking the filter cap is the issue? Could anything else cause this?

[Enzo]

70 MILLIvolts ripple on a 50v supply? I doubt that is a failing cap.

[lowell]

Agreed. But I don't know what else it could be. Any tip on what to check next is greatly appreciated. To summarize, the high v rails are good if the filter cap common is used, but not good if referenced to chassis ground.

I guess it's possible one or more output transistors aren't fully functioning. I only replace one as it was the only one I found dead short. Should I replace others?

I'm still unclear on how exactly this power amp works. It seems that the output is via the caps from the main rails. And that modulates the whole power supply. ? I find it odd that the output is coupled directly to the main filters as well, but I guess signal passes through them and swings the common up and down.

[Enzo]

OK, rereading all this...

30v across R136
45v across R105

Each of those feeds a 15v zener. Note there are 15 more volts across one of the resistors, just about what we'd expect if its zener were shorted.

So is there 15v across both CR112 and CR118? Is either shorted? (presumably CR112)

Note that the 15v supplies at top and bottom are not part of the 15v zeners I am discussing.

Meahwhile the zeners I AM discussing set up a reference for the tow limiters. Disable the limiters by lifting their diodes CR107, CR119. That gets them out of the way Now do your main rails swing back into parity?

[lowell]

Gotcha. Neither cr112/118 read shorted with diode test function and both have similar readings around 900mv.

I lifted the diodes 107/119 and rails still measure +54v/-49v.

[Enzo]

They read normal forwards, but zener action is reverse bias. Do you get 15v across each while the unit is running? One could be "zening" at 15v while the other might zene at a volt or two, there are many ways a diode can fail.

[lowell]

Getting +14.5 and -14.7 on the zeners.

[The Dude]

Yes, ^^^^^^ that. Check the voltage drop across the diodes. One could be just slightly leaky and/or breaking down.
BTW, Enzo, "zening".....nice one.

<simulpost>

[Enzo]

To zene the impossible zene
To fight when the bias is low
To try with a clear head tomorrow
To strive just to make the amp go...

This is my quest
To follow that star
No matter how hopeless
No matter how far


With apologies to some guy in La Mancha

[g1]

I'm still unclear on how exactly this power amp works. It seems that the output is via the caps from the main rails. And that modulates the whole power supply. ? I find it odd that the output is coupled directly to the main filters as well, but I guess signal passes through them and swings the common up and down.

You've been through this before . http://music-electronics-forum.com/t34497/#post321427

Also, there was mention elsewhere about the need for fairly matched caps in some flying rail designs, not sure if it applies here as it seems your voltages across the caps are equal but the center point is not at ground potential.

[lowell]

Right on G1. It's been long enough I forgot about that repair. But there was a small voice in my head saying I'd been here before.

So...being that the zenersame have those (mostly) correct voltages. Where shall I poke around next? By the way I measured from chassis to those zener voltages but not across the zeners. Does it matter?

[lowell]

So after studying the circuit more, I see the temperature comp diode MZ2361 which is mounted to the heatsink in a hole. I'm getting about 1.088 voltage drop across it. I hit it with my freeze spray and the asymmetry of my main rails declined further. Could it be that this thing is faulty at 25degC ? The datasheet suggest a min Vf of 1.24v at 25deg. Also of note, is that my driver bases are -1.2v and +1.1v. This is what led me to investigate the thermal diode. But I've never ran into a repair that dealt with troubleshooting one of these temp diodes. Could it be causing the asymmetry? I appreciate everyone's help and sticking with me on this!

https://www.digchip.com/datasheets/p...MZ2361-pdf.php

[Jazz P Bass]

The voltages on the driver bases sound normal to me.

[lowell]

Thanks! If you were fixing this where would you look next? Also of note is that there are -15mv on the output PNP emitters. Seems to me that the negative side is pulling too much current. The bases are -620mv and +470mv. I replaced R113/129 driver emitter resistors initially because I fried one while troubleshooting in the beginning. Could this be the cause? Wouldn't think so because I do have +/-600mv on both driver bases. The following makes me think the positive outputs aren't ON enough and that the negative outputs are working right.

Could one of the POSITIVE side output devices be pulling excessive current through its base? Is this an instance where I can replace the power amp with resistors to see if the rails oblige?

[nickb]

Just a reminder to do this

OK, rereading all this...
Disable the limiters by lifting their diodes CR107, CR119. That gets them out of the way Now do your main rails swing back into parity?

If you still have 15V across the speaker jack after doing that, then measure, relative to chassis ground, the voltages on pins 4,8, 5,6, & 7 of U1.