Peavey SC400 PA brain (No Output question)

[freeformfx]

yes that's what I was thinking, I'll need to recheck but I thought I was getting low resistance between Drain and Gate.
The -25v is getting to the 1M resistor (r275) but the voltage on the other end is like -25mV . It's reading pretty high resistance in crkt also. I think I'll swap it out in case it's went open from excess heat or something.

[freeformfx]

alright , been at this half the day. The the 1M r275 was fine, but I unsoldered the "x" end feeding the control voltage to the FET gates and viola! there it is. (a bit high around -24v rather than -22v but there) And with it in circuit the voltage on this side becomes -24mV

So I'm hoping someone much smarter than me can look at that schem and tell me where that voltage is getting shunted out on this end. I'm perplexed. I mean other than the FETs being bad themselves. I'll look for some J111 but don't think i've got any.

[Jazz P Bass]

For troubleshooting purposes, if Q105 (J111 JFet) is suspect, take it out.
It appears to be a mute at power on delay.
The J111 is a depletion device.
It is normally on, D to S.
When the -24V ramps up it will turn the JFet off.
As to checking opamp signals, I prefer to look at the output pins.
Trying to monitor the input pins can be decieving, depending on the circuit topology.
(something along the lines of a virtual ground)

[freeformfx]

"as to checking opamp signals, I prefer to look at the output pins"
sound thinking. however I can definitely confirm there is no signal getting in or out of the U100/U101 chips. And as I stated there is no -22v signal on the gate of the FET. Though the -22v IS present on the side of the 1M resistor that is NOT connected to the FET circuit.
:
"For troubleshooting purposes, if Q105 (J111 JFet) is suspect, take it out.
It appears to be a mute at power on delay."
This is basically what I'm seeing it as here too. So you're saying it shouldn't cause any trouble to just remove it and see if the signal passes then? that's something I wasn't sure of. Not knowing exactly what's wrong with this unit the last thing I want is to cause any extra troubleshooting and repairs. I think this is what I'll try, it's just curious both of these muting FETs for the Main and Monitor would fail.

Hey I'd wondered where Enzo was at , he's usually all over it. Then I just read on here he's been sick, Hope he's doing alright...everyone think a positive thought for him. Great guy!

[freeformfx]

wow this thing is turning into a complete bugger!
I removed the Q105 fet and fired it up, within seconds the fan ramped up and warm air poured out of the heatsink on the PNP OT's! they took like 10minutes for to cool off to touch.
I did inadvertently leave the -22v feed disconnected but I can't imagine that'd cause e overheat (??)
I've retested several times now with everything back in place and those PNP's get blazing hot immediately and read like shorted C-E until allowing to cool which they seem fine. In the short time I have I do read the -60v rail is reading only -52v.
No ideas what else is happening here?? I can take any measurements and post if someone has ideas to help. I'm about to leave this thing otherwise...and I abhor giving up
FWIW the other Q205 FET does NOT read 23ohm S to D,

[Jazz P Bass]

So the PNP output transistors are full on.
That failed J111 was keeping the amp in mute very well.
You need to build a "lamp Limiter".
One way that the PNP's can be full on is a short.
In the driver, in the bias string...
Q213 is the driver for Q206 & 207.
They are all PNP's.
The collectors are tied to -60V.
The farther away the base gets from -60V (less negative) the harder they will be turned on.
If opamp Q100B is outputting the incorrect voltage, that may explain what you saw.
I would like to see the base voltages of the driver transistors Q200 (NPN) & Q213 (PNP)

[freeformfx]

do you mean the U100b opamp (4560)?
Yes I am gonna have to put it on the limiter to do any further work on it til I get those PNP's to stop overheating. I will try and take those voltages in a bit and post them
I'm still confused I guess on how messing with that FET caused those OT's to overheat when they were idling fine before.

[Jazz P Bass]

The FET. in it's failed state, was "muting" the output section.
This is not the normal condition.
By removing it you have exposed the real issues with the amplifier.
My bet is that you will find a very low negative voltage (or worse yet, a positive voltage) on the PNP driver base pin.
Full tilt boogie.
Do not apply a load to the outputs until this is resolved.
Are the IC's installed in sockets?

[g1]

The Fet should only be muting the audio signal. Often if one is bad all the mute fet's will mute so I doubt they were both bad.
This new problem sounds more like a DC issue. I think you may have inadvertently caused the new problem. Double check for any problems with connectors, broken wires, or power transistor insulators.

[freeformfx]

thanks guys , I'm still gonna check those voltages and post them, just sucks that now I'm gonna have to do it under protection cause these suckers get hot FAST and as I said read in a shorted state as soon as they heat up. But return to normal.
"I think you may have inadvertently caused the new problem"
yes indeed this is exactly what I was afraid of before removing the FET and investigating the missing "x" voltage, since I don't have a real good understanding of this crkt. still seems to me though nothing involved with the muting fet could screw up the outputs in this way. so yeah I guess I should check those ribbon connectors etc..
Yes the IC's are all socketed. Could I put an NE5532 for the BA4560 at least. I don't have a replacement for the 13080.

[freeformfx]

under current limiting there is soft steady pulse from the bulb.
the vltg was held down way too low w/ the bulb I had in there so I quickly took the readings from the pnp's w/o limiting and they are something like -51vdc...-2.7vdc...-2.1vdc
and i said it's strange to me that almost as soon as powered up the PNP's read shorted C-B until powered down for a short time then resistance builds back up between the junction
the npn's readings are +59v +1.7v +.1.3v

[freeformfx]

well guys , I checked the pnp drivers also and the vltgs looked pretty good to me there too. Have them wrote down on the bench but I think c -51v b -2.7 e-2.1.
I think I need to figure out what is pulling that neg rail voltage down all of a sudden and then get back to the FET problem unless somehow it and those IC could be causing it.
I don't keep a diverse supply of solid state devices stocked around here so I'll have to order but don't know yet what all is involved w/ this problem. Guess for now I'll just pickup the J111, 4560 IC and a couple pair of the driver txrs just in case. Can't find a 13080 IC from mouser thats DIP.
If anyone's still tuned in let me know what you think...appreciate it!

[g1]

It would seem the PNP output devices are what is pulling down the minus rail. I would just replace them. Sounds like one or both are shorting only when under voltage.

[freeformfx]

thanks g-one, there are 4 of them all behaving the same when under voltage, which i thought strange and was really hoping was not the devices themselves. And it's odd how they just started doing it (far as I can tell) after disturbing that main mute FET. replace them huh? that sucks, I just ordered some parts for it but didn't wanna spring for those just yet.
anyone else agree with this method of action?

also anyone know best place to get 2SA1302's , there's a bunch on ebay but like my previous experience with fake Sanken's etc.. I don't trust 'em and it's too much $ to get screwed on a part

[Jazz P Bass]

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