Peavey KB100 no sound

[Tone Meister]

1987 model KB100 was dead shorting and wouldn't power on initially. The correct schematic is attached.

What I've done thus far

• Replaced both (shorted) 6392 outputs Q6 & Q9 with matched MJ15015G
• Removed U2 87478 compressor chip
• Replaced U1, U4, & U5 (4558)

Current Conditions

• No speaker load
• Amp is powered up on light bulb limiter
• No appreciable DC on output
• Unable to get sound when injecting a signal into POWER AMP IN on rear panel
• 40v and 15v rails intact

Rechecked U1 and U5 and can still measure DC even with new 4558 chips.

DC readings on new U1
pin1 -14v ... pin 8 15.1v
pin2 .47v ... pin 7 2.9v
pin3 -.51v ... pin 6 -.08v
pin4 -15.4 ... pin 5 .08v

DC readings on new U5
pin1 .002v ... pin 8 14.8v
pin2 .002v ... pin 7 13.2v
pin3 .002v ... pin 6 14.4v
pin4 -15.5 ... pin 5 8.2v

• All .33/5W are good
• 47 ohm resistor good
• All 5W resistors are good

Q3 (5332)
E 41.9v
C 1.4v
B 41.1
Q5 (5331)
E .78v
C 42.0v
B 1.4v
Q8 (5332)
E -.39v
C -41.4v
B -1.0v

Q1 (3904)
E -.54v
B .05v
C 38.2v
Q2 (3904)
E -.54v
B .08v
C 38.8v
Q4 (6531)
E .15v
B .13v
C -.95v

Something is causing DC to be present at U1 & U5 even after new 4558s are fitted. I didn't have it passing audio for a short time, but it was low on power and distorted. I removed the U2 compressor chip for troubleshooting. I have no idea if it's good or bad, and i don't know how to test it.

[Enzo]

First, one problem at a time, preamp or power amp. I pick power amp. U5 and U5 are in the preamp. having said that, pin 5 of U5 is to be a ground, yet you have 8v there. That speaks out to me.

back to power amp.

Your Q5,8 voltages don't make sense, I suspect you have confused the pinout, you can't have 42v on the base and 1v on the collector. I might believe the other way around. Try E-C-B left to right.

U7a is the input, I also like to pull U2 during service, and doing that means whatever U1 and the other half of U7 are doing is irrelevant. The test for U2 is to get everything else working, then install it. If the amp still works, U2 is likely OK.

Apply a steady signal to the PA in jack. Does it appear at U7-1? Is there DC on U7-1? If so, then check those diodes on the input, a shorted one puts 15v on pin 3 of the IC. Is signal present at base of Q1? Signal at collector of Q3?

[Jazz P Bass]

Q3 voltages look 'pinout' incorrect.
The Collector should be at 1.4V.

[Tone Meister]

Q3 voltages look 'pinout' incorrect.
The Collector should be at 1.4V.

You and Enzo are both correct. I mislabeled those pins again. I will double check all voltages and verify pinouts and check the points that Enzo suggested and report back when I can.

[Jazz P Bass]

Honestly, the posted voltages do not look 'off'.

You are not trying to run the amp on a limiter are you?

If you are, ditch it.