Peavey Valve King 100 head

[jondoe]

Yes it should be there on pin 1. Measure the DCV on pin 1,2,3,4 and 8 wrt ground please.

I did all of them while I was at it

Pin;
1= 0.71
2= 0.00
3= 8.55
4= 0.00
5= 9.32
6= 9.42
7= 9.65
8= 18.99

(made a typo above, edited.)

[nickb]

Pins 1, 2 and 3 should all be about same i.e. 9V or so. Either the NJM4560 is bad or the ground side of the reverb tank is shorted to ground.

Unplug the cable to the IN of the tank i.e. that goes to U101 pin 1 via P108/P105 and redo the DCV measurements.

If it's now OK then check the DC resistance from the shield side of the on the cable that goes to the tank to ground. The out lead must remain plugged in to the tank and PCB. If this is just a few ohms then remove the tank and find out why the input shield is shorted.

[jondoe]

Unplugging the IN side of the tank makes pin 2 rise to 0.72 volts, every other pin is the same as before. I swapped out the NJM4560 for a brand new one made and it made no difference. The existing one doesn't show any shorts on any of it's pins out of the board.

[nickb]

The tank bringing pin 2 down to 0v suggests there is a short from ground to the shield side of input the tank.

Lift one end of D118, unplug the input cable to the tank and retest the DCV.

[jondoe]

I made a slight mistake on the pin 2 reading, it's always 0.72 VDC, not 0.00. I lifted a leg of the D118 the readings remain the same

[nickb]

Then look for pin 1 being shorted to something. Wasn't the PCB resoldered at one time? Maybe there is a solder bridge.

[jondoe]

Then look for pin 1 being shorted to something. Wasn't the PCB resoldered at one time? Maybe there is a solder bridge.

I did put a socket in for the opamp, I tested for shorts on the sockets earlier and double checked those solder joints, they are pretty neat and tidy and no shorts. That leaves C125 I guess? I think I took that out already and tested it on a component testers, but I can always pop it out again. R129 and R34 both read as they should.

[nickb]

C125 being shorted would cause pin 1 to be high, not low. With the amp off remove U101 and measure the DC resistance on the socket from pin 1 to ground. Try both ways around and then repeat on the diode test range. Measure the DC resistance from pin 1 to pin 2 on the socket.

[jondoe]

C125 being shorted would cause pin 1 to be high, not low. With the amp off remove U101 and measure the DC resistance on the socket from pin 1 to ground. Try both ways around and then repeat on the diode test range. Measure the DC resistance from pin 1 to pin 2 on the socket.

Between pins 1 & 2 I have 22k resistance, in diode mode I get nothing in either direction. I put the opamp back in before testing resistance from pin 1 to ground and now I have higher voltages on Pins 1 and 2 and the reverb output on pin 7 is now carrying a signal....

I'm going to knock on the head for the night and start fresh tomorrow, somethings fishy here!

[nickb]

Didn't I say intermittent == pain! At least it's narrowed to a very small area. I think at this point it's up to you to find the connection issue.

[g1]

Then look for pin 1 being shorted to something.

I'd think D118 could be disconnected for test purposes?

[nickb]

I'd think D118 could be disconnected for test purposes?

Already done see #49

[jondoe]

I've resisted the temptation to take this out into the street and thrash it with a tree branch, but I'm holding the option open.

I've had the board out again this morning, I've been over every solder joint with a magnifying glass and anything that looked at me side ways has been touched up. I've had the opamp in and out a few times to ensure the connections are good, I've put the board back in and gone in double chopsticks prodding every exposed part, so far, so good, I still have an output on the reverb tank.

I *think* the humming on the reverb has reduced a little, I've also taken the AC reading on the speaker terminals again;

reverb 0

vol down 50 mv
vol up 53 mv

reverb 10

vol down 67 mv
vol up 72 mv

[nickb]

I've resisted the temptation to take this out into the street and thrash it with a tree branch, but I'm holding the option open.

I've had the board out again this morning, I've been over every solder joint with a magnifying glass and anything that looked at me side ways has been touched up. I've had the opamp in and out a few times to ensure the connections are good, I've put the board back in and gone in double chopsticks prodding every exposed part, so far, so good, I still have an output on the reverb tank.

I *think* the humming on the reverb has reduced a little, I've also taken the AC reading on the speaker terminals again;

reverb 0

vol down 50 mv
vol up 53 mv

reverb 10

vol down 67 mv
vol up 72 mv

You do not have a hum issue. Those are well into the normal range. You do have a bad connection on one or more pins of the opamp though

[jondoe]

You do not have a hum issue. Those are well into the normal range. You do have a bad connection on one or more pins of the opamp though

Huh, well in that case I'll stop being concerned about the hum and try and go for improving reliability. I'll take a few trips out with it in the boot of the car, take up and down the stairs a few times and back to the shed to see if something fails. If it passes a week of rough handling I'll tick it off the completed list, I can only do so much.

Thanks again for your input nickb and others, I'm learning, slowly