Phaser woes on Peavey Classic VTX

[woodmedic]

I just purchased this amplifier. Got it home and everything including the footswitch seems to be working correctly with the exception of the phaser circuit. At first the rate knob would make the sound change like it was in the pulled out position even though it was not. Now the rate know does nothing pulled out or pushed in. The depth knob only makes the amp louder as it is turned clockwise. I checked U5 pin 14 and I have no signal there. In fact, I have no signal on any pin on that side of U5 (8-14). I do not know where to look from that point. Any on where to look next? I am having trouble following the schematic for some reason. I can't figure out where the signal enters the phaser circuit and where it leaves it. Thanks for any help!

I posted this on another forum but they have to approve my posts before they will appear so thought I would try over here. Enzo had started helping me over there but I can't post a reply on the other site so maybe this one will work better.

[dmeek]

U5 pin 8-14 side is part of the modulation LFO. there should be a slowly changing DC voltage on pins 8 and 14. I would guess if it's not oscillating the problem would be the pull switch, Range pot or C62.
Sounds like the audio path is OK. Input to the phaser starts at U4 pin 8, output is at the Depth pot

[woodmedic]

On U5 pin 14 I have a steady 14.3 volts dc. Pins 8 is -.02. With the rate switch pulled out I have roughly 14 volts on 14 and -14 on pin 8. Not sure how to check a cap with a DVM but can tell you that I have -14 volts on one side of it and .16 on the other. Seems like that could be it?

[Enzo]

Try a new IC.

[woodmedic]

I have some new tl074 ics on the way. I have swapped the two of them with no change but I’ll try installing two new ones when they arrive.

[woodmedic]

I just noticed that C62 is supposed to be a NP cap and it looks like someone put a regular polarized cap there. Could that cause this problem?

[dmeek]

It should still oscillate with a polarized cap when the pull switch is in. You could lift one end of R87 to eliminate the switch as a suspect. Can you sweep the phasing effect with the pull switch out?

[woodmedic]

I will try taking R87 out of circuit and see what happens. The range knob does nothing to the sound whether the pull switch is in or out. No sweep effect is taking place at all.

[dmeek]

Let's get it working in manual mode first. With the pull switch out you should have +14V on U7 pin 7 and -14V on U5 pin 8

At the junction of R102 and R103 you should have a voltage from 0V to about -13V as you turn the range pot.

[woodmedic]

With pull out I have 13.5 on u7 pin7 and -14.1 on U5 pin 8. At the R102 R103 junction I do indeed have 0–12.8 volts.

[dmeek]

Good so far. Now check U5 pin 14. Range pot should cause 0v to +4V. If you find +14V there one of your opto/ldrs may be bad or not connected.

[woodmedic]

That is the case. I have 14v on U5 p14 regardless of the position of the range pot. The LDR’s are the 40101’s I’m guessing?

[dmeek]

Yes, unfortunately. If you're lucky it's a bad connection. More likely an LED is open. I don't know what replacement would work, you would have to replace both so they match.
Possibly a Vactrol VTL5C2/2 if they are still available.
I wonder if you could carefully extract the old LEDs and glue in new ones.

[woodmedic]

I’ll research those and see what I can find. Thanks a bunch for the help!

[g1]

40101 = 21L565 = (approx.) VTL5C3/2 which are currently reproduced by xvive. One of the places that sells them is small bear:
http://smallbear-electronics.mybigco...vive-vtl5c3-2/