Peavey stereo chorus 212 chorus to loud when lead or crunch is selected

[kanks]

I've got this Peavey stereo chorus that ive been asked to look at. It came to me making a horrible 100hz hum that I quickly found to be broken solder joints on the filter capacitors. Then I moved onto another issue regarding the switching.

The issue with the switching was that it would not switch out of clean. at this point I had not connected the foot switch as it had a burn mark on the bottom. the switching issue was due to a bad Q1 that hold the switch contact low without a foot switch connected. I've now opened up the foot switch and the wiring was a mess and all burnt up so I assume that 15v as been sent to Q1 and that's what fried it.

Now I'm using a know good foot switch until I can get round to rewiring the old one but another problem as shown up that with the foot switch connected turns out to be the chorus but only with the lead or crunch selected. Without the foot switch the chorus cant be completely disabled so without it the lead and crunch are useless.

Whats happening is that with lead or crunch selected and chorus turned off they function correctly but with chorus on even with the lead and crunch turned completely down there is a rather loud distorted sound (while playing guitar) still from the speaker. this when you turn the lead or crunch channel up becomes ear shattering and sounds awful.

Now Im wondering if this could also be a issue caused by the faulty FS or if its something completely different. The fact the amp works correctly even with chorus in clean is confusing. Im not exactly up to speed as how the chorus is being mixed in to the channels and the schematic is a little confusing to me so if anyone as any pointers they are very much welcome as this as got me a little beat at the moment.

Thanks kris

[Enzo]

Is your known good footswitch specifically for this model?

Clean up the switching mess, that may solve most of your other issues. First, do not assume anything. Q1 has a pullup to the base to +15, so it is normal for it to face 15 volts. The collector sees 15v through the relay coil. 15v won't hurt it.

You say the FS is burnt up inside? We have to wonder what on earth would do that? It sees 15v for the LEDs coming in pin 5. otherwise it is just wires and switches and a few LEDs. I can't imagine the stock switching circuit in this amp burning those up. Any chance it was plugged into something other than the PV amp model?

In the absence of a FS, Q1 grounds the bottom end of the panel switch for channel change. So is it doing that? Is there a very low resistance from collector to ground while the amp is on? Have the switch open so the relay voltage doesn't confuse things. Does the panel switch open and close electrically? Does the panel switch click the relay off and on?

My concern here is the Q1 burnt up and so did your pedal, so some energy from somewhere came through there. It is possible damage was done on the main board. For example, the FS jack in the corner, feeds a bunch of long thin traces along the edge of the board up the side. Right after the little row of jumpers. One of those could have burnt open like a fuse. Whatever killed the FS could have run up the chorus switch line and burnt up some of the JFETs that do the actual switching, Q9,11,13.

[kanks]

The foot switch I'm using is for this exact model. As for Q1 I changed it and now it grounds out the channel switch so the switch now works without the pedal.

Looking at the burn on the FS now it looks like it could of been made by a cigarette but the cord as been changed by some hack but when compared with the other to be fare seems correct even if messy.

The only burnt trace I found was for the digital reverb board (looks like someone plugged it in wrong ) and that as now been fixed and the reverb is working correctly.

Im going to start checking the Jfets. Would some scope shots help.

[kanks]

Tested Q9,11,13 out of circuit and all test fine.

[Enzo]

Not sure I would have pulled them, but OK. So they all have low D-S resistance and the gate will turn each off? The thing is to test in circuit, are the gates getting turn off voltage when the FS is activated? And in fact I usually measure D-S resistance while the circuit is live, to see if it is being switched. A JFET can eb good and still not get switched if the gate circuit has failed.

The amp is stereo, each speaker has its own Power Amp. In dry mode, ie chorus off, the same signal goes to both. Turn on chorus and those JFETs route the signal. The one side remains dry signal, the other side is chorused. The chorus effect is to delay the signal some amount that varies cyclically. That signal in one speaker and the dry in the other allows them to work off each other. So one speaker is dry, the other speaker is effect only, no dry. If you look at the JFETs and control circuits, hopefully it will emerge for you how that comes together. Note the gates of some are controlled by the same control voltage, so more than one JFET work together. Note Q9,13 are opposite types P channel or N channel on the same control line, which can swing positive or negative. SO whichever way control swings, one or the other JFET turns on.

[glebert]

Probably not the issue, but are you testing with the two cabinet speakers? I recently was working on a Fender M-80 chorus and got this awful distortion when I turned on the chorus. Turned out the speaker cabinet I was using, which was sold to me as a stereo cabinet, was not! So the two channels would play together OK when there was no phase difference, but when chorus was on they were beating the heck out of each other.

[kanks]

New finding is that it is only coming out of power amp 2.

So to recap its producing signal on power amp two whit all the volumes at zero when the lead channel or crunch channel are selected and the chorus is on.

Also the chorus works fine when used with the clean channel.

The cabinet is the original combo and the wires are as they were. thanks for the suggestion.

[Enzo]

I have seen amps where the speaker wires got mixed up too. Each amp has a hot and a cold lead, but the cold lead is NOT ground. There is a sampling resistor in the ground lead for current feedback. So if someone took the hot lead from one and and the cold from the other, you'd get sound, but the amps wouldn't work right.

You have a power amp in jack for each power amp, plug the guitar into each to see if both power amps work. And make sure to test both speakers in case one speaker is bad.

Also try the simple test of a cord from preamp out to power amp in on the dead amp side.

RUn a cord from preamp out of each side to some other amp and speaker to see if the preamp out is making signal.

[g1]

So to recap its producing signal on power amp two whit all the volumes at zero when the lead channel or crunch channel are selected and the chorus is on.

R57 is a dual pot for the level control. Verify that there is nothing getting through either side when the volume is down.
If something is getting through one side of the pot, it is either dirty, defective, or not getting a proper ground connection (likely poor solder connections).
If nothing is coming through either side of the level pot, then there is signal leakage into the power amp through some other path.

[kanks]

just to finish this thread off as I forgot.

The amp was fixed apart from making poor ground connection on the input jacks to the chassis because I had not put the nuts on the jacks.

installed nuts problem disaapered