Peavey Triple XXX Effects Loop issue

[bnwitt]

Well this amp is back again doing the channel changing thing like before. Evidently it wasn't the switch like I thought. Now I have to figure out why it jumps from crunch to Lead all by itself. Evidently the K2A relay is energizing on its own. Could this be Q11 shorting?

When this happens, the amp volume oscillates and the Lead channel LED comes on.

[Enzo]

Is it toggling between lead and crunch? Or is the lead by itself coming on and off? Q11 won't affect Q7. If Q11 shorts, then it turns on K2 and CR22. But that won't change what Q7 and its friends are doing.

Look, pretend it is a 12AX7 instead of a transistor. The relay is the plate load. Now the relay is going off and on on its own. What would you do? Youd check the grid. If the grid is turning off and on, then the tube is just doing its job. If the grid stays where it should be then the tube is shorting. OK, but it is a transistor. SO is the base togling hot and cold, thus controlling the relay as it should, or is the base steady, leaving the transistor as the only suspect?

The only thing that controls Q11 is Q7 - when one is on the other is off. Whenever Q7 is on, it turns Q11 off. LDSW - "leadswitch" to me - is just a path to ground. If that were open, NEITHER Q would turn anything on. SInce they work, it isn;t at fault. LDSW won;t affect just one.

SO I am now at the point - correct me if I am wrong - where the whole Q7/11 thing is being turned off and on when not desired. What is the base of Q7 doing? Left alone R11, R75 would pull up the base and turn it on. To switch channels, we turn it off. How? Pull down the base. PIns 4,5 of S1 do that when together. We are going to ground off R75 by way of R72, S1, and Q8. In the absence of a foot switch, Q8 is held on by R78, R83. They keep Q8 on, so it completes the path to ground for S1.

You replaced S1? The line from S1 has to cross over to the FS jack board via ribbon. SO does the +17v for R78. If any of those connections fail, it will do funny things.


But basically the task is this. For K2 to operate, Q11 must control it. FOr Q11 to operate Q7 must control it. FOr Q7 to operate, Q8/S1 have to control it. Unless Q11 all by itself is intermittently shorting out - unlikely, it shorts or it doesn;t mostly - track back through all that and see where a voltage is flipping around. I'd be betting on the FS board and its ribbon.

[bnwitt]

What is happening is when he is on crunch channel and playing hard the lead channel light comes on steady and the amp produces lots of distortion with the output oscillating in volume like would be heard if the relay was chattering between the two channels. I will ponder your explanation. Unfortunately it is hard to get the amp to do it and when it does it is really quick, so its difficult to take readings. I'll check the footswitch cable. Remember I'm having problems with the effects in/out too so the problem may be all related to the effects/footswitch board or cabling.

[bnwitt]

Finally Fixed

Ok, so I finally fixed the problem with this amp changing channels on its own. It turns out that the S1 switch has its pin 3 ground connection through the switch body to the chassis face. Without the switch nut being really tight and without a grounding washer in place, that ground is intermittent as the inside of the chassis has light paint overspray near the switch hole. By sanding the chassis contact point and really tightening down the switch with a star washer, the problem went away.

The odd thing about it though is I could get the amp to toggle between crunch and clean by wiggling the switch before the fix but I never got it to do what it did with the owner which was have the Lead relay come on simultaneously with the crunch relay and produce the chattering relay sound. Anyway the owner played it for almost an hour and couldn't get it to do it again.

[KevinOConnor]

Hi Guys

The loop is a series type only driven by a discrete buffer circuit. There are closed-contacts on the return jack so if you simply plug a cable into the return with the loop engaged, then the amp will be silent. If you patch from send to return there will be sound. If there is nothing plugged into the send and return jacks and both the level controls are up, then there should be signal.

The signal level should not be altered by the loop controls if relay K5 is not energised. It is normal schematic convention to show the non-energised relay state.

Those diodes tied to the signal path and to the supply rails are there to protect the input of the buffer. You see similar diodes wired at opamp inputs. If a signal is extremely high, it will be clamped to 700mV above the positive rail and 700mV below the negative rail. Normally these diodes never turn on and present no problem to the sound.


It is often the case when jacks are not used, or are used only in one mode for a long time, or for switches under similar conditions, that the contacts become oxidised and intermittent. In the case of jacks, the fouling can act like a diode, where a strong signal can break through but a small signal cannot. Cleaning may help but in most cases the jack has to be replaced.

Just noticed how old this thread is ! How slow am I?

Have fun

[Enzo]

perhaps it takes longer for some threads to get all the way to Thunder Bay.

[KevinOConnor]

Hi Guys

You might be right... we are isolated up here... really an oasis though... sssh... don't tell anyone how nice it is.

In my defense, the thread was listed in the "similar threads" window below one I had replied to. I implore you for mercy this one time and to belay the order for whipping, Thy Liege.

Have fun

[bnwitt]

oh my lord! I'm in a thread with both the great Enzo and KevinOConnor! I'm not worthy, I'm not worthy. Kevin, I have all your books. No wait not all because after I buy the latest one you keep writing more.

[KevinOConnor]

Hi Guys

bnwitt: You are interpreting your own greatness reflected off of us back to you as ours. Only the truly great are that generous.

Have fun