Peavey 212 Classic VT Series

[Enzo]

I think you misunderstand "thermal issues."   The whole amp warms up.  That means the metal chassis and mounting hardware warms up, metal expands some.  The solder on the connections warms up and metal expands.  The thermal issue at the pot is more likely the solder connection widens up a micron or two as the whole thing warms up, and poof:  your symptom.  It isn't about 1.6v somewhere heating anything.

[Roly]

you would think that 1.6v at U5A would not be enough to cause thermal issues at R46 pot.

No, I certainly wouldn't, I was thinking of indirect heating from some other section (e.g. a zener dropper for the preamp power supply).

But whatever, at some point you have to stop mulling over the circuit and discussing how many teeth a horse has, and go out to the stable and count them.  This fault isn't on the circuit and will only be found by close investigation of the actual assembly, and there is only one of us who can do that.

[tubeAMP]

true.  discussion redundant.  thermal.  solder.  got it  :duh

[tubeAMP]

for those interested in the ugly beast
http://www.sacmusic.de/amps/reload.html?pea_cl.html
now you know what were talkin about :dbtu:
what did I know about amps back in the day.  thought that it was longer ago than 79 but I must have just been coming out of the fog of the 70s.  a favorite gig piece borrowed frequently by a buddy locale venue musician.  the clean channel or BRIGHT in Peavey land is pretty good actually when the thing is working.  NORMAL whatever that is channel is kind of gritty by tube amp standards.  I am now learning solid state after favoring tube amps, old radios and such for so long.  hope you dont mind the dribble but I am having fun with new interests  :lmao:
still more to come  :duh

[tubeAMP]

this may be helpful for anyone trying to find Peavey parts
http://www.ampix.org/albums/userpics/10003/PV_Semi_Cross_Ref.pdf
this is pretty cool op amp illustration
http://www.williamson-labs.com/480_opam.htm
I may get a chance to bench the Peavey this weekend  :dbtu:

[ChewyNasalPrize]

For what it's worth, I had an "intermittent" failure going on with my Randall head that is the same age as your Peavey. It would drop volume and go fuzzy sounding. Sometimes come back to normal, sometimes not until after switch-off. Almost always failed after a bit of playing at above a certain level of volume.

Roly and DrGonz helped me diagnose and it turned out to be a bad solder connection on one of the filter caps. I cut fresh leads and resoldered and so far (fingers crossed) it hasn't failed again and sounds great.

Just took a lot of poking around and testing.

[tubeAMP]

still trying to figure out the foot switch with the OP amp and CMOS data.  looks like COMBINE has something missing in the foot switch diagram.  combine what.  connects pin-1 to F.S.-1 at Q2 then SELECTOR selects between pin-2 and pin-3 connecting F.S-2 or F.S.-3 at U3 and U4 CMOS.
all I can figure is that plugging into J4 AUTOMIX is always connected through J3 BRIGHT.  U4 is switched ON/OFF cutting BRIGHT amp U1A into U5A or U2 that mixes or not with NORMAL amp U1B.  U3 switches NORMAL ON/OFF.  the foot switch does this somehow while making hard connections between F.S. points.  something else U2 has pin-9.  there is no pin-9 in a CA3094 OP amp.  must should be pin-4 Vcc-

[Roly]

HOLD THE PHONE!  One problem at a time (per thread) you are going to get bamboozled and confuse us as well.  Stick with your intermittent Mid problem, then start a new thread about your footswitch (problem).  This also makes problems/repairs easier to locate for those who come along later.

[Enzo]

Agree^^^^^


Nothing is missing, I will be happy to explain how all that works, but let's not go there until what we are already working on is fixed.

[tubeAMP]

interesting.  another forum says do not start a new thread for the same device.  different strokes folks.  the only confusion is keeping the trends straight  :loco
otherwise Im pretty good with discernment.  probably bench the amp this weekend and practice with the pot connections as small and delicate as they are on PCBs.
but knowledge of operation flow wouldnt hurt.
I see that AUTOMIX plug lifts CT GND off of Q2 base turning it ON that turns Q1 ON.
must be sending + voltage to either FS-2 or FS-3 through FS-1 to operate the CMOS switches ?  this is what is confusing to me.  the switches, CMOS and foot :loco
pretty straight forward after signal hits U5A then its OP amp to hop amp til it gets to the power amp stage.  goes through two driver/phase inverter IQ1 and IQ2.  one is getting inverted signal from OT secondary.  then 6L6GC pp output :duh

PS:  at the same time AUTOMIX Q1 ON is sending bias voltage to U2 that beefs-up the signal from U1A while U4 is OFF...

hmmm...  more thinking.  maybe Q2 is sending CT GND to FS-2 or FS-3 through Foot Switch.  U2 does the combining while foot switch connects FS-1 GND to either FS-2 or FS-3 turning each OFF/ON

[Enzo]

I don't think we said start a different thread, we just want to work on one problem at a time, so we don;t get confused by references to various unrelated sections of he amp.   In other words we fix channel 1 before we start on channel 2.   Once the first issue is solved, I will be glad to explain in detail how the footswitch and automix work.

[tubeAMP]

just sitting at my desk killin tyme looking over this schematic waiting for the next request to go on.  lots of tyme to put to good use 8|

[g1]

 Have you tried cleaning and resoldering the mid pot yet?  If so, what was the result?

[tubeAMP]

I am going to get to it this weekend.  it is good to have feedback for best focus :tu:

[tubeAMP]

you guys must have done this before.  I dont want to say that its working so it doesnt quit when I go check it again.  but its been working for at least an hour now with all controls functional.  there I said it knock on wood :trouble
took it down to the PCB for a solder touch-up and pot cleaning.  probably didnt get enough cleaner in there because I didnt take all the connections off to be able to turn the board up-side-down.  they are working well enough turning smoother after lubing the shafts and some in the pot openings.  the solder job is the main attraction and is doing well.
so now Im a candidate for more knowledge 8|
this means that I get to learn the flow of operation through the AUTOMIX jack to at least the tone stack hopefully :dbtu:

muchas gracias