Peavey 212 Classic VT Series

[Enzo]

Sometimes I need to clean pots, and I see looking inside that the pots stick up from the board, so I would have to spray cleaner up into them from underneath somehow.  A simple trick is to flip the chassis over on the bench, so the open side is down.   Knobs facing away from me.  Now I lift up one end of the chassis, so I am now looking up into the chassis.  Now the pots are hanging down from the board, and the opening is on top.   

With the extension tube on the sprayer, I put the end of the tube against the exposed edge of the pot wafer - where the legs come out.  I gently spray, and the liquid then dribbles down into the opening.  Turn the pot back and forth, et voila, clean pots.    Forgive me, but it is not unlike pissing on a cement wall and seeing the fluid roll down towards the ground.

[Enzo]

OK< Classic VT footswitching.

Note the reverb and phase footswitches common to ground, as you might expect.  The channel switches common to R34 over by the automix jack on the drawing.

The channel switches do not enable the channel in question.  Rather they kill the other channel.  YOu want channel A?  Then kill channel B, or vice versa.  You want both?  Then kill neither.

The two input channels are more or less identical.  The output of each is switched on or off by U4, U5.  After those the paths converge to one path through the amp.  There is a footswitch control input to each of those ICs.   grounding either of those turns off the corresponding IC, so it kills the channel.    That would be simple enough.  You can plug into the individual channel ins, and play through just one channel.   But then you'd have to be careful not to have that channel turned off - killed.   Plugging into a channel input pretty much means you want it on, so there is no need for channel switching then.

Automix allows you to play through both channels at the same time.

Note the Bright input jack has its ground shunt through the automix jack.   Plugging into the automix ungrounds the bright in, which will be the input channel used.  More on that in a moment.  The automix jack also keeps the base of Q2 grounded.  That turns it off, so the collector and thus R34 rise to +15v.  Since that is the common for the kill switches, they too will have the positive voltage on them, so the channels remain live.   But when you plug into the automix, then the ring contact shunt opens and the base of Q2 is pulled up by R31, turning the transistor on.  That drags the collector down to ground, or at least close enough to it.  That means R34 and the kill switch common are now grounded.  NOW the kill switches work.   You only want kill switches working when both channels are available, ie when using automix.   And that combiner?  It simply breaks the ground connection so neither channel can kill.

Back to the signal path.   Automix only feeds the bright channel, but we need to get that signal over to the other channel too.   The signal from the bright input is also routed through U2, to cross feed to the normal channel input.   We don;t want that crossfeed all the time, so The Q2 controls the footswitch, but it also controls Q1, which turns U2 on and off.

That was a conceptual overview.  If I glossed over anything too thinly, let me know.

[tubeAMP]

Thanks Enzo.  I think that I was getting it more or less in my second analysis.  close anyway.  looks like the ICs are held-up with voltage until the FS kills the power source with Q2 ground.  not used to these logic circuits.  hurts my brain.  I did see the simple ground (CT not chassis) circuit at FS-G for REVERB and PHASE.  sounds better when someone else says it as well.  keeps me focused.  confidence mo bedda :dbtu:
it is pretty cool design using transistors and IC switching :duh
thanks for taking the time with the walk through
happy trails to you

[tubeAMP]

thought that I would check it before chassis going back into cab.  failed :grr
good thing that I can recreate the trouble by moving TREBLE and MID pots.  PCB flexes too but is isolated to this area.  checking continuity between resistors and capacitors looking for opens.  nada.  thought maybe a voltage change might indicate intermittent... poking C28 while amp is ON is creating the trouble.  reheat solder joints.  no improvement.  found a 600v .03uF in my stash for a replacement.  big and fat but fits.  Ragu, its in there.  is working good senior.  maybe problem solved.  C28 must have been going open.  everything mo bedda :dbtu:
tomorrow a new day will make some more checks before reassembly.  while testing the amp without C28 in the circuit the trouble is recreated exactly.  so I am pretty sure that was it but wont hurt to check it-out once more

[Roly]

while testing the amp without C28 in the circuit the trouble is recreated exactly.

This is the sort of check back we like to do wherever possible.  If you can re-create the fault on demand you can be pretty sure you've found it.   :dbtu:

[tubeAMP]

all the parts that I have are high voltage for tube radios and tube amplifiers.  these low voltage solid state pieces are tiny and delicate.  looks like it was an electronic component failure after all.  capacitor replacement is noticeably larger but functional. 
PCB is kinda flimsy.  makes crinkling noises when flexed.  I wanted to re-seat the connectors but not coming off easily.  crinkle-crinkle
thanks for the input guys.  looks like we are on our way

[tubeAMP]

the old beast was looking kinda dusty.  cleaned it up with some cleaner, brushes and a sponge.  vacuumed it out.  brushed-in a coating of ArmorAll into the Tolex then polished with a towel.  looking like a brand new 70s combo now.  sounding great.  in a godda da vida baby.  actually more like Howlin Wolf and Elmore James tunes

[Roly]

PCB is kinda flimsy.  makes crinkling noises when flexed.

We try to avoid doing that because it can lead to hair line cracks in the PCB traces.   

[tubeAMP]

yes we do :loco
the thickness of the board to its length ratio is flimsy.  pots mounting is delicate.  connectors are tight fitting.  there is no way not to stress the board when touching it.  this is what was partially responsible for finding the trouble area.  the board flex when moving it.  closer inspection revealed the trouble.  in this case a failed capacitor :duh
checking the amp this morning it cranks.  looking good sounding good.  I am much more familiar with it now :dbtu:

[tubeAMP]

this is what I am working on lately.  attached is 8- transistor radio.  had an open IF transformer T4.  repaired it with a strand from 24AWG stranded CAT6 patch cable.  screaming loud now :dbtu:

[JHow]

Transistor radios are addictive.  Stop now before you end up with a box full.  :tu:

[tubeAMP]

it is getting to where I am working on more solid state than tube type apparatus.  its all good :loco
BTW I do have a variety around the house.  some people decorate with vases and lamps.  I decorate with radios and amplifiers from the early 30s to mid 60s :duh