Here are the U11 pin voltages excluding pins 1 and 7.  Hopefully this is what you need?

Clean on: 2: -16.46
              3: -.346
              4: -16.48
              5:  +.352
              6:  +1.656
              8:  +16.06

Overdrive on:  2:  -1.659
                    3:  -.344
                    4:  -16.63
                    5:  +.656
                    6:  +.085
                    8:  +16.35

Ok.  I have re-started my efforts to find the issues with this amp.  Symptoms are the amp will not switch to the overdrive channel at times after switching to the clean channel.  I started the testing all over again, carefully.  I'm getting +7.74 volts at U12 pin 16, -8.03 volts at pin 7 and 8, no changes for any of the three pins when the overdrive switch is pushed on or off.   Pins 9 and 10 switch from +7.77 volts to -7.59 volts when overdrive switch is pushed.  Pin 4 to pin 3 is reading 63.0 kohms, pin 4 to pin 5 is 61.5 kohms, neither changes when overdrive switch is pushed.  Pin 15 to pin 1 is 76.9 kohms, pin 15 to pin 2 is 78.5 kohms, neither changes when overdrive switch is pushed.  U12 is a new chip (4053) if you remember. 

I know the amp failed to switch to the overdrive channel again last night when I was playing it, but with the board out of the amp, and no speakers connected, I am not wise enough to figure out how to tell if the amp is actually misbehaving or not when I test.  I had the board plugged in and turned on for 20 plus minutes when I took the above numbers today, and I retested each several times to confirm they were accurate.  This is more than the time it typically takes for the amp to fail to switch.  Last night it started to malfunction in under 5 minutes.

I also tested TP16 at U11, (pin 7) and for one test I got -15.18 volts when overdrive is off, +15.56 volts when overdrive is switched on.  Upon a later re-test, the voltage stayed at -15.18 when the overdrive switch was repeatedly switched on and off.   After I got the unchanging TP16 test, I powered the amp off and back on, and retested TP16.   The voltage repeated the first test. that is, it changed when the overdrive switch was pushed on or off. 

TP17 at U11 (pin1)  read +15.50 volts when Chorus is off, -15.21 when chorus is on, and tested the same at either time of the different TP16 tests. 

Does any of this help at all?  Is U11 bad?

I would really like to hang onto to this amp.  Any ideas based on the above numbers?

Thank you all.

I decided to start simple this morning.  I pulled the board again and went through all the solder joints on the board with a magnifier.  I found 4 or 5 suspect solder joints, including one pin on U11 and one pin on the overdrive switch on the amp, and re-soldered them with new solder after removing the old stuff.    I also cleaned off a bunch of old brown crud from around a bunch of the solder joints, probably old flux?  You can eat off that board now....

Put the amp back together, and after several hours of playing today, I cannot get the amp to fail to switch between clean and overdrive as it is supposed to, including using both the footswitch and the amp switch.  The problem is gone.  I guess the old car stereo troubleshooting trick worked on this amp (check and resolder, check and resolder....).   

I think it's fixed.  If the problem does come back, I'll re-post.  Again, thank you all very much for your help. 

Unfortunately, after about 20 minutes of playing, the condition has returned, just as I described it in the original post.  The overdrive will not turn on even though the switch is activated and the LED on the amp is lit.  I must have misinterpreted something in the meter readings or the problem is elsewhere.  Anything else I should try?

I did measure ohms between the pins for B and C when the overdrive switch was turned off and on.  One meter probe on each pin.  For instance I put red on pin 4 (or15) and got no changes when black was placed on 3 or 5 (or 1 or 2)  and the overdrive switch was turned on or off with the footswitch or  the switch on the amp.  I hope that was what you instructed me to do??   I'll put some time on the amp tonight.  But the few minutes I managed last evening showed all good.  I'll know more tonight.

One other thing I noticed was less noise on startup and shut down of the amp.  Is that in my head or could this chip effect that as well?

Got the meter out and Voltage at pin 16, 7 and 8 was steady and present, SELA voltage changed as the switch was turned on and off, but no switching volt or ohm changes when I put the meter on the switching pins of U12B and C.  My local electronics store had MC14053BCP chips for .49 apiece.  Cheapest thing I have bought in a long time.  Swapped the chip,and the amp is behaving as it should for a short session this evening.  I'll get some more time on it later this week, but it does appear the amp is fixed.  Enzo and G1, I am eternally grateful for your assistance.  If I can help either of you with any Quadrajet problems or V-Twin issues, please let me know!  I'm better with a wrench than I am with a soldering iron or a multimeter....  Many Thanks.

I see that now on the schematic.  If any of those tests fail, replace the chip, or do I try something else?  Thank you again for all this insight.  I really appreciate it. 

Thank you for the responses!   I can see two 4053's on the schematic, U12B and U12C, but one only U12 on the diagram, which I think means U12 is the 16 pin chip and the U12B and U12C are the two half's of U12?  If so, the chip is a Motorola MC14053BCP, Date Code? FFMY9009.  Is this the same as the TI chip?  Now is where I get lost.  Any advice on which pins would give me the info I need?  I'm attaching photos of the front and back of the board in case that's helpful.  If you need more info I'll provide all I can. 

What is the basic process for testing?  Power up and measure what pin or pins?  voltage +/-when overdrive switch is pushed on or off?  Something else?  Sorry I'm not more competent on this stuff....

For my first post here, I have a stumper.  I just picked up a Princeton Chorus red knob amp.  Great cosmetic condition, the clean channel is great and the overdrive channel works, for a while.  After having the amp on for a few minutes and using input 1, the overdrive channel will just stop working.  You can push the amp switch or the footswitch, and the overdrive led lights up on the amp and footswitch, but the clean channel is actually working, the clean volume is working, and no overdrive.  If you turn the amp power off and back on, the overdrive is immediately back.  It does this with or without the footswitch connected.   I've cleaned pots and cleaned jacks.  I'm not a tech.... but I do own a soldering iron!  I'm just not sure where to start on this one.  I got the amp at guitar center, so my 40 days to return is running.  I love the amp for what it is, and I want to keep it. 

Anyone ever seen this before?  Any ideas?  Thank you all.