1979 Princeton Reverb squeal

Hello, I have seen other posts around this site related to fender amp reverb "squeals" but not sure any of them match my issue. Please direct to appropriate forum if this question has been answered.

Here is my issue... while playing at moderate volume/settings my amp suddenly started to have a loud, oscillating squeal. I took it to the bench and here is what I learned.

1. No squeal when reverb knob is at zero, but it kicks in around 3 on the reverb pot, squeal stays the same volume and it is still present with the actual volume knob at zero.

2. I notice that the oscillating squeal fluctuates with the tremolo speed.

3. The squeal is present with or without the reverb pan plugged in.

4. I pulled the tubes one at a time and lose the squeal when I remove V3, so I replaced that tube with a known working tube, no change, so I replaced the other preamp tubes one at a time with known working tubes and still no change.

5. Visual inspection inside shows nothing obvious, and I measured everything possible (while attached to the board) and show nothing unusual as well.

6. With reverb at zero amp functions perfectly normal.

So... any thoughts where to look next? Thanks!

 

Thanks, good thought. Just checked them, and they are all tight.

 

Cables: I unplugged from the chassis to check, but no difference and unaffected in or out, switched around, etc...

V3: I will check the voltages and see if that shows anything goofy.

Filter Caps: I have a new set of caps for it (as they have never been changed in my ownership and they look stock) and that was my next invasive step. I thought it might be something with the caps, and will probably change them anyway, but with the reverb on zero the amp sounds perfect... quiet, no hums, crackles, drop outs, etc...

Mike_LA, you know I have had loosie goose pin sockets in the past, once had the creamy overdrive at 3 until I noticed a 6V6 taking a nap at the bottom of the amp.

Gonna look at all of these suggestions tonight... Thanks for everyones sharing of their knowledge and experience!

 

So I looked at all the wonderful suggestions you all have given me, everything checks out. So, frustrated I broke out the wooden stick and started tapping and pushing things, and when I push down on the board next to the tremolo caps I can make the problem go away. So, thanks to CBS quality control, can't seem to figure why they put this thing here and the thing there, but at least all of my patch wires are on the surface (nothing running under the board in that area) I am guessing I have a a bad soldier joint somewhere? Tomorrow I will give the eyelets a re-soldiering and give that a check. After that I guess I will need to pull the board to check everything underneath. As an amateur enthusiast his has been a challenging (but fun) trouble shoot.

Dan40 - So the OT date is period and the soldiering looks factory, so I don't think it has been changed to the best of my knowledge, however, now that you mention it I remember some time back I had noticed a "loose" wire and following the schematic I reattached it. I was recently looking back at my pics and notes, but realized that I mention the reattachment, but nothing on what/where it was. (As a military guy and I think I need to haze myself!) Anyway, that was about 7-8 yrs ago and it has worked fine ever since, but maybe it is contributing to the issues now? Thanks, I will definitely keep that in mind.

 

Update for those who are interested, a closer inspection I noticed one of the ground wires connecting to the brass buss plate near the reverb pot "appeared to be soldered, but was actually just sitting (loose) in an imprint of the original solder point. A quick re-soldering and presto. Thanks again to all the wonderful advice and responses. Jam On!