Princeton 112 plus- royal pain in the highness

[Reno Rico]

Hello everyone, new to the form and hoping that one of you guys can help me with a problem that's had me wandering the net uselessly for far too long.
     I bought a fender Princeton 112 plus from a moving sale for certain low key gigs. It worked for a really quick test before I bought it and for about half of the first gig when it started a faint ticking that grew to be fairly loud over the next 20-30 min.
     It remained after letting the amp cool down and powering back on and after I got it home. Whether or not something was plugged into it, or if the volume was turned up, or what channel, or reverb, or outlet, or location, a persistent tic-tic-tic as steady as up tempo 1/8 notes and as loud as A bad ground hum.
     I took it apart to see what I could find and noticed a very light scorching on the board so I pulled and tested the big ceramic resistor next to it to find it below spec. I ordered and installed a replacement and reassembled with my fingers crossed after not finding anything else obvious. My electronics skills are somewhere between beginner/intermediate. It sounded great just long enough to give myself a pat on the back before it started creeping back.
     I pulled it back apart and tested the new resistor which was OK. I tested the old Resistor again and this time it was up to spec. I'm competent with a multimeter but guess I must have made a mistake the first time except Why was it good for 10 minutes where before it started right on power up? I made no more progress after that and put it in a corner while I stocked back up on patients for a couple months. 
     I started messing with it again the other day and while it's still too sick to go out and play, the symptoms are different since my repair attempt so it seems I'm on the right track. Now when I power it on there's a fairly loud hum, regardless of channel or volume or anything plugged in. I can play through it with no additional noise if I keep the volume low and more importantly avoid low frequencies. As soon as I add a little bit of bass to the tone I get A noise similar to the tick but more of a modulation. It's almost like a tremlo effect but not on the guitar signal just on the noise that appears. By the way the amp has no tremlo or chorus effect, it's a basic 2 channels with reverb. There's also 2 different speeds that it taunts me at depending on which notes I'm playing. Only base heavy notes cause it like anything on the low E  and A strings . Some notes make the noise like quarter noted and others make eighth notes but playing higher up the neck on the lighter strings(and thin eq settings),I don't get any of that noise although random light static clicks and pops.
      Everything seems mechanically sound and I feel like I've eliminated the possibility of something outside of the amp affecting it like power issues or other devices. I almost always just plug in and play without anything in between the guitar and the amp except a cable, of which I have tried several, all of them good. I have a basic but limited understanding of the circuitry but not enough to know what to look for if I started probing around. I don't know what a bad transistor sounds like but stabbing in the dark, I'm thinking this may be the problem? Maybe a bad ic?
     The wise thing would be to count my losses and  turn it into spare parts because I've already wasted way more time than it's worth but I'm emotionally invested now and I'm not gonna give up until I've either fixed it, dismantled and tested every last component or given in to my urge to use it and my three other broken amps for target practice. If it helps, I'll make some time tonight to open it back up and get a picture of the board where the scorch mark is.
     If you've made it to the end of this post, I thank you for your patience and welcome any input.

[g1]

Do you have any electronics on your person?  I don't know many people who do not.  Turn them off and see if it has affected anything related to the fault(s).  It may seem like a long shot but it's something that must be eliminated as a possible cause.

[Reno Rico]

Nothing. A funny thought did just pass through my mind though. Maybe an idea for a short story or something about someone who's trouble shooting reveals
the interference is coming from them because they're really an android. It's no wonder I can't figure it out with crap like that constantly going through my head and hogging up my cpu.

[Jazz P Bass]

Have you tried attempting to isolate the issue.
Do Any controls affect the ticking?
Try using the FX Send to another amplifier.
That is the preamp circuit.
Also, you can plug a guitar into the FX Return which is the power amplifier circuit.

[Reno Rico]

Wow, it never occurred to me to plug into the effects return and bypass the pre amp. That could come in handy in some other situations. Do I understand correctly that one could use just the preamp or the power amp of an amplifier by going in or out through the effects loop? This is something I should have known long ago.
     Yes some controls do affect the tick, any tone adjustments that increases the lower end. Turning up the bass or mid or turning down the treble makes it worse, similar to the way that playing lower frequency notes does.
     First chance I get I will see if I can narrow it down to the preamp or power amp.
     Thanks for the help, that little tid- bit was a valuable revelation, although I am a little embarrassed that after playing over 20 years I hadn't put it together.

[Reno Rico]

Plugging the guitar directly into the effect return does not help the problem. I believe that means the problem is in the power amp section?

[g1]

Normally I would say yes, it is in the power amp section.  But you also said the tone controls affected the sound of the ticking.  If the tick was just in the power amp the tone controls should not affect it.

The item that is common to both the preamp and power amp is the power supply.  But it would still be good to run the preamp out (FX send) to a different amplifier and verify if the tick is there or not.