'65 Princeton Reverb Reissue Randomly Detunes Notes

[Stevem]

It's not detuning notes, it's added another pitch tone to what your playing and modulating that into a different tone.

[alnight]

Yeah, these intermodulation distortion gremlins can be quite frustrating. Every time I've had one it has been a capacitance issue, but not always a bad cap. I've had bad ground connections, balance resistors, solder joints, etc.

Just a stray thought. Did you check that bias cap?

[sssmoka]

... The amp was manufactured in October of 2024, according to the hand-written date on the sticker that's on the PCB. An amp so young shouldn't be having problems. ...

That amp should be returned to the retailer for warranty replacement. It's literally the sole reason to pay retail on a new amp.

I agree. However, not my amp. Not my call.

Let's say my car developed an issue, and I took it to a mechanic. They fix the problem and give me the bill, which I pay.

If I later discover my repair should have been free because the vehicle is still under warranty but the mechanic didn't inform me, I'd be pissed. If I found the mechanic was aware my vehicle was under warranty, and they just didn't advise me to take it to a warranty repair center, I'd be doubly pissed. I would probably think they just wanted my money, and played me for a sucker. I would probably tell everyone I knew about my bad experience with that mechanic shop, and how I was charged for something that should have been free.

If you're repairing this amp free-of-charge as a learning experience, good on ya. But since you're hanging your shingle out there as amp repair, sometimes "good service" is about guiding the customer to their best outcome, even if that means directing them to take their amp elsewhere. Just a thought...

Honestly, I can appreciate where you're coming from, B Ingram. Did you know...

• None of our local music stores perform tube amp service - they have no tech

• The man that was servicing tube amps locally suddenly shutdown shop - he retired due to medical issues

• The only option we have in my region is to ship an amp off for service - costly and timely

• I do not need to lie - any client I've gotten, or positive review left is because of customer service

Thank you for bringing these pertinent issues to everybody's attention. I want folks to know how I run my business. That way, they know what to expect if they choose to patch the hole in their roof with my shingle. I hope you have a fantastic day, sir.

[Lauri]

Have you tried poking it with a stick?

[sssmoka]

Yeah, these intermodulation distortion gremlins can be quite frustrating. Every time I've had one it has been a capacitance issue, but not always a bad cap. I've had bad ground connections, balance resistors, solder joints, etc.

Just a stray thought. Did you check that bias cap?

I did check the bias cap for proper value, sir - it was at 100uf. I'm going back in to (CAREFULLY) check the live circuit from input to output with the meter and the scope to see if I can isolate where this intermodulation distortion is originating from. Then, I'm flipping the board to check for bad solder joints. I do sincerely appreciate your council, alnight.

[sssmoka]

Have you tried poking it with a stick?

A chopstick? I've tapped the tubes. Haven't poked at the board components, yet.

[sssmoka]

It's not detuning notes, it's added another pitch tone to what your playing and modulating that into a different tone.

Hmmm... adding another pitch and modulating into a different tone? Not detuning. That changes the manner in which I think about this amp's issue and how I approach it, i.e. where it may be occurring.

Much obliged, sir.

[Lauri]

A chopstick? I've tapped the tubes. Haven't poked at the board components, yet.

Yes, try poking the pcb and components to see if poking somewhere makes it better or worse.

[xtian]

A chopstick? I've tapped the tubes. Haven't poked at the board components, yet.

Yes, try poking the pcb and components to see if poking somewhere makes it better or worse.

This is excellent advice. You can also use both hands to torque and twist the chassis while in operation--sometimes this will reveal broken solder connections.

[sssmoka]

So, whilst I was probing and scoping gain stages, I noticed that my scope was displaying odd readings (intermodulated frequency included) at V1A pin 7 during my signal generator input - before I even cut the amplifier on. What was in line between input jack 1 and V1A pin 7? Nothing...except a 1 M Ohm 1/4 W resistor according to the schematic - which I replaced. I also had to replace two 68K resistors in a voltage divider off of input jack two. Eureka - they were all bad. How they got that way is beyond me, but they're not that way anymore.

Thank you ALL for your advice, assistance, suggestions and help. I appreciate everybody.

[xtian]

Wait, whut? How did a bad voltage divider cause IM distortion?

[sssmoka]

Wait, whut? How did a bad voltage divider cause IM distortion?

Ultimately, I had to replace the entire input jack PCB. Original jacks still introduced IM distortion even after swapping out both 68K resistors and the single 1M resistor on the input PCB. As a resolve, I took two stereo jacks and wired them accordingly to the schematic, hooked them to main PCB using the input jack PCB hookup, and the frequency intermodulation has stopped. The amp's been on for nearly an hour, and I've run test tones and my Strat through this PRRI and it is definitely repaired.

Not sure what to make of a set of input jacks on a PCB causing frequency intermodulation distortion... but I am damn sure open to an explanation.

Strange things have happened... this is one of the stranger ones.

Thank you, again, xtian.

[sluckey]

As a resolve, I took two stereo jacks and wired them accordingly to the schematic...

A stereo jack is functionally different from the jacks on the schematic. They are not interchangeable just because they each have three lugs.

[sssmoka]

As a resolve, I took two stereo jacks and wired them accordingly to the schematic...

A stereo jack is functionally different from the jacks on the schematic. They are not interchangeable just because they each have three lugs.

Thank you for pointing that out, sluckey. Perhaps, for clarity's sake, I should elaborate. I took two stereo jacks and wired a 1M resistor in with input jack one and two 68K resistors in with input jack two so that they are functioning as they should - not necessarily identical to how the schematic shows the jacks connected in the circuit (as you are correct - the jacks are different) ... my improvised solution suited the owner and solved the problem - which was my intent. I appreciate your shedding light on my erroneous description of the implemented resolve.

[sluckey]

I took two stereo jacks and wired a 1M resistor in with input jack one and two 68K resistors in with input jack two so that they are functioning as they should

The stereo jack has no switch so you lose the Hi-Lo input selections that are provided by the jacks on the schematic. I don't doubt that both jacks pass signal but they ain't functioning as Fender intended. In addition, the jacks are no longer shorted to ground when no plugs are inserted. IMO, that's an issue.