'65 Princeton Reverb Reissue Randomly Detunes Notes

[sssmoka]

Good evening, everybody!

I do hope this memo finds you at the conclusion of a pleasant day, or at the start of an enticing night, and in good spirits.

I've a situation with an amplifier that has me stuck, stumped and stupefied - hence, I'm here to ask the brain trust for advice.

A newer '65 PRRI came in for service due to what the owner could best describe as a warble with every note played. 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. Nonetheless, using a frequency generator, I fed the amp a 440Hz signal and gave it a listen. I heard what I can best describe as an intermittent, off-pitch secondary note.

Intensity, Speed and Reverb were all at 1 on their respective control knobs during these tests.

The amplifier's owner prefers NOS tubes; as such, that's what's in there. Testing my consumables first, I (carefully) pulled and swapped every tube in this amplifier (from pre-amp to reverb driver and recovery to phase-inverter to power-amps and rectifier) with known working tubes. There was no change in the apparent symptom of this out-of-tune audio signal.

I put the amp on an oscilloscope and found that with every detune - there was a consistent, measurable drop of approximately ~2.7Hz. The 440Hz signal at input is coming out as a ~437.3 Hertz signal at random intervals: both before and after the output transformer. When doubled to 880Hz at input, same ~2.7Hz drop at output. Same with a 220Hz input.

I'm puzzled as to where in the circuit this frequency fluctuation is originating. I thought it was coming from a capacitor at the end of its' life beginning to short and allowing some DC into the AC signal. All the caps appear to be in good condition.

I even went as far as to short the footswitch jack to deactivate the vibrato and reverb - the detune remained.

I'm simply lost for answers at this point in time and could certainly use some direction, guidance and advice. Thank you for your time and assistance. They are both incredibly valuable and greatly appreciated.

- Chris Kennedy
- Soda City Amp Repair

[xtian]

Failing filter caps, and easy to diagnose. Simply clip in an additional 47u/500v cap at the first (reservoir) filter, and see if this improves things.

[B Ingram]

... 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.

[sssmoka]

Failing filter caps, and easy to diagnose. Simply clip in an additional 47u/500v cap at the first (reservoir) filter, and see if this improves things.

Thank you, xtian, for your response!

A/C filter caps... that was my first thought. There are a total of four 22uf/500V caps in the A/C filter cap bank. None leaking electrolyte. All things appeared intact. Using my multi-meter, I took a capacitance measurement on each filter cap while in circuit. The 1st, 3rd and 4th caps read fine. However, the second cap's value read high and kept jumping from 31 to 37 - I thought this was an odd reading for 22uf Illinois Capacitor brand cap (being so high out of tolerance - seen a 22uf F&T read 30uf but never a 22uf I.C. cap read higher than ~26...maybe 28. That aside, the inconsistent and bouncing value of capacitance being read made me think I'd found the problem.

Wanting to alleviate the issue once and for all, in my excitement and haste, I recapped the amp. Four new 22uf/500V F&T caps installed. Yet, the random, dissonance remains. So, I fixed a problem with those I.C. caps that was sure to happen - it just hadn't, yet. What I didn't do was rid the amplifier of the symptom of randomly producing detuned output.

Measuring the F&T caps, they're reading the same as the I.C. caps - the second is bouncing between high values of capacitance. The rest are reading steady.

What is causing this bouncing capacitance and is it indicative of the source of the frequency downshift irregularity?

I'm stumped, said the tree to the lumberjack.

[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.

[xtian]

Just FYI, ESR (capacitance) meters often fail to flag caps that behave badly under high voltage.

Second thing I would expect is lead dress (coupling or oscillation)....but not very likely in a Fender production amp!

Definitely try playing this amp through another, external speaker, to rule out the speaker itself.

[alnight]

You have verified the capacitor ground point, yes?

[sssmoka]

Just FYI, ESR (capacitance) meters often fail to flag caps that behave badly under high voltage.

Second thing I would expect is lead dress (coupling or oscillation)....but not very likely in a Fender production amp!

Definitely try playing this amp through another, external speaker, to rule out the speaker itself.

xtian: I genuinely appreciate all of your help. Thank you.

Dually noted - can't trust a cap meter on high voltage caps. For my understanding and curiosity, I'm guessing this is because the meter can't manifest the high voltages under which the cap is failing?

Following suit, I hooked the PRRI up to my Deluxe 112 cabinet and the amp itself is still producing the erroneous frequency shift - even through an external speaker by itself.

As for lead dressing, here are the before cap job and after cap job pics of the PCB. Things look pretty tight in there... but there may be something obvious I'm simply overlooking.

Before:




After:




Currently dazed and confused...

[xtian]

Right about cap testing. Most meters do not use high voltage to test, and some caps fail only under the stress of operating conditions.

You can improve the lead dress in this amp. For each of the preamp sockets, the wires running to pins 1,2,3 can be grouped together, and 6,7,8 also, but try to keep these two groups away from each other.

The grey wire running across the PCB is your NFB wire, I think, but what is the white wire it's wrapped around?

And it looks like the OT primaries (red/brown/blue) are zip tied to the heater wires!! Def separate those from each other. Also try to improve the path of the green OT secondary wire--looks like it's brushing the PI socket and PI pin 3 (yellow).

[sssmoka]

Right about cap testing. Most meters do not use high voltage to test, and some caps fail only under the stress of operating conditions.

You can improve the lead dress in this amp. For each of the preamp sockets, the wires running to pins 1,2,3 can be grouped together, and 6,7,8 also, but try to keep these two groups away from each other.

The grey wire running across the PCB is your NFB wire, I think, but what is the white wire it's wrapped around?

And it looks like the OT primaries (red/brown/blue) are zip tied to the heater wires!! Def separate those from each other. Also try to improve the path of the green OT secondary wire--looks like it's brushing the PI socket and PI pin 3 (yellow).

I attempted to improve the lead dress by following your suggestions, xtian. The grey and white wires running across the PCB are for the footswitch jack. I made a go at untwisting them. I also seperated the OT primaries from the heater wires and moved the OT secondary wire away from pin 3 of the PI socket.

Still, the amp has that infernal frequency shift originating somewhere...

[xtian]

Good work, sorry it didn't help. If you just sit and play the amp normally, is the malfunction audibly obvious?

[sssmoka]

You have verified the capacitor ground point, yes?

Yes, sir - I verified the capacitor ground point. I think I performed this procedure correctly; I connected the negative probe of my multi-meter to neutral wire connection at chassis and measured capacitor values and voltages.

[martin manning]

Possibly a cracked or bad solder joint?

[Craig B]

I am totally ignorant of oscilloscopes so take my idea with a pound of salt.
Does the output of the phase inverter stage show the same signal change? How about the stage before that??
Good luck with your brain teaser!

[B Ingram]

... 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...