'65 Princeton Reverb Reissue Randomly Detunes Notes

[Stevem]

Without that short to ground when neither input is used the amp will hum if that channels volume is up.

[Healer amps]

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...

Sorry but that lead dressing is not good not even in the after shot..
Here is what I would if it was on my beach.

1. V1 the blue and green wires are up in the air together, no good ! tuck the blue wire close to the chassis and make it as short as u can. The green wires I would remove and use coax cable direct from input and volume.
2. u have 4 wire blue, green, red black, going to the bord, they should ether be twisted or put together whit traps.
3. V2 disconnect the green wire and make it as short you can and make the blue wire go around the socket instead like they did in the old days = https://el34world.com/charts/Schematics ... layout.pdf
4. The twist to V1 can be better.
5.fix the "spaghetti" on V3 and V4 make the blue and green wirer as short as u can and not close together, yellow wires (cathode) are not as sensible but can be shorten for better look

There more in this amp to improve, but I have not read all the answer so they have probably told you .
I have been on this forum for a very looooong time. But being from Sweden and have dyslectic I have not respond to so many questions.

Best regard OJ

[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.

OK... I'm back with egg on my face. Amp owner finally got around to testing his equipment. The frequency intermodulation distortion is still present - I can't hear it and haven't seen it on a scope, yet, but I do trust the owner. Hence, I replaced the OEM input jacks and PCB. Back to square one - hunting down the source of this frequency IM distortion.

D'oh!

[xtian]

Just want to point out that IM distortion is present in EVERY amplifier to greater or lesser extent.

[alnight]

You know, I would cut that zip tie where your black and white heater wires are tied to what looks like the output transformer leads and move the heaters well away from everything. Might not solve the problem, but couldn't hurt to try.

And, you know, double check and possibly clean all the chassis grounds...

As xtian says, there will always be some intermodulation, but it shouldn't be awful.

[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.

OK... I'm back with egg on my face. Amp owner finally got around to testing his equipment. The frequency intermodulation distortion is still present - I can't hear it and haven't seen it on a scope, yet, but I do trust the owner. Hence, I replaced the OEM input jacks and PCB. Back to square one - hunting down the source of this frequency IM distortion.

D'oh!

OK... cut zip ties and moved B&W heater wires away from other wires as much as limited lead length would allow. Checked chassis grounds. Checked individual gain stages (pins 2 & 7 on the 12AX7/ECC83S preamp tubes and pin 5 on the 6V6GT power amp tubes). All things look good. Here's the humdinger - my ears can't hear. nor can my scope see, any irregularities in the signal. Period. I'm thinking the amp's owner was mistaken about the frequency intermodulation distortion still being present. I simply do not and cannot hear it with my ears or see it with my oscilloscope. Hence, I've concluded that said amp is indeed repaired.

[sluckey]

Maybe there is an issue at your customer's location. Ceiling fan, fluorescent light, router, etc.

[alnight]

Agree with sluckey. Dirty power, dimmer switches, etc. can create sonic artifacts. In listening tests, I usually find intermodulation most noticeable on the B/G strings from about 7th to 15th fret. In some cases it becomes more apparent at higher volumes/current draw. Increasing the value of the primary reservoir cap(s) can help mitigate that, but will also make the amp feel more 'stiff'.

Realistically, this is a new amp and all of these things should be purely academic. Perhaps you should ask the customer to bring his guitar, cable, and whatever else he regularly plugs into the amp to your location to see if he can demonstrate/recreate what he's hearing at home. That could help narrow things down.