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Author Topic: Noisy Princeton Reverb 65  (Read 815 times)

Erich

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Noisy Princeton Reverb 65
« on: August 05, 2017, 04:10:50 PM »

Ok,
    I may have posted about this before, but I cannot find it. I have an old Princeton 65 SS amp. It is so noisy it's unusable. I've resoldered and then replaced the power caps and no change. I've resoldered all of the connections on the PCB and it made no difference ('refreshed' the solder). I've tried several guitars, cables, etc, and can not change the noise that way. It's 60-cycle hum. I have conditioned power to plug it into, a shielded studio (HAM radio operators made that a necessity. I basically have a Farraday cage in my basement HAHAHAHAHA!), have tried it in several circuits, in several locations (inside and outside of my house), and no change.

   I've also swapped out the input jacks and have not had any luck. Fender SS amps are 'noisy' out of the box but this is insane. ANY help would be appreciated. I was an electronics tech in the Army and work in the industry now, so I'm not a novice, but I simply cannot figure this thing out! I love the amp (when it works) but am about ready to set it on fire! Thanks in advance!
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phatt

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Re: Noisy Princeton Reverb 65
« Reply #1 on: August 06, 2017, 07:05:37 AM »


Assuming it has an FX loop then divide and conquer. ;)
Get some high gain pedals direct into the return which bypasses ALL the preamp circuitry and if the noise is not present then you know the issue is inside the preamp circuitry.

To double check your problem you can take a signal from FX OUT and send it to another poweramp and see if the noise is still present.

The hum can be caused by the dreaded ground loop which wreaks havoc with hi gain circuits. The issue might be from a repair not done right or a missing or corroded earth tag.

Of course Finding the ground plane path in some circuits can twist the mind of even the best technicians.  :duh

If the issue is the preamp then you can inject a signal and follow the signal path until you find the noise. I use a 10nF cap on an alligator clip sent to a small bench amp to track down just where in the circuit the issue presents.
HTH,, Phil.
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Erich

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Re: Noisy Princeton Reverb 65
« Reply #2 on: August 06, 2017, 07:34:55 AM »

THANK YOU! I don't know why I didn't think of that. I'll be on it this afternoon.  :duh
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Erich

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Re: Noisy Princeton Reverb 65
« Reply #3 on: August 11, 2017, 10:42:46 PM »

Hey, I think I may have found he problem: The transformer outputs look like they have been hooked up wrong. I've checked the schematic I have and I can't tell from it (it's a crappy copy and I can't find another one) if the Xformer leads are attached to the board correctly. Does anyone have a picture of the fully connected board / power supply?

There are several 'options' to attach it and I just don't want to bung something up. Thanks in advance!!!
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Enzo

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Re: Noisy Princeton Reverb 65
« Reply #4 on: August 12, 2017, 09:05:31 AM »

There are no options for the secondary, the red wires go to P3 and P5.  The red/yellow goes to P4 on the board edge.

If you live in the USA with 120v mains, ther are only the two black wires, and they go to P105 and P109.   The schematic calls out the posts for each wire in the 220/240 setup with export transformer.
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Erich

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Re: Noisy Princeton Reverb 65
« Reply #5 on: August 12, 2017, 02:38:17 PM »

Thanks. The board in the one I have has 10 different spade terminals in the area where the power supply hooks up and none of them are marked with a color / voltage. You post helps. THANK YOU!
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Enzo

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Re: Noisy Princeton Reverb 65
« Reply #6 on: August 12, 2017, 11:00:41 PM »

Are not the posts numbered, Like P3, P4, P5...?

If you have the export transformer, the wire colors are on the schematic, with the post # each belongs on.  If you have the 120v transformer, there are only the two black wires, and the schematic says where they go.
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DrGonz78

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Re: Noisy Princeton Reverb 65
« Reply #7 on: August 13, 2017, 03:42:40 PM »

The board in the one I have has 10 different spade terminals in the area where the power supply hooks up and none of them are marked with a color / voltage

I think we need a picture to understand better what you are looking at.
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“A person who never made a mistake never tried anything new.” -Albert Einstein

Enzo

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Re: Noisy Princeton Reverb 65
« Reply #8 on: August 13, 2017, 04:08:27 PM »

I posted the service manual in post #5, it includes the layout.
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DrGonz78

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Re: Noisy Princeton Reverb 65
« Reply #9 on: August 14, 2017, 04:47:05 AM »

The board in the one I have has 10 different spade terminals in the area where the power supply hooks up and none of them are marked with a color / voltage

Oh yeah Enzo I am with you 100%. I know those amps pretty well and they are clearly marked with P3, P4, and P5. I only requested a picture as the OP was almost implying that there are no markings on the board or something. Of course they are not marked with a color or voltage and that does not mean he said they are not marked with a P3 or other designators. However, he never even acknowledged that input. It's almost as if Erich thought Red and Red/Yellow on the board but those are just the color of the wires.

Hey, I think I may have found he problem: The transformer outputs look like they have been hooked up wrong. I've checked the schematic I have and I can't tell from it (it's a crappy copy and I can't find another one) if the Xformer leads are attached to the board correctly. Does anyone have a picture of the fully connected board / power supply?
But to look at a picture of his amp we all could tell him right away if the transformer is connected correctly.
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“A person who never made a mistake never tried anything new.” -Albert Einstein

J M Fahey

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Re: Noisy Princeton Reverb 65
« Reply #10 on: August 15, 2017, 08:06:29 PM »

For one I do not yet what kind of noise we are talking about, there´s many different types with different origins.

And in any case primary connection would have little or nothing to do with "noise"  so don´t mess with it.

I)f possible, post an MP3 showing the noise, and move controls to see whether any of them has some effect.
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Erich

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Re: Noisy Princeton Reverb 65
« Reply #11 on: August 16, 2017, 05:43:00 PM »

    Sorry for going dark for a bit. Also sorry about the post about the XFormer leads. What I was trying to say (poorly) is that you can't read the writing on the board. After looking more closely, for some reason, mine's marked on THE BACK! So, that's sorted....
    Ok, I have the Xformer hooked up correctly (just wanted to cross that off of the list). I appreciate all of the help. I went through some other steps (isolating the -pre, checking grounds, AFTER re-soldering all of the joints, etc (and testing after each step to isolate variables).
   I then hooked the amp up without the reverb tank attached. No change
   The hum is present with nothing plugged in. It is constant in volume and can somewhat be overcome with signal (basically turning up the amp). It is also exactly the same regardless of what cord, guitar, etc., is plugged into the amp. I've measured the outputs of the transformer and they are well within normal range.
I plugged into the 'Pwr In" circuit to see if there was any change and there was none.
I replaced the caps about 2 years ago and it did thing to change the hum. I still, with all of the information I have,,am leaning towards the caps  being bad.. I checked my receipts and cannot find the one for two 2200uuf, 50V caps. These have no markings on them and I'm beginning to wonder if I have some shady crap I bought when I was broke. I'll have the new Nichicon caps (direct replacements) in hand tomorrow. I truly appreciate everyone's help and will post an update when I get them in.
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Erich

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Re: Noisy Princeton Reverb 65
« Reply #12 on: August 17, 2017, 07:49:18 PM »

SUCCESS! So, I installed the new filter caps and one of them blew immediately. I took a wild guess and I began testing the diodes in the rectifier circuit and BAM! One of them was shorted open. I thought this only happened with a massive overvolt condition, but apparently it was weak (or something).

SO, the amp is MUCH quieter. I think I need to repair the shielding in the cabinet and make sure the ground from the amp to the cab is good and I believe I have repaired this thing.

Thank you to everyone who weighed in. I've never experienced a diode shorted open before!
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J M Fahey

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Re: Noisy Princeton Reverb 65
« Reply #13 on: August 20, 2017, 02:24:13 AM »

"shorted open" ????????????????????????????????
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Erich

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Re: Noisy Princeton Reverb 65
« Reply #14 on: August 21, 2017, 03:19:10 PM »

OK. JM Fahey, Wrong terminology.. . . .The diode 'shorted' (blew) for want of a better term so that it acted like a resistor as opposed to a diode. Sorry for the faux pas.
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