Fender Princeton 65 DSP - Low Volume and Hiss

[earache]

Hi

I've got a Fender Princeton 65 DSP that has very low volume and much hiss on both channels. I poked around inside with a chopstick when it was powered up and found that the diodes, resistors and other components near the input jack are microphonic. They make quite audible noises thru the speaker when tapped or touched with the stick. I flowed new solder to these components and to the input jack but found little change. Does anyone have a schematic and some prior experience with this amp's problem? Thanks in advance

[Jazz P Bass]

Fender support page

http://www.fender.com/support/amp_sc...ton_65_DSP.pdf

[earache]

Thanks

Thanks Jazz P Bass, shoulda looked there myself...appreciate your help.

Should I always suspect devices on the board that are microphonic as I noted in my previous post?

[pecorporation]

Do you get the same problem with nothing plugged in to the input jack?

[earache]

Solved!

I figured out the problem. I took another look at the soldering on the PC board near the input jack and found that I didn't do a good enough job fixing the solder joint at the input jack - the jack had pulled off of the board somewhat and so the tip connection wasn't adequately making connection to the circuit.
What made me look again was the fact that the components that were microphonic were low numbers ("C1" etc) and so were probably at the beginning of the signal chain, with nothing between them and the jack (so the jack must be the problem!). Thanks for the help.

[Jazz P Bass]

Princeton 65 DSP

I would think poking around the front end of the circuit may be irrelevent.
Those first stages are increasing the feeble 100 mv signal from your guitar.
Try plugging your guitar into the Power Amp In jack.
If this does not produce noise, then we have isolated the problem to the preamp.
If it is still noisy, the problem is in the power amp section.
If you have another amp available, plug a cord from the Fenders Preamp Out into that amps input.
This test will prove out the preamp of your amp.
Isolate the problem.

[Sowhat]

I just repaired a Fender Princeton 65 dsp with this problem and aside from the input jack having cracked solder joints the input jack has a small barely noticeable ground spike coming out of the front (face that mates to the chassis) which makes the ground to chassis connection. These plastic input jacks are so flakey that the threads are very susceptable to getting stripped making the connector move around with use breaking solder joints. This spike also wows out the contact point on the back face of the chassis eventually loosing ground contact. Even if you do good solder joint repairs, if this spike does not make contact with the chassis your problem will persist. I solved this problem by putting in a little .010" aluminum shim that the spike digs into between the connector and chassis and then that contact face can move around a bit and contact is maintained.

[Enzo]

And the jack with the ground spike looks just like a plain one if you are not paying attention. I don't have any of the spiky ones in stock usually. SO I tack a short piece of bare solid wire to one of the ground terminals on the jack, so it stocks out past the board edge. I then bend it up against the face of the jack so it will be pinched between the jack and the chassis when installed. Seems to work well.

[Sowhat]

Hi Enzo, excellent tip, as you can see that was my first post here although I have been fixing instrument amplifiers for over 50 years now. I just hated to see other posts dancing around the problem here without a definitive solution and explanation of cause because this type of failure is so typical. Didn't these crummy plastic jacks once come with a complete metal bezel up front at one time, I've seen that type used on inexpensive Marshall's like the now obsolete MG series combos... the spike is a ridiculous design, but having plastic input jacks is even worse. I failed to mention that sometimes I purposely ignore the PCB solder pads and instead I flex wire the input jack pins to the board so when the inevitable loosening happens the PCB solder joints won't crack or even matter, the very short wires will just move around with the loose jack making the thing that much more road worthy like the good old point to point amps from the 50's and 60's.

[Enzo]

The plastic jacks are OK in my view. I like to caution folks when they want to replace the plastic jacks with "good old metal Switchcraft" jacks, that in so doing they will be grounding the signal chain at the chassis there, and that MAY upset the designed ground scheme.

The spike thing sucks, yes, but in its defense, it is one way to get grounding to the panel if you want it without having to add a star washer or some other wire connection. They mount the jack on the board, and they are done.

The Cliff style jacks are all plastic, but the other jacks usually come in both plastic and metal bushing types, and we select them for design based on whether we want grounding or isolation at the chassis mounting hole.

[Sowhat]

The plastic jacks are OK in my view. I like to caution folks when they want to replace the plastic jacks with "good old metal Switchcraft" jacks, that in so doing they will be grounding the signal chain at the chassis there, and that MAY upset the designed ground scheme.

The spike thing sucks, yes, but in its defense, it is one way to get grounding to the panel if you want it without having to add a star washer or some other wire connection. They mount the jack on the board, and they are done.

The Cliff style jacks are all plastic, but the other jacks usually come in both plastic and metal bushing types, and we select them for design based on whether we want grounding or isolation at the chassis mounting hole.

I get cranky without my coffee and say stuff that may be a bit misleading, sorry. But that aside, I really do dislike plastic to human interface parts like input jacks and even pot shafts because humans are so much stronger than small injection molded plastic parts that the human, especially with the leverage of a nice sturdy instrument cable always over-stress especially when they get physical doing that great guitar solo onstage in front of thousands of people. In any case, of course you're right, you don't want to screw around with the designed grounding scheme even though there are thin fiber insulating shoulder washers for those traditional metal jacks should you need to float that connection... you don't even want to get me started on my opinion of right angle mounted PWB panel controls, I'm very old school and probably a bit out of step with new fangled inventions for manufacturing convenience. I worry more about real world usage and durability. Maybe I'm crazy or just a bit senile.

[Enzo]

Oh yes, the fiber shoulder washers. First thing the customer loses.

yeah, there is a plus/minus chart for each jack type. What really breaks plastic jacks is people stepping on the cords. I don;t worry about them yanking the cord or shoving into it. But step on the cord and you can lever your jack right out of the circuit board. My feelings on CLiff jacks is that I have seen DARN few broken ones over the years. They work. And yes you can bend the contacts on them, but when you put that much stress on them, a metal bushing jack would have been bent as well.

And I always looked at it this way. Customer breaks plastic jack by stepping on cord or however. Wants metal bushing jack. OK. next time he steps on his cord, his front panel is bent, and the metal jack is bent in the process anyway. And jacks that don;t crumble put more stress on the pc board.

And then there are the ones where we cram a switchcraft #11 over the circuit board with jumper wires down to where the old plastic pc mount jack was. The old plastic jack could lose a nut and still work until the wiggling cracked the solder. That #11? It has pushed back inside somewhere, and the star washer fell off it and is rolling around amongst the circuitry.

And I don't even want to get into which we would prefer: a broken jack, or a $1200 amp stack falling face down on stage.


Now I am not really trying to make anyone like plastic jacks, but when someone gripes that they want metal ones instead of the "cheap" ones they originally had, I think it is unfair. The plastic jacks work and have a purpose, they are not in there for economics.

[pecorporation]

sheez