Fender Princeton DSP

[Harley]

I bought a damaged Princeton DSP digital amp.  The effects pot was broken.  I replaced the pot and all was fine.  Then the volume pot on the drive channel started popping at the 4 mark on the control knob.  I just replaced the pot... now I can't get anything other than static through the speaker.  The static level rises and lowers with the voume controls on both the drive and clean channels.  I don't know anything about electonics.  I can replace parts - that's about it.  Any suggestions where I should start?

Okay... I pulled it apart again and ran jumper wires over to the speaker cabinet... plugged in a guitar and it worked on both channels.   I put it back together... nothing.  I pulled it apart again tried it on the bench and saw some smoke... looks like I should sell it for parts... what do you think?

[DrGonz78]

So what parts got smoked?? Is this the Fender 65 DSP?? Yes it's worth fixing and it's worth selling for parts. What's it worth to you? Do you want to troubleshoot it??? Or just Shoot it?  :trouble

At least lets get specifics of what amp we are talking about and find a schematic to assess what components got smoked.  :crazy2: :blank:

[Harley]

Well it is a Princeton 65 DSP - The service diagram I have is 005741900 Rev D. 
As far as what's smoking - the solid state circuit board is discolored next to a 150 Ohm Dale resistor CP-5-3 10% 5W.

What has me confused is how would replacing a volume pot stop the amp from putting out sound.  The speaker is attached and I can hear hum through the speaker.  But, nothing from the guitar.

It looks like someone replaced the instrument input jack at one time and the circuit board was broken where the effects switch is located.  I fixed this with super glue.  Could super glue on top of the copper circuitry cause an issue?

I also soldered the new snap-in b100K pot into place.  I was going to remove it and put the old one back in but it's seated better now than from the factory. I'm afraid it will break if I try to pry it back out.

I would like to fix the amp.  I'm buying some books and would like to get better at this.

Thanks for any help.

[Enzo]

Your new pot could have been defective, you could have damaged the new pot when installing it.  When removing the old one, you could have cracked a copper trace, torn a solder pad off the board, ir similar.  Sometimes it is not the part one installs that causes a problem, it is the flexing of the board and other things that crack conncetions.

[DrGonz78]

Well it is a Princeton 65 DSP - The service diagram I have is 005741900 Rev D. 
As far as what's smoking - the solid state circuit board is discolored next to a 150 Ohm Dale resistor CP-5-3 10% 5W.

Please refer to part on the board such as R103 or R105 etc etc... Took longer to locate what part you are referring to here and I see it as R122. As far as where the smoke came from... That resistor generates heat, overtime the board will look a bit brownish. Probably not where the smoke came from... Look really careful for a fried component and even on the DSP daughter board too.

It looks like someone replaced the instrument input jack at one time and the circuit board was broken where the effects switch is located.  I fixed this with super glue.  Could super glue on top of the copper circuitry cause an issue?

Damage to the board in the way Enzo described is a most likely culprit. If you have a DMM, test using the continuity setting and see if there is a short between traces/pads around the problem areas. Soldering small jumper wires between damaged trace points on the board can restore functionality. If the pads are torn off you have to get more creative and careful figuring out how to solder wire to the components. There are many techniques to use as you can rebuild traces too and not just using jumper wires.

Foremost, go check for continuity between all the traces that are known to be repaired items first and see what we find. Also, very curious where this smoke came from...?

[g1]

the circuit board was broken where the effects switch is located.  I fixed this with super glue.  Could super glue on top of the copper circuitry cause an issue?

If the board is cracked where there is copper circuitry (traces), then super glue will not fix it.  You must inspect the crack with a magnifier and repair any cracked traces with jumper wires. 
  It is possible flexing of the board when you had it out did further damage to the cracked area so you must make sure all the cracked traces are repaired.
  As far as the smoke, that sounds like a different problem.  If you were running the amp on the bench without the heatsink all bolted to the chassis it is possible some of the output devices may have fried.

[Harley]

I appreciate the help and feedback.  I got a chance to mess with this thing a little last night.  I have gone all over the circuit board and cannot find any breaks in any of the copper trace.  I checked a lot for contiuity and found most read .000 a couple read .001 but they were long runs. 
I checked the new installed pot and when checking for resistance on the two outside legs it read open.  I am under the impression that the two outside legs should give me the total ohm rating for the pot.  I checked the outside and center leg and the ohm reading ranged from 000 to over 99 as I turned the knob.  So it looks like the pot works. 

I checked the resistance on the resistors and they are all right on spec.  The brown discoloration is probably from normal heat from the resistor. 

New Question - would a bad pot on the drive channel also knock out the clean channel?

Another thing I may have screwed up is the multi-pin connector that connects the DSP to the mother board may have been reversed when I put everything back together.  I don't remember how it came off and think I have it right but may have installed it backwards.

As a novice I'm not sure what you guys are referring to as pads.  When I removed the pot I softened (but did not remove) the solder.  The snapin pots do disengage pretty easily.  Is that how I may have torn a pad?

Thanks again for the comments.  If nothing else I'm having fun learning some new stuff.  I should have gotten an electrical engineering degree.  Maybe I'm getting ready to get one.

[Enzo]

A pot should indeed measure its rated resistance between the outside legs.  If you measure from zero up to 0 to 99 from the center to one end, but it also has to go from 99 to zero on the other end to wiper.  If you get open, then the pot may be open.  Probably right at one end.   Make sure your meter is set so the rated resistance is within its range.

The end of a copper trace is usually a round area with a hole in it for a component lead to solder to.  That round area is called a pad.   It is not rare to have the round pad crack free of the end of the trace.  Moving a component lead before teh solder is fluid can tear the pad off the board.  It can visually appear intact when it is not.

[Harley]

I set this project aside for a while but recently tried again.  I found a broken trace on the input jack and soldered a jumper to attain continuity... and I replaced the volume pot of the dirty channel with another new one.  I get a very faint distorted sound through the speaker when playing a guitar through the amp.  Both the clean channel and the effects channel are distorted.  The effects switch does not change the sound.  I have tested the pcb for trace continuity and cannot find any issues... very tedious work.  The amp lights up and gives me a little sound - no volume - any ideas?

[DrGonz78]

Do we know if the speaker was damaged?? Probably not right? Well disregard if you have tested the speaker, but I think you should run the preamp out to another amp. Got to cover our bases and would definitely use a another amps preamp out to the power amp in to test output & speaker of your Princeton. Just got to know... Still keep looking for traces that might be broken but it sounds like you spent quite some time looking so far. These cracked traces can be really hard to find.

First do those test then we'll see if we need to signal trace the preamp next...

Edit: Oh yeah did you ever figure out what got smoked? Still I am leaning towards a bad trace somewhere in the preamp or a damaged dsp board...