Fender Princeton Chorus DSP Crackle and Pop

[Sowhat]

I just finished up a job that many of us have encountered many times, a flakey connection problem. When the job came in the door and the customer demonstrated the problem by whacking the top of the amp with his fist anyone would immediately know from the sound of things that something cracked in there somewhere. My initial guess was those pesky spikes on the plastic input connectors so I pulled the chassis, jumper grounded that location and slapped the chassis back in for a test. Well, that wasn't the problem.

The chassis went back up on the bench and I finally decided to actually diagnose and hopefully trace the problem. Visual inspection showed a few cracked solders on the input jacks, no real surprise there, I fixed those up and still the thing crackled and popped like crazy. In checking things a little further I realized that only the left amp was making a fuss. I broke out the magnifiers for my magnifiers and made a close inspection of the board. Nothing looked funny, all the solders looked perfect. Undaunted I resoldered the big resistors anyway, you know the ones that normally get a bit hot, still no cessation of the noise.

There's really not a lot to the circuit with this being a 1 chip wonder… well, in the case of the Princeton Chorus, 2 chips. I figured there would be no harm in resoldering all the leads to the left side TDA amp chip. They looked perfect and shiny but what the heck, I resoldered them anyway. I did a quick check of the crackle and pop and it had magically vanished.

So, in conclusion it was an invisible flakey solder joint on the TDA amplifier chip. With the amp going from it's cabinet to the workbench 3 times during this exercise and hooking up all the probes and connections multiple times this made what was in reality a super simple quick fix, a full hour of bench time.

[The Dude]

Glad you got it. Nothing worse than an intermittent. I've learned over the years that often times it's less time consuming to just get out the iron and solder a whole board or section of a board than to hunt for hours for that one tiny intermittent connection. Yes, it's satisfying to know exactly which one it is. On the other hand, it often takes much longer to "hunt and peck". Plus, you have better odds of it staying fixed if you just rework the whole thing. Sometimes you think you have that one illusive connection, only to find out later that it just magically worked for a while.

[Sowhat]

Often times I will indeed shotgun solder a board, many times that fixes the problem but I am left with an emptiness of not knowing what exactly went wrong and what caused it. I did fiddle around here a bit I admit, and I already had the shotgun in my hand as the next resort, but I wanted to at least narrow down the problem to satiate my own curiosity. I only narrowed it down to 8 or 10 or however many pins those TDA's have but not the specific pin.

My conclusion is that my customer is well practiced in whomping and banging around his amp based upon the strikingly artistic presentation he made demonstrating the problem. As for what exactly went wrong, who knows, but you can certainly speculate with some confidence.

[52 Bill]

... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

I guess that should be $1.00 to solder a pin and $49,999.00 for knowing which pin to solder.