Lightbox circuit

[Nigel Goodwin]

Okay, I see it on the other schematic. I guess you figured out I was just focusing on the input board. I'd like to know why the circuit needs it. I'd say (after looking at the main input circuitry), that C20 is used to block the DC voltage. I still believe the pot on the input board needs a GND connection. I'm waiting for Splitdog to answer my question about the outer pins of the pot.

C20 prevents the DC getting to the opamp, but passes the cracking caused by adjust the sensitivity resistor.

You're still thinking the pot is a volume control, it's not, it's mostly an adjustable load for the mike, plus a bit of crude level adjustment as well. The input also isn't floating, it's got R45 and R47 preventing that.

 

[splitdog]

So the pot is not grounded?

 

[GCook]

Nigel- I'll be quiet, and just sit back and watch.
Splitdog- Your meter says it's not, so I guess it isn't.

 

[splitdog]

Should I try a jumper with alligator clips from the 3rd leg to sig grnd?

 

[GCook]

If you have a 1/4 watt, 50K resistor, just jump the SIG input to GND. The successful result will be that the resistor remains cold to the touch, and the noise goes away.

 

[splitdog]

Could you clarify? I'm not an engineer.

 

[GCook]

Connect one end of a alligator clip jumper to SIG. Connect the other end of the jumper to a 1/4 watt, 50K resistor. Connect the other end of the resistor to a second alligator clip jumper that you will attach to GND.

 

[splitdog]

On the pot? Or the inputs and outputs on the board?

 

[GCook]

The main board. Not the input board.

 

[splitdog]

So between sig input, and 'neutral'?

 

[GCook]

On the main board schematic there is a J2 input connector with two terminals (SIGL and GND). Connect there.

 

[splitdog]

Ok. I have to get one. I'll let you know. Thanks!

 

[splitdog]

Any more thoughts?

 

[splitdog]

UPDATE: Problem solved! I put a 1 uf cap on the sig line (+) in series going into the main board from the small board. That solved the pot noise. There must have been DC coming up from the main board.
Then, I rewired the incoming signal using quality shielded cables, and diverted the AC wires away. And the crackling to the music stopped. Thank you for your help!!! Jim