Lightbox circuit

Nigel Goodwin

Super Moderator

Most Helpful Member

• May 17, 2025

• #21

GCook said:

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.

Click to expand...

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.

 

S

splitdog

New Member

• May 17, 2025

• #22

So the pot is not grounded?

 

G

GCook

New Member

• May 17, 2025

• #23

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

 

S

splitdog

New Member

• May 17, 2025

• #24

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

 

G

GCook

New Member

• May 17, 2025

• #25

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.

 

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splitdog

New Member

• May 17, 2025

• #26

Could you clarify? I'm not an engineer.

 

G

GCook

New Member

• May 17, 2025

• #27

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.

 

S

splitdog

New Member

• May 17, 2025

• #28

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

 

G

GCook

New Member

• May 17, 2025

• #29

The main board. Not the input board.

 

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splitdog

New Member

• May 17, 2025

• #30

So between sig input, and 'neutral'?

 

G

GCook

New Member

• May 17, 2025

• #31

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

 

S

splitdog

New Member

• May 17, 2025

• #32

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

 

S

splitdog

New Member

• May 19, 2025

• #33

Any more thoughts?

 

S

splitdog

New Member

• May 28, 2025

• #34

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