Problem in preamp (midboost) Charvel Model 6

[Rubiolus]

Hi from Spain

I believe model 6 (the pot is integrated on PCB)...I´m repairing this guitar for a guy...........when I connect the positive of the cell in his board position the guitar doesn´t work anything :think:....and I put a "bridge" to the pickups and the guitar works (the tone pot too), but the booster and the volume pot don´t work (the pot doesn`t turn down , and always is at maximum).....Pots and electronics are in good condition I proved it.....I think the problem comes as a consequence of the bad operation of the Operational AMplifier (electrolitic caps look fine)

Somebody had this problem in the past?

This pics is from Kramer Forum (the preamp is similar or the same I think)



The red wire is the point when I conect, doesn´t work

And this is the schematic



Excuse my primitive english

[Rubiolus]

Somebody to love?

[dg]

If I'm reading correctly, you are only getting it to work when you completely bypass the active circuit (the volume pot on these is part of the active circuit). That means there is either a bad component on the circuit board, a break or short in the circuit, or the volume pot has gone bad. I'd start by trying to rule out the volume pot, since that is often the problem on these. You may be able to just put a jumper across the connection points for the pot. If it works when you use a jumper, then you know you need to replace the pot. If it still doesn't work, then you need to troubleshoot or replace the whole circuit. Lots of people just remove the circuit completely and start fresh with new volume & tone pots, especially when they upgrade their pickups.

[bratfink]

Finn Guitars does a replacement for this circuit.

A few years ago you could get OEM replacement JE-1200's from Fender. I think it might be the same circuit that is useed in a Clapton model or something? Either way at a UK dealership I got a new replacement circuit. No-one over here in the US can tell me of the same thing being available here so it could be that it was just old stock left in the warehouse and I got lucky asking for it.

[DonP]

Somebody had this problem in the past?

Sorry, but no. I had to wire mine up but it worked fine. Make sure nothing is shorting out. Double check that your work matches the schematic. Other than that, try replacing the opamp.

[Rubiolus]

Thank`s to everybody

If I'm reading correctly, you are only getting it to work when you completely bypass the active circuit (the volume pot on these is part of the active circuit). That means there is either a bad component on the circuit board, a break or short in the circuit, or the volume pot has gone bad. I'd start by trying to rule out the volume pot, since that is often the problem on these. You may be able to just put a jumper across the connection points for the pot. If it works when you use a jumper, then you know you need to replace the pot. If it still doesn't work, then you need to troubleshoot or replace the whole circuit. Lots of people just remove the circuit completely and start fresh with new volume & tone pots, especially when they upgrade their pickups.

The volume pot are OK....I tested boost pot and volume pot with the multimeter (tone pot works)

A short in the circuit is a posibility I had not thinking about....is a good point to look before change the op amp

[Chief_CrazyTalk]

I'd put a socket in there if you do change the opamp. I did like that circuit but eventually came back to passives.

[Rubiolus]

I'd put a socket in there if you do change the opamp.

Of course it`s a thing I had in my mind, if we´ll try this point