[sdiy] Tone control questions

[David Ingebretsen]

Hello everyone,

I'm building a Moog 984 4 channel mixer and I'm having problems with the
tone controls. I found and fixed two errors in the schematic already, the
resistance of an input attenuator pot was mismarked and one of the resistors
in the bass control was mismarked as 47k when it should have been 4.7k.

Anyway, I simulated the circuit in Multisim and with the correct resistor in
the bass control, it simulates perfectly. When I assembled the PCB, the
mixer part is fine, but the bass and treble controls don't work right.
Instead of going from an attenuation at full counterclockwise to neutral at
mid-point, to boost at full clockwise, I get attenuation at both clockwise
and counterclockwise and either neutral or boost at midpoint for both
controls.

I think I get the general picture of what the tone controls are doing, but I
don't understand the details. Has anyone any thoughts why the simulation
would work but not the actual circuit?

I've done the following:
1. Triple checked all the component values on the PCB 2. Quadruple checked
my schematic against the Moog schematic and the Multisim schematic.
3. Replaced the transistors
4. Replaced the tone control pots
5. Played around a lot with the Multisim simulation trying to get a handle
on what the various components do and what would happen with a wrong value
in the tone control circuit.
6. Tried to check the dumb things like power supply voltages, etc.

If anyone wants to see my schematic capture, the moog schematic, or my
multisim schematic/project I'm happy to send them along.

Thanks everyone.

David
 
David M. Ingebretsen
Collision Forensics & Engineering, Inc.
2469 E Ft Union Blvd, STE 114
Salt Lake City, UT 84121

dingebre at 3dphysics.net
dingebre at CFandE.com