[sdiy] Tone control questions

Carsten Toensmann carsten at analog-monster.de
Tue Mar 11 08:48:19 CET 2014

Leaving the question why the simulation is ok and the prototype shows that 
quite obscure behaviour...

-----Ursprüngliche Nachricht----- 
From: Terry
Sent: Tuesday, March 11, 2014 8:58 AM
To: David Ingebretsen ; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Tone control questions

Quite a few mistakes in those Norlin issued Moog modular schematics. These
seem to be just simple copy mistakes where whoever copied these over from
the original blueprints, did not proofread very well. Bottom line, if you
are using the Norlin schematics, you are going to find these silly
mistakes like 4.7K when they should have copied over 47K. Best to use the
originals or a combination of the two copies.

The Norlin issue 984 schematic has a few differences from 1967 the
blueprint. You noticed the one pot labled 2.5K when they are all 25K pots.
There is the bass circuit where the 4.7K is mislabled as 47K. The 80uF cap
on the -6 rail is missing the +/- signs. And perhaps where you may be
having a problem, the first resistor after the 22K mixer resistors is
mislabled as 4.7K where it is 47K on the original blueprint. Could try
subbing in 47K for that one?

Another observation, the old circa 66/67 Moog circuits mostly used 2N2926
for NPN's and 2N3702 for PNP's. These were found in the early 901's
oscillators, 902 VCA, 904 filters. By 1969 Moog was using 2N3392 for the
same NPN's, and 2N4058 for the PNP's. Most schematics were updated with
these, but that 984 schematic is the same revison from 1967. Wouldn't
surprise me if those later 984's used the 2N3392's for the NPN's in that
circuit. Guess you figured out those odd value filter caps are those
Mallory tropical fish type. Would like to see a close up of an original
board layout. The 984 had two circuit cards, each with two identical mixer

> 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
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> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
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