Moog Music Forum

Hi, I'm trying to get a bank of 3 vintage 901B's to all have the same 50% pulse width at the same time. The 901A sends the 901B's a voltage to perform the pulse width modulation on the 901B's. The 901A is correctly sending the 0 to +12 voltage spread to the 901B's. But, the 901B's all have slightly different sounding "50"% pulse waves. Somewhere in the 40 -50 % range, instead of all being at the desired 50% pulse width at the same time. Looking at the Norlin Technical Service Manual for Modular Systems, on page 9, there is a schematic of the 901B oscillator. I don't see a trimmer anywhere for the pulse width control, like there is on the 921B's. Can someone please help me with where to place a Bourns trimmer in the 901B circuit to allow for calibration of the pulse widths? Again, the 921B's have a trimpot for the pulse width, but the 901B's do not.
I suspect a trimmer might be placed near R30 which is the first resistor downstream from connection point "C", which is the pulse width signal from the 901A.
What ohm value of trimmer should be used, and where should it be placed?

Thanks a bunch for your help,
-MoogDriver

I exchanged emails with Gene Stopp, the Moog Modular Product Manager, and Gene provided an excellent solution for this question.
Here is his response, and an image of the modified 901B schematic he provided with a trimmer for the pulse width adjustment on 901B's.
Thank You very much, Gene for providing this.

Here's Gene's response:
First you should adjust the various waveform shape trimpots as described on page 8, which may make things better right off the bat, then you can add a trimpot to each 901B according to the modified schematic I have attached. I would probably use a Bourns 3319P and use a Dremel to drill the 0.040” holes somewhere convenient.

Here is the image of the modified 901B schematic, with the pulse width trimmer added.

cool!!

I added a PWM solution to the 901B circuit in my clone consisting of a "Moog-style" control voltage mixer in the PW / PWM section mixing the voltages of a PW pot and a PWM input jack. The result is fed into the same location of the original circuit like Gene proposed.

LTSpice representation:

Circuit:


Plot:


Front panel extension:


-Carsten