Hi everyone,
I have two questions about my Moog Modular, please…
1: Should the 904A filter “self oscillate” (like my Mini’s do)…?
My 904A doesn’t… although it all seems to work fine otherwise. If it should self oscillate - would this more than likely be a calibration error, or most probably a fault…?
2: The power on bulb. It’s a largish red dome which unscrews. Is there a removable bulb in there or do I need to source a complete “red-dome” replacement…?
Cheers,
TOM
I am convinced the 904a should self-oscillate when the resonance is raised to maximum. There might be a calibration trimpot inside that needs some adjustment.
There are techs here that can most probably tell you what to do about it.
As for the power light, on the Minimoog it’s a neon light (operating from the mains input power) that can be replaced by similar ones of other colours. As for the one on the modular, I suspect it might be the same. But then again, there are people here who could confirm or deny this…
If the pilot light is like the one on the 1969 Moog here, it uses a Dialight brand lens cap. You can still get the lens cap in red/green/clear if you need that part. Newark sells them.
Newark replacement Moog modular pilot light lens
If you just need to replace the bulb, it is a type 327. It has a flange and slides into the back of the lens cap. Mouser part # MS25237-327 (Although that part now shows EOL?)
When I found the 1969 Moog I have here, the 904A did not self-oscillate like it should. Just like the Minimoog, it should start to oscillate around 7 or so on the regeneration dial. On the schematics the resistor is listed as “Selected”, although the value is also given as 1.8K. That resistor goes directly to the regeneration potentiometer, so it should be straight forward to locate it. There are a few different schematics, the early 904A using a different feedback loop with a 25K pot. And later revisions like the 1969 one here, using a 50K reverse audio pot. And the Norlin era 904A’s look nothing like the board layout on this one.
Anyhow, after locating the correct feedback resistor, I used a deca box (Resistor substitution box) to find the value I needed. I did not remove the existing feedback resistor, since disassembly would be needed as there are two circuit boards back to back with two “C” frames. So I just clipped the deca box to the existing resistor, and dialed in (Selected) the regeneration response I needed. As it turned out the existing resistor was a 1.8K and I just tacked on a 2.2K to get the oscillation to start where I wanted it. Starts around 7.
Photo here, but your 904A may look different from this circa 1969 one.
(Highlighted resistor “piggybacked” before soldering in place.)
Funny story about the 904A feedback. A somewhat known modular manufacturer had a 904A clone that started feedback at around 4 1/2. When asked about this behavior, since most users found this to be less than usefull, he responded this was just like the original. The original they had must have either been out of spec, or someone “Selected” a new value that started this oscillation at 4 1/2. So all these kits were distributed with the error, I still have one. We were told to make a resistor substitution to push it back further, like on a Mini, but it wouldn’t be stock. 
Guess if you are making clones or circuits inspired by, you should have a properly calibrated cookie cutter, before you make all those cookies. My “perfect” Moog filter clone still starts to oscillate at 4 1/2. Have to fix that some day.
So how about some nice high resolution photos of those circuit boards on the 701?
EDIT:
Mouser new part link for USA style Moog bulb:
327 bulb at Mouser
Thanks Terry,
I got it to self-osc today. Not as drastic and pronounced as the Mini but I’m happy.
And I found a supply of those bulbs here in the UK… so all’s good…!
By the way here’s a little thing I played today… and I had the tape running…;c)
https://soundcloud.com/q-uazimodo02/modular-moog-minimoog-psycho
Cheerz,
TOM
I have seen several 904A’s that didn’t oscillate, but likely due to age. I’m not sure if they originally planned it to - it’s a gain issue. I have a really old one here from 1965, and it does not. Will see if the feedback gain is different by design.
The Dialight lamp is quite correct for the Portable (P Cabinet) systems like the IIP and System 10 and 15. I think the regular modular versions (C Cabinet) used a flat front lens, although many have been changed by now.