October 14, 2008
Today I worked on my first 280 Quad Envelope Generator. I was told that the top envelope seemed to get confused when it got pulses from certain modules. Once it got into this confused state, it was inconsolable. I didn't understand this condition at all. 2 of the 4 lamps are burned out as well.
There are 2 versions of the 280 module. I'm not sure what the deal is, since they were designed only 3 months apart, on March 23rd and June 21st of 1971. I guess this means the first one was a dud of some kind, and I don't know how many, if any are out there. This one was of the second type. (I'm guessing there was a third design as well, since the pictures I got from www.electricmusicbox.com show one with no lamps or pulse outputs and both schematics I have show these.)
Anyway, the problem turned out to be that someone had replaced the input comparator opamp with an LM307 instead of a uA741! I never would have guessed that this would be a problem, but when I opened it and saw that only the weird channel had a different opamp, obviously I was suspicious.
The lamps are still dead. Since they are enclosed in a lens apparatus that is panel mounted, they are not an easy replacement job. I have been told that cutting the back off and sticking another lamp inside the lens is the only way to keep the look. The owner and I have decided to replace all 4 with LEDs inside the lens, since the lamps are so slow they don't even light on short envelopes. Plus, I really don't want to replace them again.
Oddly, the 292 on the bench today had a bad uA741 as well. They must suck. ;)
Overall, I have developed a fondness for this module. On the surface, it would seem that a 281 is better than a 280 or 284, and capable of all of their tricks plus more. But, this little guy does his job with a swagger that can't be duplicated. Oh, and the output swings a FULL 0-15 volts.