October 14, 2008

My first 280

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.


wavingpalms said...

Only seven posts so far, and each a gem.

That sound you heard after posting 'standards' was a thousand a-ha!s happening...

This is a wonderful resource- please keep it up, and thank you very much.

Reed said...

In my little world, the one exception regarding lamps is the 265. Nothing says "random voltages" like those mesmerizing, candy-like lamps. The "fluctuating random voltages" LEDs on the later 265's & 266's have a tendency to look either on or off unless you stop what you're doing & stare at them for a while.