LEDs for Lamps

I dug into the 280 quad envelope generator's indicators. Originally, Buchla used lamps for the indicators on his modules. This evolved into the use of LEDs in later and current modules. Don couldn't decide how to mount LEDs either, some modules had them mounted to a "motherboard" PCB, some had them mounted to terminal strips bolted on the panel. The owner of this 280 decided to have me replace the lamps with LEDs, so they would be faster as well as last longer. I decided to change all four, even though only two were blown.

The offending lamps are sealed into a panel mounted enclosure that was intended to be disposable. Sadly, 40 years have passed since they were installed and they are no longer being manufactured. The only solution that will keep the aesthetic of the panel is to cut off the back of the enclosure and put a new light inside it. This is the cute lamp that was inside.


Luckily, a 5mm LED fits inside perfectly. I have chosen to use RED diffused type. They light up nicely with no extra resistor! Here is the job partially finished. The sawed off lamp on the left and an unaltered one on the right.


And here all done.



I forgot to snap a picture of the LEDs lighting up, but they look great. Now the fast, "blip" type envelopes can be seen, which was not the case with the original lamps.

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.