I have a CGS-83 Dual Positive Slew, version 1.0, built into my Serge/Bugbrand blue panel rack. I was looking at Prism Circuits and discovered that several versions of the dual positive slew have a problem. The two sections interact. Changing the rate of one messes with the rate of the other one. The problem was due to the two sections sharing a single LM3900. I tested mine and sure enough, it did. So I ordered the fix kit at the link above. The kit provides a small PCB holding two LM3900 chips, so that each slew circuit gets its own LM3900.
The instructions from Prism Circuits are for their dual positive slew PCB, not the Ken Stone original that I have. I had to do a bit of extra work to get it installed.
The LM3900N was in a machined pin IC socket, which is my custom when working with single sided PC boards. That socket had to be replaced with a double header socket that would accept the pins from the fix.
With some good solder wick and some patience, it was quite easy to remove the solder from the socket pins.
The next step was to assemble the kit and find some inline sockets. I took care to examine the kit PCB that holds two surface mount LM3900 chips to insure the correct orientation of everything.
The pins mount on the “this side facing up” side. However, after installation that side is actually facing down.
The fix worked, as expected. This was a timely find and fix, because my first patch of the new year is going to be an attempt to replicate the patch I took to a Modular On The Spot with the Serge/Bugbrand.
I see the dual 3900 kit mock up , but asking did you use another lm3900 or MC3401 for the other chip . I Ken stone used an MC3401 but it is comparable as a LM3900 and Elby says use 4 lm3900 on her new pcb of this exact nature , I was just trying to read the other chips number and figured another lm3900 would suffice. Happy New Year . thanks for answering
Hi Robert,
The other chip is an LM3900. It’s for pulse logic and works fine. Belated Happy New Year!
thanks just checking some ic’s need to be the correct ones. I have circuits that use ic’s that are very odd and parameters are sketchy too.
thanks for this! caught it just before soldering in sockets.
2nd article dual slew would not go to changes he found, and you implemented before you soldered.
i’ve tried the Prism Positive Slew fix in the CGS 83 v1.0 but still found some interaction between the 2 slews when cycling. any ideas? would doing the same thing to the other LM3900 potentially resolve this issue?
Hi Jason,
I don’t know. If you describe the interaction, I can try to replicate it. The other LM3900 deals only with pulses. Have you tried swapping out that LM3900 for a different one?