Quirks & Build Issues with Topps' Buchla 281

[camelneck]

I finally started building my Buchla 281 Quad Function Generator clone .
Unfortunately, I was unable to obtain any of Topps' Dual 281 PCBs. Thus, I had to use 4 of the Single 281 PCBs (two Version 2 PCBs and two version 3 281 PCBs.)

So far, I have only wired up one of the boards (a version 2 PCB). I wanted to do a thorough checkout on this first FG before I wired up the other 3 PCBs.

The FG appears to be working except for one possible issue. I say possible issue because I'm not exactly sure how this FG suppose to work.

In a post on the Muff forum, someone said that if the module is powered up with the mode switch of a particular FG in the cycle (or loop) position, the FG will begin cycling without the need to send a trigger pulse to jump start the cycling. However, if my FG's mode switch is in the cycle position when I power up the module, the FG does not begin cycling until I send a pulse to the trigger input.

QUESTIONS:

1) Could someone verify how the cycle mode suppose to work and if my FG is working properly? Do you have to jump start the cycling with a trigger pulse if the 281's mode switch is in cycle mode when you power up the module?

2) If the FG should begin cycling automatically (without the need for a trigger pulse to jump start the cycling), when the mode switch is in the "cycle" position when the module is powered up, then my FG is not working properly. Does anybody have any suggestions on what I might need to do in order to fix the problem?

3) Does anyone know if there are any operational differences between Topps version 2, version 3, and the dual 281 PCBs?

Thanks!

[camelneck]

The 2nd question I have is how does one set the trim pot.

I've read a couple of posts where people have used the following procedure, but there seems to be an issue about whether CV voltages should be applied to the attack and decay inputs when setting the trim pot:

1) Turn the attack and decay pots fully counter-clockwise for the shortest attack and decay times.
2) With the attack and decay times set to minimum, adjust the trim pot so the output signal has a frequency of 1KHz.

My question is should CV voltages be applied to the attack and decay inputs. Perhaps they need to be grounded to insure that the CV voltage applied to both inputs is 0V.

I would appreciate it if someone would clarify the calibration procedure for the trim pots for me. Thanks!

[Luka]

i need to trigger mine for them to cycle

which makes sense as the end out needs to retrigger the input trigger

not sure if there are differences in the versions - possibly just in how the OR circuit is implemented - i think there was talk of people using an active OR (really vague on this could be making stuff up)