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Is this Buchla 281 working properly???
MUFF WIGGLER Forum Index -> Buchla, EMS & Serge Goto page Previous  1, 2, 3 [all]
Author Is this Buchla 281 working properly???
ym2612 wrote:
I applied this mod on my A and B channels, and their transient/sustain behavior is now correct, but they no longer automatically start cycling when switched to cycle mode. I have to kickstart them like on the Toppobrillo 281. I haven't looked into why this is.

That's interesting... Did you remove the parts indicated for removal on Dave Brown's diagram, or leave them in?
Prompted by this, I played around with the mod again.

I can confirm that on my 281:

- making the mod as drawn on Dave Brown's page, which means removing three parts and installing three additional parts, works as described by ym2612 above, i.e. when switched to cycle, the envelope will only start cycling once a trigger is received.

- making the mod "my way" aka "the lazy way", which involves adding the three parts drawn on Dave Brown's page but not removing the other three, retains the "cycling starts immediately" functionality.

There was some discussion of cycle-starting on the Toppobrillo 281 here torder=asc&start=75
with at least one claim that a "real" 281 needed a trigger to start the cycling. Can anyone confirm? (I mean a vintage 281, not a 281e)
Maybe I don't understand the problem(s) - it is late and I'm a few beers in - but my "vintage" 281r works in transient, sustain and cycle modes without need for modification. As well, in cycle mode I can get envelope A pulse out to modify B (where the rate of B does NOT change the rate of A) with a simple change: put a diode at the pulse input (or I guess you could do it at the pulse out?).

I'm sure somebody smarter than me will come along and tell me how this won't work...

- Jim
Photos of my modification. I'd be great if somebody else with V1.1 could test this out and confirm/deny that it works for them as well.

well I'll be damned - sometimes the simplest mods work the best! jimfowler - I just installed your mod on my vintage version 281 - need to spend some time with it to be sure, but upon first impression I can confirm it works as expected applause
This is great news!

The mod for the v3.0 is really little more than installing this diode, while simultaneously fixing the transient / sustain which does not work as the PCB is designed.

The only possible issue I can see with this mod is if there is a capacitor somewhere that can't discharge properly with the diode in place. This could make something lock up or become unresponsive after a while. If that ever happens then we can take a closer look at the circuit and install a bleed resistor somewhere, but for now I reckon it is highly likely to work just fine.
worked with the module for a few hours last night - things still looking good, though I did manage to lock it up once (nothing a flip of the toggle couldn't restart - it's happened a few times pre-mod as well) still, I think I'll have a crack at adding a bleeder resistor and seeing if that helps things further. what would you suggest cygmu? 4.7m or so from the cathode of the diode on the trigger-in to 0v?
Hmmm, I am not really comfortable advising without having seen the circuit (and even then...!) but something like that is worth a try.

If it seems to help but you still get lock-ups when cycling really fast then you could reduce the resistor a bit. 1M or even 100k might be all right, depending on the value of the
non-discharging capacitor -- if there even is one of those!
I will probably get a set of the 1.1 PCBs to build and put behind the panel of my current rev3.x, which always was and still is sick.
For those with V1.1 how are you getting the module to “lock up”? I can’t get mine to misbehave but maybe I’m not asking the right questions.
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