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Is this Buchla 281 working properly???
MUFF WIGGLER Forum Index -> Buchla, EMS & Serge Goto page Previous  1, 2, 3, 4  Next [all]
Author Is this Buchla 281 working properly???
cygmu
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?
cygmu
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
https://www.muffwiggler.com/forum/viewtopic.php?t=64874&postdays=0&pos 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)
jimfowler
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?).

https://vimeo.com/277581932
https://vimeo.com/277581939

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

- Jim
jimfowler
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.



livefreela
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
cygmu
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.
livefreela
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?
cygmu
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!
ym2612
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.
jimfowler
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.
Triglav
cygmu wrote:
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
https://www.muffwiggler.com/forum/viewtopic.php?t=64874&postdays=0&pos 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)


Could you please post a fixed version of the original Buchla schematics (with all your mods)? I'm trying to get my Toppobrillo 281 to start cycling reliably without a trigger, but I'm getting a bit confused with all the versions floating around and I don't have a Roman PCB to test against.
djs
Triglav wrote:
Could you please post a fixed version of the original Buchla schematics (with all your mods)? I'm trying to get my Toppobrillo 281 to start cycling reliably without a trigger, but I'm getting a bit confused with all the versions floating around and I don't have a Roman PCB to test against.


Is yours cycling with a trigger? All other functions working nominally?

I have a topp 281 which works fine with the capacitor mod and nothing else.
Triglav
djs wrote:

Is yours cycling with a trigger? All other functions working nominally?

I have a topp 281 which works fine with the capacitor mod and nothing else.


Yes, it is working fine otherwise. I haven't tried the cap mod yet, as I was under the impression that it only works for a few minutes after powering up. Is that incorrect?
djs
[quote="Triglav"]
djs wrote:
Yes, it is working fine otherwise. I haven't tried the cap mod yet, as I was under the impression that it only works for a few minutes after powering up. Is that incorrect?


Mine will keep cycling happily for as long I let it?
Triglav
camelneck wrote:
You implemented my mod correctly and it seemed to be working just fine.
The only problem was this: you set your expectations a bit too high.

When I came up with this modification, I wanted to find a very simple way to make the 281 start cycling upon power-up (when the mode switch is in the cycle position) without the need for an external trigger to jump start the cycle process. This modification accomplishes that ... plus some.

Plus some? Since the capacitor remains charged for several minutes. the mode switch doesn't have to be in the cycle position at the moment of power-up as long as you move the mode switch to the cycle position before the capacitor discharges.

After several minutes (actually the time varies depending upon the leakage requirements of the capacitor that is used) this mod ceases to work because the capacitor eventually becomes discharged.

As long as you make sure the mode switch is in the cycle position during power-up (or as long as you move the mode switch to the cycle position shortly after power up) this modification will work fine.
cygmu
Where is that quote from?

You’ve asked me to draw out the modified schematic, and I’ll have a go, when I have a spare hour or so to remind myself.

Reading back about the camelneck mod, I think the latest version of the various mods will deliver the best of all worlds, a non time limited instant cycling no weirdness trigger section. If you have Roman boards I’d definitely recommend making the mods as shown on Dave Brown’s pages. Note that he’s updated his account now so it’s prettt straightforward to follow.

https://modularsynthesis.com/roman/buchla281/281_qfg.htm
Triglav
Thank you so much!

I've got the quote from this thread: https://www.muffwiggler.com/forum/viewtopic.php?p=1792603#1792603

I'm kind of confused because looking at the Toppo schematic, it seems to already include the Dave Brown mod (although with a 1.8nF cap for some reason):


Or am I interpreting it incorrectly?
pre55ure
My understanding of it is that if you go back to page 1 of this thread cygmu posted a schematic of the mode switch from the Buchla/Roman version.

It has a capacitor, a 4.7m resistor and a diode connected between the + rail and the cycle side of the switch.

The rest of the schematic is wrong and needs to look like the Toppobrillo schematic to work properly, but you need to retain the resistor/cap/ diode section from the original (buchla) schematic. This is what provides the pulse that starts the cycling behavior. Though I havent tried this with euro levels. It may need some component values to be adjusted.

I did this on my V2 (4U) boards and they all work properly. Well - to the best of my knowledge, I have never owned an original 281.
cygmu
Triglav wrote:
Thank you so much!
I'm kind of confused because looking at the Toppo schematic, it seems to already include the Dave Brown mod (although with a 1.8nF cap for some reason):


Or am I interpreting it incorrectly?


I think you are interpreting correctly.

When I worked out the mod requested in p1 of this thread, which is the mod reported on Dave Brown's page, I used the Toppobrillo schematic to help me figure it out.

What you end up with if you do the mods as now recommended on Dave Brown's page is a sort of combination of the Toppobrlllo setup and an improved version of the camelneck mod which should be quite robust.

I think pre55ure's account of the situation in the last post is right (thanks for reminding me how it works!)
cygmu
I think this is what it looks like after the mod.



Note that the switch is actually a SPDT. The central position marked "transient" is actually the "off" position of the switch.

The upper resistor/cap/diode combination is already on the Roman boards but not the Toppobrillo ones. Its job is to send a pulse when the cycle mode is engaged, to get the cycling started. The camelneck mod adds the capacitor, but not the parallel resistor. The addition of the resistor means that the cap recharges when cycle mode is not engaged, so that it works again next time.

The lower resistor/cap/diode combination replaces a 100k resistor on Roman's boards, but was already present in the Toppobrillo schematic. Its job is to turn an incoming gate into a pulse when the unit is in either Transient or Cycle mode. The fact that it works in Cycle mode is where we came in in this thread, because it lets you re-trigger a cycling 281.

In sustain mode, the lower resistor/cap/diode is shorted out by the switch, so the incoming gate is coupled directly to the rest of the circuit, and therefore the gate can hold the 281 envelope high for its duration. The Buchla drawing has a 100k resistor between the gate input and the rest of the circuitry in this mode, but the Buchla drawing is definitely wrong, and this works, so I think we can be happy.
ArguZ
I am loving this smile
That is exactly what we need...

Shawn, are you taking notes ?
Can the next batch of PCBs have all those necessary fixes included ?
Peake
I've just today received a 281 V1.1 set of PCBs from Shawn to build one for a customer; they have gold traces instead of the tinned on the last version? Interesting, have they been revised with this fix?
jimfowler
Shawn would be the guy to ask but I doubt the boards have been updated.
Triglav
cygmu wrote:
I think this is what it looks like after the mod.


Thank you so much, all is clear now!
I've applied the mods and channel A&C are now perfect, B&D however now stop cycling at certain settings. (Side "B" on each board) Patching the END into the input makes them cycle for the whole range though. I thought the two sides of the Toppobrillo board were identical. Any ideas on what might cause this?
cygmu
Triglav wrote:
cygmu wrote:
I think this is what it looks like after the mod.


Thank you so much, all is clear now!
I've applied the mods and channel A&C are now perfect, B&D however now stop cycling at certain settings. (Side "B" on each board) Patching the END into the input makes them cycle for the whole range though. I thought the two sides of the Toppobrillo board were identical. Any ideas on what might cause this?


Does it latch high? Maybe the modified camelneck mod is misbehaving somehow. The two sections aren't quite the same because of the quadrature circuitry. I am not sure what the issue could be though.

Does the issue arise with fast cycles, or slow ones, or just particular subtle settings?
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