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218 Easel keyboard buffer mod to keep stable tuning
MUFF WIGGLER Forum Index -> Buchla, EMS & Serge  
Author 218 Easel keyboard buffer mod to keep stable tuning
papz
I posted this in the VCS3 Card thread and it helped with a cOC tuning issue as well, so it's good to rather start a own thread. This should help in any situation where a 218e controls several devices.

With an early BEMI 218e Rev1 keyboard that I have here, switching the VCS3 Card's "key" switch slightly upsets the 208 CO and MO pitch and tuning.

The cause is the 218e's "main" output is not buffered.

I added a simple buffer to this output and now the CO and MO pitch is stable regardless of the VCS3 Card "key" switch position.

It's a very easy upgrade without permanent or risky modification of the 218e.
All needed is a TL071 opamp, a 100nF cap and cable. Total cost for the parts is less than 1€.
Any person with basic soldering skills should be able to do it.

1. Disconnect the blue banana tab from the PCB and bend it
2. Snip pins 1 - 5 - 8 of the TL071 to ease mounting
3. Bend and connect together pins 2 and 6 of the TL071
4. Solder the TL071 to the bent banana tab using it's soldered pins 2 and 6
5. Solder pin 3 of the TL071 to the PCB tab where the blue banana tab was formerly soldered
6. Connect pin 4 of the TL071 (black cable) to a -15v pad on the 218e PCB : the via spotted in red on the pic or where the power connector is wired
7. Connect pin 7 of the TL071 (blue cable) to a +15v pad on the 218e PCB : the via spotted in red on the pic or where the power connector is wired
8. Solder the 100nF cap across pins 4 and 7 of the TL071




ArguZ
Thank you papz, that is exactly what i mentioned in the other thread half a year ago smile
Let's just hope they incorporated that fix in the new 218e.
papz
Great minds think alike thumbs up
papz
I noticed a similar issue on a customer's 218r rev2 : the "key voltage" output voltage drops when connected to the 208 input, a buffer fixed it.
The "portamento" output is stable.

On my 218r rev1 it's ok.

Maybe this actual 218r rev2 is faulty ?
Another issue is the "key voltage" output is lower than the "portamento" output : when set to 1.2V/oct, there's only 1.1V/oct on the "key voltage" output and the buffer doesn't help of course.
I couldn't investigate deeper, no time and no schematics.

Would be good to know if this occurs on another 218r rev2. Could someone please check on his 218r rev2 ?
Thanks
eveldave
Any idea if this is an issue on the BUSAs?
papz
I don't know but it's easy to check : patch the "main" output to several destinations and see if it upsets tuning and tracking.

If you don't have a VCS3 Card yet it's a nice opportunity to get one. Mr. Green boops get out of this body !
ArguZ
It is definitely and issue on the 218e, yes .
They are just not made for high impedance inputs.
All e modules seem to be low impedance.
So if you stay in the ecosystem all is well, if you go dark you need to bring a light.
lumin
So after seeing Sascha post the graphic below, I did some testing and found a slight issue with the cardOC and the buffer mod. The tuning stays consistent when engaging the Keys switch, thank the good lord, but it seems that when using the sequencer app, having the 218 connected to main prevents the changes in pitch (voltage?) the app is providing. Removing the banana from 218 to main, the cOC sequencer pitches behave as normal.
Any thoughts as to why this may be happening?
Thank you both


ArguZ wrote:
lumin wrote:
I have a quick question regarding the PSU upgrade papz did above.
Somewhat specific to the NLM CardOC, but I can imagine it would apply to other cards as well.
When CardOC control over pitch is activated, I get proper tuning. If I plug my 218 into the pitch jack on 208, my tuning goes out of whack. Would doing this upgrade prevent this from happening? or is it inherent due to the introduction of the 218 circuit and component values into the whole mix which I assume drops voltage somewhere?

Thank you!
Lu


Hello,
yes , papz solution for the 218e is exactly what you want in the case you have a cardOC V1.
The outputs on that one are buffered as they should be, but the 218e are not.
As we can not change the way the 218e is produced we made a new version of the cardOC which is shipping now and that one has buffered inputs as well.
If you use the 218e with that, it compensates for the unbuffered outputs.
So two solutions for the same problem, depending on what one already has.

One more benefit of buffered inputs is that one can use the internal 208 sequencer to play a little melody and the 218e to transpose that melody for example.
Check out the picture...




The blue line is the sequencer playing an octave jump..
The big orange one is the keyboard transposing it by 2 octaves
And the yellow one is the output going to the pitch circuit shared by both oscis.
ArguZ
Oh, that is normal...
the cardOC is overriding the pitch input.
You need to use the black banana in the card itself.
The problem is actually that the pitch input is present on the card bus, the front panel pitch and at the slider itself.
So, if you use one of the old resistor card and the slider you also get a mix of voltages.
Same with the iProgram card or any other CV provider.
That is why we made a new cardOC that has a buffered input adder so you can use the sequencer or envelope and precisely add voltages using the bananas on the card.
Maybe i should make another video wink

PS: that only works with the cardOC 2.0 , the one with the blue Sullins connector
The older ones that are just a PCB do not have the buffered double input, but still override the pitch, so just connect your keyboard to the banana in the card and let the quantizer do the rest.
If you need to disengage the pitch from the card, use the switch.
lumin
I think I understand. I just remember prior to doing this buffer mod that I could, using the 218, transpose the cOC sequence that was playing, albeit with unstable tuning. It did not sound very good, but the capability was there.
Now, having the 218 connected, I get no changes in pitch from the sequence playing from the cOC until I remove the banana from 218 pitch to Main.
I did try the banana out from cOC to the Main and it does the same as flipping the Keys switch.
Plugging banana from 218 pitch into a CV input of cOC looks like the best move to transpose. I for some reason didnt think about that.

I actually use an external sequencer to control my easel, but I found it interesting that this was something new that arose after doing the buffer mod.

Thanks for the help!


ArguZ wrote:
Oh, that is normal...
the cardOC is overriding the pitch input.
You need to use the black banana in the card itself.
The problem is actually that the pitch input is present on the card bus, the front panel pitch and at the slider itself.
So, if you use one of the old resistor card and the slider you also get a mix of voltages.
Same with the iProgram card or any other CV provider.
That is why we made a new cardOC that has a buffered input adder so you can use the sequencer or envelope and precisely add voltages using the bananas on the card.
Maybe i should make another video wink

PS: that only works with the cardOC 2.0 , the one with the blue Sullins connector
The older ones that are just a PCB do not have the buffered double input, but still override the pitch, so just connect your keyboard to the banana in the card and let the quantizer do the rest.
If you need to disengage the pitch from the card, use the switch.
ArguZ
Yeah, the thing is to get a proper transposing action you need two buffered outputs.
By modding your 218 you added a buffered output.
If that output is 0V it is actively holding down any other voltage that is not buffered.

From the 218 into the cardOC and then out of the key bus is the way to go.
Try to add the sequencer as a CV modificator in the input selector.
Not sure if that works, but it just might wink
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