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CV droop with MFOS 1V/oct Keyboard Controller
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author CV droop with MFOS 1V/oct Keyboard Controller
Matt C.
I'm experiencing some pretty serious CV droop issues with my MFOS Single Bus 1V/Oct Keyboard Controller (link to schematic below), I'm wondering if anyone has had similar issues with this board and whether there are any possible fixes.

Right now the pitch CV will drop a half step within probably 4-5 seconds. I tried cleaning the PCB with isopropyl alcohol and a toothbrush but didn't see any obvious improvement. Do I just need to use better cleaner? I'm pretty sure the S&H circuit block is to blame, but if there are other possible causes I'm all ears. Any ideas? Thanks.

Praxis
I'm guessing leakage from C19 or C20
Nerone
Exactly the same problem. Could be an issue in the last stock of PCB???
Help! cry
Matt C.
yeah very frustrating. I've tried everything I can think of, it's gotten slightly better maybe, but the droop is still audible. real bummer.

I'm thinking I might need to install a midi-to-CV converter in order to make this thing actually playable.
Nerone
I'm using a National LF444, and you?
I'm looking for a test with an IC from another brand (f.i. Texas instruments). Maybe the input resistance is different???
Matt C.
can't remember what brand my LF444 is. I did try a different chip but it was probably the same brand. Worth a shot to try TI

I had assumed the problem came from a dirty PCB, but after many passes with flux-off, the problem persists.
Nerone
Me too, tons of flux remover...
So, I'll try it with other components (IC and caps).
Anyway, the analog triggering is fantastic!
flagada
What kind of cap did you use for C19?

The analog triggering in mine was actually terrible. If I pressed the same note 10 times very shortly, I could clearly hear pitch differences, sometimes upto half a note. I also had a CV drop, but not within 4-5 seconds, it took longer.

However, I built it on stripboard, so I may have done something wrong. I was glad to switch to a MIDI2CV converter.
diablojoy
Quote:
What kind of cap did you use for C19?


this !
I have built a few of these boards
you will get some droop eventually but should be after minutes not seconds
for C19 I have always used 10nf polystyrene 0.5% = D tolerance code
obtained from RUSHONLINE

https://www.rush-on-line.co.uk/cap_index.php
this one
LCR SC1276 SK710
Nerone
I was referring to the triggering speed, in the sense that I feel the difference with the MIDI latency.
In my case it works perfectly, even in fast repeating notes.
The attack pitch is ok, then a very fast droop occurs.
I started with a polystyrene 10n, then I tried with larger caps (not polystyrene), the droop decrease but of course the variations of pitch are not correct, especially in large intervals.

diablojoy, what LF444 did you use? (TI, National ...?)
tojpeters
I have built 3 of these and they worked great.
JanneI
I've built one which worked great too!
diablojoy
National LF444CN
I have currently 2 of these boards in use working fine

Quote:
I'm looking for a test with an IC from another brand (f.i. Texas instruments). Maybe the input resistance is different???


hmm I am thinking this is unlikely to be the culprit

texas instruments aquired national sometime ago so these days should be the exact same chip
data sheet comparison both show 1012input resistance
note this figure is fairly common for JFET input opamps
eg TL074 is the same
LT1058 is perhaps a better option as a replacement for LF444 if you wish to try - more expensive though
also note that polystyrene caps are prone to heat ie keep the leads with enough length to add a heat sink clip component side during soldering and allow time for cooling between doing each leg and be quick.
diablojoy
Quote:
The analog triggering in mine was actually terrible. If I pressed the same note 10 times very shortly, I could clearly hear pitch differences, sometimes upto half a note. I also had a CV drop, but not within 4-5 seconds, it took longer.


That sounds more like the keyboard contacts side of things - KBUS
it is a constant current source through a divider
if your key contacts resistance varies at all you will get pitch differences
what are the contacts made of ?
flagada
diablojoy wrote:

That sounds more like the keyboard contacts side of things - KBUS
it is a constant current source through a divider
if your key contacts resistance varies at all you will get pitch differences
what are the contacts made of ?


I used an old Korg Poly-800 keyboard I got from a friend. It had these rubber with carbon contacts. I had cleaned them well with (I think) IPA.
diablojoy
Quote:
It had these rubber with carbon contacts. I had cleaned them well with (I think) IPA.


Yep that would explain it then.
that's a known issue with this design - carbon contacts won't work
better off finding an old home organ keybed with metal contacts from the 70's
on ebay quite often you can get them free to good home.
flagada
@diablojoy: Just saw your reply. Too bad I did not know that before! I spent a lot of time making a nice wooden case for the keyboard and removing a few bad keys. Still managed to make some nice music with it, so it was not totally in vain, though.

Ok, I will keep the module in my modular just in case I find a keyboard with metal contacts.
tojpeters
Maybe you can add reed switches and magnets to what you have
diablojoy
Quote:
Maybe you can add reed switches and magnets to what you have

Yes that may also work well
would need to epoxy magnets to underside of keys and figure a way of mounting the reed switches , It's nothing that someone somewhere hasn't done before.
the home organ keybed thing from the 70's is probably going to be a lot easier, though the inverted keys do look nice thumbs up
Nerone
FIXED! It's peanut butter jelly time!
Most likely I cooked TWO pairs of Jfet (Q2, Q3). A carefully soldered new pair and all works fine (just a very slow drooooop......)
Probably the voltage in C19 was sucked back from the -12V line through the gate of the (burned out) Q3 and R14
d'oh!
johnstilton
Old thread I know....

I also have this issue on two of these boards that I just finished.

One droops bad. the other droops VERY bad.

I used J310s instead of the recommended 2N5457s. I am going to get some of the latter and some Wima Polyethylene caps and see if either improve the situation.
devinw1
Be very careful when you solder JFETs. Follow good ESD protocol, and for best safety, clip a shunt onto the 3 leads which shorts D, G, and S together while you are soldering and then take this off when you are done soldering.
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