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1V/oct Keyboard PCBs : leaky S&H
MUFF WIGGLER Forum Index -> Music From Outer Space  
Author 1V/oct Keyboard PCBs : leaky S&H
johnstilton
Tonight I (partially) successfully completed my first build: added series resistor networks to the cleaned switches of this Lowery keybed and two MFOS 1V/oct keyboard PCBs.

The left side is 20 keys and the right side is 37 keys.

The sample and hold circuits are very leaky though. The output voltage starts to drift away immediately after a keystroke. Worst on the right hand circuit.

This voltage is supposed to be stored on C19 a .01uF polystyrene capacitor. I wonder if they are leaky? Also I used J310s in place of the 2N5457s for the sample switch FETs Q2 & Q3 and I wonder about this substitution now.

I made sure to rinse off the flux thoroughly around the S&H components as Ray wrote “any leakage path to the capacitor will cause it to (droop).”

I’m going to walk away feeling OK about this progress for now but I wonder what other builders of this circuit have to advise ?
Bodo1967
johnstilton wrote:
This voltage is supposed to be stored on C19 a .01uF polystyrene capacitor. I wonder if they are leaky?


I doubt that, tbh. These caps usually don't cause any problems and don't leak (unlike electrolytic caps). I have them in several circuits and never encountered any "strange" behavior.

johnstilton wrote:
Also I used J310s in place of the 2N5457s for the sample switch FETs Q2 & Q3 and I wonder about this substitution now.


Using another FET type might at least be worth a try.

What about those red mats on the picture? Is it possible that these are not 100 % non-conducting? Personally, I'd place the PCBs freely for testing so they don't have any contact with any surface whatsoever. You could use M3 screws and nuts in each corner to give the PCB "legs" (like a table). Or just have 'em dangling in the wind from some string or wire wink.
johnstilton
I found a thread for the same issue :

https://www.muffwiggler.com/forum/viewtopic.php?t=177166&highlight=

It doesn't look like the original poster found any resolution but the last writer did resolve their issue by installing new FETs.

I'm going to order both a polypropylene caps and the specified obsolete FETs and see if either improves the operation.

I did try it with the pcbs resiting on a pair of plastic pens.
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