e-licktronic Forum

measuring continues...
the m5218 have correct power - ~13.4V on pin 8 and ~14.8V on pin 4

the cd4069 has no power on its supply pins.
0V on pin 14 and -0,013V on pin 7

Now I have to wrap my head around the schematic to understand where the error could be... or get some sleep first

these measures are from mid tom.

hi tom was easy. there i was missing the 68k resistor

so only mid tom to fix

Currently I suspect the problem lies with IC20.
I tripple checked the soldering and the resistor values of the VCO control voltage generator.
But the m5218 is outputting a constant ~11.4V on pin 1 (before R12A).
The input voltage from the tune knob is correct, as it gets 2.0V to 3.6V or something like that on the non inverting input 1. inverting input is keept at 0V as it should be.

Pin 1 on the 2 working toms is more or less near the input voltage from the tune knob.

I replaced the m5218 IC20 with a tl072 rigged onto a pin header to test if the opamp was faulty.
same result. so the opamp was not damaged.

now I'm lost at what to do and have to leave the board alone for a while to get a clear head

would it be possible to get a screenshot of the mid tom section layout where the traces are visible?
would make debugging so much easier!

Daaaang! I found the problem!
The right 'standoff' from VR14 is placed directly above a via carrying the signal from R12A to the 4069 VCC pin (14).
Since the standoff is connected to GND and touches the untented via when the pot is fully inserted, it shorts the output of the opamp down to GND, eliminating the 4069 supply voltage.

This was a hard one to find! but at least the mid tom is working now, too

comboy wrote:Daaaang! I found the problem!
The right 'standoff' from VR14 is placed directly above a via carrying the signal from R12A to the 4069 VCC pin (14).
Since the standoff is connected to GND and touches the untented via when the pot is fully inserted, it shorts the output of the opamp down to GND, eliminating the 4069 supply voltage.

This was a hard one to find! but at least the mid tom is working now, too

Nice one! Good work.
Yeah, maybe one should put a thin layer of insulation material under the VR-pots.

VR14 is not near any joints? Can you explain further?
I had no problem with the Mid Tom though

Ah, do you mean that tiny little hole on the right side?

exactly - the tiny little hole.
That's what made it so hard to find.
The corner of the pot poked exactly into this tiny hole

All missing parts listed except for the seq part that I will complete during this weekend:

Hihat:
C159 - 100pF, ceramic disk

HiTom:
R386 - 68k
C95 - 0.1 uF Film cap

Handclap:
R226 - 68k

Bassdrum:
C4 - 100pF ceramic

Ride:
C140 - 100pF ceramic disk

Crash
RA11C - 20K
C50 - 100pF ceramic disk