SMR4 MKII - no sound at pin 3


I’ve built the SMR4 MKII but i’ve get no sound at test pin 3.
All transistors and ICs are in the right position.

Can you post a good pic of top and bottom of the filter board, especially the area around IC7 (the IC where you are testing the signal) ?

here are the photos.
i even checked every trace around pin 3, looks fine.

Looks like you burnt one capacitor…

the red?
well, i replace it.

replaced it… still no sound.

  • Voltage on IC4 pin 7 when cutoff = 0 and cutoff = 127?
  • Remove IC6. Current between IC6 pin 1 and IC6 pin 6 when cutoff = 0 and cutoff = 127?

looks like the backside needs a shave :wink:

IC4 stuck at 0 and 127 at 0.39 V
IC6 at 0 0.73V
IC6 at 127: 1,38V

There’s a reason why I put current in bold in my second sentence. I am asking for a current measurement.

what do you mean with current?
do i have to set something special up for that?

you’re measuring Voltages (V), what you need to do is current (mA) if your meter has that section

oh, now i know what you mean…will do this tomorrow.

current at 0: 0,00 mA
current at 127: 0,04 mA

The current measurements look extremely low… You should read a value between 400µA (cutoff at 0) and 400mA (cutoff at 127). Do you have the same reading even when playing a C3 or C4 note on a MIDI keyboard connected to the unit (to shift cutoff up with key tracking?).

IC4 pin 7 should be in the -100mV … 100mV range.

So from there I conclude that your CV scaling circuit is totally off. First thing to check is the trimmer - maybe you have swapped the trimmer of the control board with that of the filter board - or maybe it is incorrectly soldered. Then check all solder joints around IC4.

it’s a 20 k Trimmer. checked all soldering points, no effect.

any idea?

No, sorry! It’s obvious it’s with the CV scaling circuit, so look around all parts in block C-D 2-3 in the schematics.