No resonance on SM4 mkII (SOLVED)

Everything seems to be working except resonance. It will not self-oscillate.

pictures removed.

I’ve resoldered around IC4 as one post suggested. I’m hoping someone can spot a problem. I want my candy!

No resonance at all or it’s going very close to self-oscillation without reaching there?

There is no change in sound from 0 to 63. Filter sounds good and is definitely cutting off.

okay… Is the rest of the filter working? Are you sure that the two boards are correctly connected and that the 1x8 connector on the filter board is correctly soldered?

I am running it with a jumper, but I checked the continuity between boards on “Q” and it has a good connection…

Can you confirm that the rest of the filter is working? Because there are three things that would prevent resonance:

  • The resonance CV isn’t transmitted correctly to the resonance control VCA.
  • The resonance VCA doesn’t work.
  • The resonance VCA works and it receives the right CV, but something is broken in the filter core itself so that feeding back the output into the input doesn’t cause any resonance behavior (such as one of the 4 cells being at a different frequency from the other ones).

I’ll do my best to describe the filter’s behavior.

The filter cutoff control works. I can make the sound darker or brighter with it. The ADSR that drives the filter is sending a signal; all of the parameters work. The contour amount also works.

Is there a way to trouble shoot the options you listed with a meter?

Can you post a good photo of your filter board (both sides)?

I had one in the first comment, can you get the links to open?

Remove IC3. Measure current flowing from pin 1 to 6 for the minimum and maximum resonance settings.

And measure the voltage side on each side of the resistor between C17 and C18 - again, for minimum and maximum resonance.

Current between pins 1 and 6: 0 for both resonance settings. I don’t read current very often, so I hope I was doing it right. I removed the IC, set the meter to 200 mA and pushed the probes into the IC socket. With IC socket “facing” left, 1 is the bottom left and 6 is the 6th one across the bottom.

Voltage across resistor: Res 0: 0.02 V, Res 63: 4.85 V. For both readings, the end of the resistor away from the IC’s is positive.

Did the pictures open for you?

1/ On most meters, you need to connect one of the probes to a different terminal when measuring current instead of voltage.

2/ I was not expecting a measurement of the voltage across the resistor, but two voltage measurements, one between ground and the first point, and another between ground and the other point.

Thanks for the ammeter refresher course.

Current through pins 1-6 (red on 1, common on 6): Res 0: 0mA Res 63: 0.2 mA
I assume that means the current is flowing from 1 to 6.

Voltage vs. ground, pin near IC: Res 0: -0.009V Res 63: -0.009V
Voltage vs. ground, other pin: Res 0: 0.011V Res 63: 4.84V

I checked the voltages both with the IC in and with it removed. Same voltages either way.

It is now working! The oscillation is there and the filter is now tuned. The little beast sounds great!

possibility 1: The gremlins, realizing that a systematic effort was underway to hunt them down and evict them, vacated the Shruthi.

possibility 2: after removing IC3 the second time, I noticed a bent pin on the chip. I assumed it was bent when I removed it, but it could have been the culprit the entire time.

Thank you pichenettes for your assistance and patience. Now I’m going to eat my candy and re-read the manual.