Shruti filter pot responds reversed

Hello everybody,
my 1st post here, just received and assembled the Shruthi. This is not my 1st DIY kit and I’m not a newbie in electronics, but this time I’m experiencing something weird going with this thing, everything seems to work correctly except the filter pot that responds reversed: when it is turned clockwise, I see numbers increase on the display but the cutoff actually decreases (the sound darkens); and when turning counter-clockwise, the numbers decrease but the cutoff increases (sound gets brighter).

Here are a few pics of my assembled boards:
https://plus.google.com/photos/114126357140158493814/albums/5949802702449278321?banner=pwa

I am pretty sure I’ve done everything the right way, I double checked every single component and tested everything before soldering it on the board, but I might have overlooked something, so I ask your help for troubleshooting this strangeness.

Thanks,
Guido.

Pics attached

What is the voltage, when you move cutoff from 0 to 127:

  • On the F< pin of the control board?
  • At IC4 pin 7 on the filter board?

If I test the Control board alone, by powering it with an external 5V PSU, I can read the correct voltage on the F< pin, ranging between 0 and 5 Vrms, and I can see the actual PWM on the oscilloscope changing the duty cycle between 0% and 100%.

Once attached the Control board to the Filter board, the voltage ranges between 0 and ~2,45 Volts and starts increasing at value 66 - 67 on the display. The voltage on Pin 7 of IC4 ranges between ~4,4\ V\ and~ 0,2 V and, again, it only spans between 66 - 67 and 127, nothing changes between 0 and 66.

I’ve tried replacing the TL074 with a new one (I have plenty of spares in my lab) but nothing changes.

So the problem is definitely around IC4B (schematics here) - the voltage at IC4 pin 7 should go from 100mV to -100mV (that’s an order of magnitude, I don’t recall the right value).

Maybe a bad solder joint on the TL074 (pin 5 not correctly grounded)? Or on R24 (which would give the op-amp a massive gain).

One thing I find weird is the 4.4V reading - the TL074 doesn’t get as close to the rails as this…

Ok, I just found one more weirdness. When I first boot up the unit, the filter only acts between 66 and 127, but after a while it starts acting though the whole range. Value 69 seems to be the exact middle, as I read an average of 0 Vrms on the oscilloscope. I can still view the PWM on pin 7 of IC4 and its above 8 Vpp! I have checked everything around IC4. The voltage for the resonance seems good, ranges between around -450 mVrms to ~ -1,2 Vrms.

The attached pics show what I see at pins 6 (the triangle wave) and 7 (the square wave) when the Cutoff is at value 69.

IC4 pin 6 should be at 0V if the op-amp operates normally. Are you sure that pin 5 of the op-amp is correctly grounded?

> The voltage for the resonance seems good, ranges between around -450 mVrms to ~ -1,2 Vrms.

Where do you measure this?

Voltage for the resonace is a at IC4 pin 1. I don’t know if that’s actually the expected voltage, but I can say that the resonance goes from 0 to self-oscillation, so I assume it’s working good. And yes, pin 5 is perfectly grounded.

Oops, sorry, the values for the resonance are not negative, I was fooled by the fact that the oscilloscope shows a straight line going down as the resonance increases, but it’s actually a positive voltage.

Look for something weird around IC4 - it looks like R24 is not doing its job in the feedback path of IC4B - causing it to act like a comparator rather than like an amplifier.

Yes, I was surprised about the fact that R24 measured exactly 2,2 K even with the opamp still it its socket, so I removed the IC and found out that pin 7 was not making contact with the resistor and with the base of Q2… At first I thought it was a rare case of pcb fail where a track is visually there but for some reason doesn’t make contact with a pad (already happened in the past), but instead, looking closely at the socket, I found out that the pin 7 was actually bent beneath the plastic frame and not making contact at all with the pad! It must have bent the moment I put it into the holes and soldered the two opposite pins for keeping it in place.

Ok, now the synth works perfectly. Thank you so much for helping out, that was really nice of you.

Guido.

Glad that you’re back on tracks! Enjoy the synth…