Help fixing a ADSR/VCA please

[ed_banses]

Hi all,

I bought an ADSR/VCA some time ago and I built it but never got it working. Then I tried to fix it myself but did not work :(. I will appreciate your help. This is what it happens:

Unit does not work in Loop mode but it does output an envelope in NORM mode. The envelope is square ash and will respond to the sustain pot adjusting the level of it. However nothing happens when I try to change any of the other pots. The VCA seems to be working because I can see that it outputs a modulated signal using that square envelope I mentioned before.I checked U2 inputs and the pots are sending variable voltage into the expected inputs so I bought a new 4052 to see if that was the issue but still behaving the same way. Is there anything else I can check? Any ideas?
Thank you so much in advance!!!
Kind Regards,

Eduardo

[Synthbuilder]

I bought an ADSR/VCA some time ago and I built it but never got it working.

Which issue board do you have? It'll be either issue 4 or 5.

Tony

[ed_banses]

Hi Tony, thanks for the quick reply Just check it in the PCB to be sure. It is an issue 4
Many Thanks

Eduardo

[Synthbuilder]

It is an issue 4.

With no gate connected, the module in normal mode (ie. not looping), and release turned to its maximum, what is the voltage on pin 8 of U3? It should be 9V. It also should change when you alter the release pot, dropping down to 0V when the release pot is at its minimum. If it is always stuck at 0V or even at a negative voltage then U3 may well be dead.

If it does change from 0V to 9V smoothly as the release pot is rotated, then the fault could either be the THAT2180, or one of the three resistors R37, R40 or R56 are the wrong value.

Tony

[ed_banses]

Thanks Tony, brilliant advise! I will check later when I am home with the unit and feedback here
Thanks again
Eduardo

[ed_banses]

Hi Tony, I have just measured in pin 8 U3 as suggested. The release pot is responding but the ranges are +6V to -5V in a smooth manner when I rotate the release pot. These values are different than the ones you mentioned. Do you known what could it be?
Thank you so much in advance
Kind Regards,

Eduardo

[Synthbuilder]

The release pot is responding but the ranges are +6V to -5V in a smooth manner.

That's not good. That suggests a problem with the 0V connections. If you are measuring this with a voltmeter and your red probe is on pin 8 of U3, where is the black probe connected to? It should be at 0V, for example, pin 2 of the MOTM power header or an IC pin like pin 7 of U2.

Make sure your power supply connections are sound. Assuming you are using the MOTM power header, that is +15V should be found at pin 1 of the power header and -15V at pin 4. There should be 0V at both pin 2 and 3.

Tony

[ed_banses]

Hi Tony. Yes you are right. I was not measuring right........not good!! :(
I have now taken ground from pin 2 of the power header as you suggested. The reading is 0V to 10.63V. A bit higher than expected but probably still ok? For completion I changed the TL074 and the TL072 from another module that it works and the result was the same. So probably will be the THAT2180 as you suggested. I have also checked the resistors that you mentioned and they are fine.

I will order a new THAT2180 and give it a go when it arrives.
Thanks

Eduardo

[Synthbuilder]

I will order a new THAT2180 and give it a go when it arrives.

One more thing before you buy another THAT chip, what's the voltage across R37 as you vary the release control? That is, red lead on one end of R37 and black lead on the other end.

Tony

[ed_banses]

Hi Tony. I have just measured and doing the same set-up (NORM mode with no gate) I get 0.348 when release is maximum and -0.119V when release is minimum.
Thank you for your help
Eduardo

[Synthbuilder]

I get 0.348 when release is maximum and -0.119V when release is minimum.

Right, that bit is working. Given those values it is quite possible that the 2180 is not working correctly.

[ed_banses]

Thank you so much Tony. I will order a replacement one. Hopefully that will fix it
Kind regards
Eduardo

[ed_banses]

And thank you!