Continuing the discussion from #1007 midimuso midi-cv:
Hey guys, I’m having trouble getting the reference voltage dialed in to 10.6 volts. The trimmer resistor only takes it to about 9.15 volts then keeps spinning. I replaced the trimmer thinking it may be bad but it still ha s the same range from about 5 volts - 9.2 volts.
Has anyone experienced something like this? Any ideas on where to start troubleshooting?
Thanks!
Hey there!
I had a similar problem. If I remember correctly, I made a bad connection. I built this module on stripboard though, so I cannot really give you tips where to look, but one thing is sure: don’t worry, you’ll get it solved eventually!
Maybe you have a similar issue, search for shorts and check the resistor values around the TL431 and the TI4504s.
If you post some images of your build, we may help you spot the faulty link.
Good luck and be patient!
Thanks for the response and the encouragement! 
Here’s some pics of the build. I haven’t found anything that looks off yet.
Some of the advice here may be pertinent:
Thanks Sonosus! I made sure to take all these steps before posting! 
Hopefully I didn’t miss something in the forum.
To clarify a bit more, the module works but drifts out of tune as you play up the midi keyboard. The #1222 VCO is calibrated, and is perfectly in tune when using a CV keyboard.
This is the relevant part of the schematic:
Assuming the 12 V ref really is 12 V, and the resistor values are correct, and the TL431 isn’t faulty or installed backwards, and the output isn’t shorted or very low resistance to ground, then you should be able to get 10.6 V.
The trimmer only needs to get down to around 1.5k to give ~11 V at the output. That should be no problem.
Have you checked the 12 V reference? Checked the resistance from test point to ground? I think those are the main possibilities. You should verify the resistors’ values — you may not be able to measure them in circuit but you can at least check the markings for correctness. They look OK to me in the photo but I wouldn’t swear to it. The capacitor probably doesn’t matter for this. The TL431 does look to be the right way around. Is it from a reliable vendor? Do you have another one you could try if all else fails?
Thank you! I’ll run through this!
The TL431 is from Tayda, I do have another it’s thats the case.
Thanks again!!
