Troubleshooting a MIDI -> CV (control voltage) circuit - Page 1
Forum section: electronics / repair

author: mike-k
author_rank: Contributor
timestamp: February 13, 2021, 08:48:47 pm
content: Hi,I'm trying to get this circuit working again - see PDF for schematic. It was working in the past.It's a midi -> control voltage circuit, for making music. So midi pulses coming in at J8 and J7 are converted to voltages in the range 0-10V.There are two parts to the circuit, one for notes, and another for controlling other elements, like filter cutoff. The outputs are at J1 and J2.The problem is that the top part of the circuit for notes is not working. I get constant 10V at J1.Everything is fine at the other controller - J2 varies from 0 - 10V according to input.The RFB in the DAC U3, at pin 9, shows a constant -9V. That seems to be wrong and I don't know where that's coming from.The components seem to be OK, and I tried swapping the ones from working part of the circuit with the non-working part. Same results. I'm presuming that the PIC IC and the crystal are good because of the working half of the circuit? I programmed a new PIC and put it in, thinking that was the issue, but of course it is possible that I have damaged it in my explorations.VCC, V+ and VREF are good in U3. 5V, 12V and 10V respectively.Any ideas?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 13, 2021, 10:22:57 pm
content: U6 is a highly likely candidate.They typically fail full output voltage, hence the -9v at pin 9.Also the output pin7 is directly connected to the outside world with no protection, where all sorts of shit can happen to destroy it.

author: mike-k
author_rank: Contributor
timestamp: February 14, 2021, 11:24:52 am
content: Sounds like a good suggestion but if I swap U6 with U8 I still get good output of 0-10V at J2, so I presume that U6 is working OK.I've also replaced C11, R3 and R4.

author: mike-k
author_rank: Contributor
timestamp: February 14, 2021, 11:49:12 am
content: Would the IC not work at all if the crystal was broken? Or would it only fail at certain tasks?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 14, 2021, 12:25:48 pm
content: If you have also swapped the DAC's over and got the same result, double check all the pins on the left side on the schematic are connected to both chips.Thats the data bus, ILE, CS, WR2 and XFER.You could just use a multimeter on continuity and connect to both DAC pin 13,s then 14's etc.WR1 writes data to the 2 DAC's using 2 different signals from the PIC, so make sure pin 2 on U3 actually does go to the PIC pin 2.Apart from WR1, the input sides of both DAC's are in parallel, and you seem to have confirmed the common VCC and VREF are present on the right side.You didn't mention GND being present also, meter for continuity between both DACs' pins 3 & 10.Edit: And make sure all the GND's are connected to each other, e.g. pins 3 & 5 of U6, and that U6 has +15v and -15v on pins 8 & 4.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 14, 2021, 03:31:47 pm
content: Or, if you wanted to see whether it is the PIC or the DAC's, cut the tracks from pins 2 and 3 of the PIC going to WR1 on each DAC, and swap them over, and see if the fault stays with U6 or goes to U8.

author: mike-k
author_rank: Contributor
timestamp: February 14, 2021, 03:39:45 pm
content: Thanks, that was a useful sanity check. Confirmed that swapping the DACs made no difference. I made a list of all the connections from PIC to DACs and they're all OK. The grounds on the DACs and U6 / U8 are also good, and all joined.I can only suppose now that there is a problem with the PIC and whatever it is sending to DAC U3. I also tried changing the crystal and that made no difference.I guess I will try getting hold of another PIC and see if that sorts it out. Alternatively I could swap the outputs, use J2 for notes and forget about the AUX CV capability but that would be a pity.I am partly minded to start again, design a new board in Kicad and get it made up, for example based on this one, which provides the same functionality:https://obsoletetechnology.wordpress.com/projects/midi-cvgate-converter/The board I have was home-made by the original seller and the quality is not great. Having said that I have checked continuity on it many times so I don't think that is the problem I'm having.EDIT, just saw your latest post - yes I will try that to see if I can narrow down the issue to the PIC

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 14, 2021, 03:52:17 pm
content: Pin 3 has a 10k pullup resistor, pin 2 does not.I haven't investigated why that might be.

author: mike-k
author_rank: Contributor
timestamp: February 14, 2021, 04:20:24 pm
content: The PIC is socketed so I just bent legs 2 and 3 out of the socket and hooked each up to the alternative DACs. The same problem showed as before, just with the DACs reversed. So I presume the PIC is the culprit. I will get a few in and see if a replacement works. Otherwise I can use them for the other circuit I mentioned if I put that together.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 14, 2021, 04:40:00 pm
content: Try a 10k resistor from VCC to pin 3.Or swap the 10k resistor from pin 2 to pin 3.

author: mike-k
author_rank: Contributor
timestamp: February 15, 2021, 12:52:53 pm
content: I'm sure the circuit design is good, as it was working before.I think the difference may be that the AUX CV controlled by pin 3 is a continuous controller, but I would need to check a working circuit to see if there's a difference.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 15, 2021, 08:46:11 pm
content: I was just curious in case it threw up some kind of "oh, hang on a minute" scenario.When you reach the end of the line, anything is worth a punt if easy to do.