VCO Troubleshooting

Hubaswift · Joined: Jul 06, 2016 Posts: 91 Location: Canada

For the past few weeks, I've been re-visiting old DIY projects that I never finished. This VCO was one of them and was also one of my first DIY attampts. Anyways, I plugged it in, and had a few issues. First of all no sound, U6 (a TL074) was getting very hot (pin 8 was outputting 13.5v which seemed too much), and neither of the BC547's were producing any oscillation, just DC. I've always sucked at trouble shooting and really want to make my troubleshooting skills better, so I'm also wondering how to properly troubleshoot. The way I currently do it is pretty much guess and check.

Things to note:
I'm using the matched BC547's
I'm subbing the OPA2137 with a TL072
I'm also using a 1K resistor as a place-keeper until I get my tempco (R12)

gdavis · Posted: Wed Jul 06, 2016 6:51 pm Post subject:

Troubleshooting isn't an isolated skill. Beyond checking for obvious mistakes like shorts or putting IC's in the wrong way, it starts with being able to read and understand the schematic so you know what to expect from every point in the circuit. Then you systematically go through and probe points to narrow down the part of the circuit that's malfunctioning until you find the culprit.

U6c is configured as a comparator so it will toggle between the two rail voltages. R37 and R38 on the output (pin 8) form a voltage divider that brings the output voltage down to the expected +/-5V.

Getting hot is a sign that one of the outputs may be shorted to something. With the power off, check for shorts from pins 1 and 7 to any adjacent pins and traces. You'll want to avoid powering up if it's getting excessively hot.

U6b is part of the saw core, you won't get any oscillation anywhere if that part isn't working. I would start looking at U6 pins 5, 6 and 7. Pin 7 only goes to pin 6, R21 and R23. Make sure it's not shorted to anything adjacent.

Checking the board for shorts before it's populated is also a good idea (probably too late now). When populated you may see a few k ohms resistance between power and ground but shouldn't have any dead shorts.

I use a bench power supply with current limiting to power up newly built boards. I set the current limit just above the expected current draw. Shorts in the power traces will cause the power supply to shut down preventing most potential damage.

The part substitutions you mentioned might affect pitch accuracy but shouldn't cause the problems you're seeing.

Hubaswift · Joined: Jul 06, 2016 Posts: 91 Location: Canada

I checked pins 1 and 7, and there was no short. It read 170k if I remember correctly. Pin 7 did not seem to be shorted to anything else, and pin 5 also seemed normal. I've looked over the board, components, etc multiple times, so that's most likely not the case... dunno

micromusic · Joined: May 05, 2014 Posts: 61 Location: england

This is what I do - Print a picture of the PCB off the Yusynth site so large it fills an A4 sheet, check for solder bridges as its very easy to solder points together that shouldn't be, check your PCB 10 times. I bet you will find a fault. Then pull the ICs and check every component and PCB trace for continuity and make sure all components are correctly placed and in the right way. Then check wiring and make sure all trimmers are not at their end stops. if all is ok check the voltage regs are working correctly and you have the right voltages at the ICs

This circuit works ! Use all the recommended components

Hubaswift · Joined: Jul 06, 2016 Posts: 91 Location: Canada

gdavis · Posted: Sat Jul 09, 2016 11:47 am Post subject:

I've built several yusynth circuits and except for one IC I put in backwards, every single problem I had was related to board etching defects. My first board took days (weeks?) to get working (and I'm an electrical engineer so not new to electronics). I learned to be very diligent about getting a good clean image transfer to the board before etching and catching and fixing any issues early. They can be very difficult to pinpoint and the further along you get in the building process, the more difficult they are to find and fix. I find a magnifying lamp invaluable for this.

I've seen others claim they had bad components and swapping in a good part fixed their problem, but I've never had that happen since my TTL days in the 90's.

Hubaswift · Joined: Jul 06, 2016 Posts: 91 Location: Canada

Just a quick update, I've moved on to a second VCO, but this one has its own problems as well. The saw outputs nothing, but all of the other waveforms are outputting dc. The first time I plugged it in, one of the 10 ohm resistors went, so I fixed the short on the voltage rails and replaced the resistor, but I am wondering if this still could have damaged any of the voltage regulators or ics. It seems like I can build any other Yusynth modules, but I just can't build an oscillator Sad

.

micromusic · Joined: May 05, 2014 Posts: 61 Location: england

Hi

My first attempt at a Yu VCO also failed to work and after many hours I scrapped the whole thing and etched another board, as gdavis says it could be an etching fault, all my other VCOs (all hand matched trannies) worked first time but I checked the PCB for faults before I populated it
This circuit works so I would recommend using the parts specified (then you are on a level playing field), no component should get hot if it does there is a short somewhere, when you read through this forum most peoples problems seem to be when parts are substituted.
Again agreeing with what gdavis says I have never had a bad component from new, only when buying second hand ICs or direct from China!
Keep trying you will get it to work