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Basic DIY Troubleshooting
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Basic DIY Troubleshooting
Hi all, I'm looking for a little guidance on troubelshooting a couple of DIY eurorack kits I've built. I've built up a really nice DIY system, but there are 2 modules that don't work as expected.. I've got a multimeter and I know I need to measure to check and see if voltages are going where they are supposed to, but I am a novice troubleshoot-er and don't know even know where to start placing meter probes to find these readings.

The first module is the Befaco HexMix VCA. Channel 3 of the module is not working. All other channels work perfect. Manu from Befaco told me to measure to see if the output of each summing section gives the same value. He also said that pin 1 of IC3 is the output of the CV summer.

Here is the schematic for reference:

I've found IC3 on the board, it is an smd component and extremely hard to reach with my probe while the boards are connected.

Is there an easier way to test than trying to touch pin 1 of the SMD chip and ground on a mini jack at the same time? I don't own any clips to help with the tests. I might buy some to make things easier. Also, can i just touch the outside metal casing of the mini jacks as a ground point or do I need to touch something else?

I'm also assuming I should be doing these tests with the module powered on but I'm worried I'm going to make a connection by accident and blow up my whole rig. Is this something I should be careful of?

Any tips or suggestions will be greatly appreciated.

IC3 doesn't seem to be related to channel 3. It is the OTA for channel 1 and 2. The output from channel 3 is pin 1 on IC201A but it is easier to just measure at the tip of the output jack for channel 3. With volume potentiometer set 0, you should measure 0V dc and ac here.

With a audio signal applied to IN3 and nothing connected to the CV3 input, the VCA should have full gain and you should see the input signal at OUT3 if you set the volume potentiometer to max. If this is not the case, look at pin 1 on IC201. If there is a signal there, you have no connection between the amplifier and the output jack. If you don't see a signal, go back one stage and look at the top of the volume potentiometer. If you don't see a signal here, go back again and look at the output of the OTA at pin 12 of IC5 or the right side of R41 or C15 in the schematic if they are easier to connect a probe to. If you still don't see anything, check the signal after the voltage divider at the input, consisting of R39 and R40. The signal level should be around 147 smaller than the input signal. IF you see a signal at the input of the OTA but not at the output, there could be something wrong with the gain control. Check the collector voltage of T5 and make sure it is far from -12V so you have a bias current to the OTA.
You've reached out to the maker, so that is a great step.

Touching the pins is probably the easiest. If you are going to continue DIY, then alligayor clips and a PCB vice are very good tools to have.

Assuming you test for continuity between ground and the mini jack and get a happy, beep, then yes, you can use that for your ground.

Yes, you need to test when the power is on. The only real tests I do with a module powered off are: 1) component continuity tests, 2) ensuring there is no continuity between power(s) and ground, and 3) looking at my build vs pictures to compare components.

Those are general tips - I'm not familiar with the module.
Most of the problems in my builds have been visually apparent. First I carefully inspect under magnification, and verify all value markings on parts, part orientations.
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