Befaco STMix help needed

[redlester]

No, those two lugs on the sides of pots are indeed mostly for extra structural support, and in theory grounding them could help reduce noise in some cases, but not connecting them shouldn't cause anything to stop working. Not soldering them down as long as you might need to make changes may even be the better way to go.

The fact that CH2 does not sum to mono suggests to me that it is the channel 2 right input that is the problem. Internally, this summing happens through a connection (so-called 'normalling') between the left and right inputs that is only active if nothing is plugged into the right input - think of it as getting a free patch cable from the left input into the right input. If all other right inputs work, the problem can pretty much only be with the channel 2 input jack, the channel 2 volume pot's right gang, or its summing resistor (R17). You can use your multimeter to check if these parts are connected together or have the expected resistances across connections.

Thanks for putting my mind at rest on the lugs. I will check the elements you suggest with a clear head later today. Cheers.

[pjbulls]

Yeah, without a clear picture of the PCB it's not easy to tell exactly what you should probe, but I can try to describe the components (my pin numbers are the same as in the schematics). These are the connections for the jacks:



Pin 3 of the left input should be connected directly (+-0 resistance) to pin 2 of the right input. Pin 3 of each input should go to one gang of the volume pot, which looks like this:



Now, I don't know which colour/gang corresponds to which input side, but pin 3 of the input jack should connect directly to pin 3 of the pot. Pin 2 of the pot should connect directly to one side of the summing resistor (R17 for right #2, R5 for left #2). All pin 1 connections (jacks + pots) should connect directly to ground. The resistance across pins 3 and 1 in each pot gang should be 100K ohms, and the resistance across R17/R5 should be 10K ohms. The resistance across pins 3 and 2 of the pot should change between 100K and 0 as you rotate it. Finally, the end of the summing resistor that isn't connected to the pot should connect directly to pin 2 of IC1 (R5) or pin 6 of IC3 (R17) - these are numbered as in the picture I posted previously.

If you've measured all of this and not found a problem, then I'm not sure what else it could be...

[redlester]

Wow that's terrific help, many thanks.

Will let you know how I get on.

[redlester]

OK here goes - see my findings in red. I have checked all inputs, pots and resistors, not just the ones for Ch2. See attached scan of the schematic where I have ticked the resistors which measure as expected and indicated the measurements for those which don't. The measurements for nearly all the resistors connected to the pots vary depending if the pot is "open" or "closed" (rotated), which is why there are two figures against them.

Where I have stated "not able to follow this on the board" this simply means I can't visually confirm as the board is so densely packed and my expertise with reading PCB's is so limited.

Pin 3 of the left input should be connected directly (+-0 resistance) to pin 2 of the right input yes, this checks out for all inputs. Pin 3 of each input should go to one gang of the volume pot, am not able to follow this on the board which looks like this:

Now, I don't know which colour/gang corresponds to which input side, but pin 3 of the input jack should connect directly to pin 3 of the pot . Pin 2 of the pot should connect directly to one side of the summing resistor (R17 for right #2, R5 for left #2). am not able to follow this on the board All pin 1 connections (jacks + pots) should connect directly to ground. Can this be checked with the multimeter? Am not sure how? The resistance across pins 3 and 1 in each pot gang should be 100K ohms, am getting about 46k and the resistance across R17/R5 should be 10K ohms. see schematic below The resistance across pins 3 and 2 of the pot should change between 100K and 0 as you rotate it. I'm getting about 46k to 0 as I rotate, for all pots (slightly different figures on each of the four pots) Finally, the end of the summing resistor that isn't connected to the pot should connect directly to pin 2 of IC1 (R5) or pin 6 of IC3 (R17) - these are numbered as in the picture I posted previously. am not able to follow this on the board

If you've measured all of this and not found a problem, then I'm not sure what else it could be...

There is clearly something very askew with all this, but unless what I've stated allows someone in the know to know the likely problem, I think I will end up just paying the fee and sending it off to Befaco.

[pjbulls]

No, sorry, the resistance values were a bit of a brainfart on my end: that's the expected situation when you are looking at just that part, but in an assembled circuit there's a lot more going on in parallel. For example, you will be measuring the right and left gang on the pot at the same time (since they are normalled together) and would expect 50K ohms (2x 100K in parallel). The four summing resistors are also in parallel and depending on the pot's positions can go as low as 2.5K (4x 10K in parallel), so actually your resistance measurements look pretty normal

The connection to ground you can check that like any other, just measure continuity (or resistance) to a known ground point such as the middle pins of the power connector. However, wouldn't expect that to be the issue given that your pot by itself seems to work.

For the other connections, you don't have to be able to 'follow them on the board', you can just probe the pins on the components themselves directly. Especially in an unpowered circuit there isn't much harm you can do randomly (but gently) poking around with a multimeter. At least for the jacks & pots it should be possible to identify which is which going by the panel if it isn't indicated on the silkscreen or build guide. One connection that I forgot but would expect to be there is the one between pin 2 and pin 3 within each jack. If it's not the direct connections between the jack & pot or pot & resistor I don't know what much else could be the problem...

[redlester]

No, sorry, the resistance values were a bit of a brainfart on my end: that's the expected situation when you are looking at just that part, but in an assembled circuit there's a lot more going on in parallel. For example, you will be measuring the right and left gang on the pot at the same time (since they are normalled together) and would expect 50K ohms (2x 100K in parallel). The four summing resistors are also in parallel and depending on the pot's positions can go as low as 2.5K (4x 10K in parallel), so actually your resistance measurements look pretty normal

Get the bunting out...the problem is sorted!

I looked at my resistance figures again, after your comments above, and logically if the figures for the summing resistors are correct as my measurements then the thing that leapt out was that I was still measuring 10k for R17, i.e. different to all the others. And as R17 is connected to the right input of channel 2 this seemed a huge give-away! So I looked closely again at my soldering and noticed one of the pins seemed to be a bit undercooked in comparison to the others. I blobbed some extra solder onto it, and promisingly it then measured similar to the others. Have just connected up and it's now all working normally on all channels.

Thanks again for your patient and really useful help on this, I've actually learned loads in the last few days by way of having to problem solve. The main thing I will take away is to check resistances for everything as I work and not after I've finished, and to have a clear idea of what figures I should be expecting. I've also learned the rudiments of following a circuit schematic. This is great experience for the future.

Right, that's Radio Music and STMix, what next? Turing Machine? Prok Drums?...

[señor-bling]

Wow, great troubleshooting thread. The STMix was one of my first DIY modules and it developed loads of issues over time. Reflowing has fixed all of them except channel 2. Will give it another go now.