Befaco STMix help needed

[finnurbjarna]

I've just built the Befaco STMix and am getting stereo output from all but channel 2, which is summing both the L and R input to mono. All the other channels sum L to mono and pan R right.

I've tried reflowing and checking my work but it's a pretty densely packed little module.

Any ideas where I might have messed up?

[brycecake]

I recommend posting some good pictures of your board, despite its component density, as there are some hawk eyes on here that I think secretly enjoy looking for solder bridges. Where's Waldo, for synth nerds.
It seems like the bridge would likely be near the jack legs, the legs of the pot, or maybe the resistors right after the pots. I haven't seen the board layout though, just the schematic.

[guest]

i agree with brycecake - the issue is most likely on the IN-R jack. the jack itself could be bad, or there is solder bridge or pcb trace short between the jack tip and switch.

[finnurbjarna]

Thank you, I will post some pictures in daylight.

I noticed there's some surface damage to R17, which is connected to INR2. Could that be enough to ruin the resistor?

[guest]

you can test it with your multimeter to see, but probably not. you can also verify the jack-short theory by plugging in a cable and seeing if the switch connection still exists.

[synth.void]

I had a similar issue due to a faulty pot. The R jack on my unit would take precedence over the L jack on one of the channels.

Drop a line to Befaco, they would give you directions for debugging. I ended up sending it to them with a happy ending kit, but maybe you won't have to.

[finnurbjarna]

Thank you, everyone, for your advice.

I checked R17and it measures fine. I also did the jack-short test and sure enough it shorts even with the cable in.

I removed the jack INR2 thinking to replace it but it's fine, it's the circuit shorting somewhere else.

Where does that leave me - solder bridge under one of the chips, IC3A or IC3B?

[synth.void]

Thank you, everyone, for your advice.

I checked R17and it measures fine. I also did the jack-short test and sure enough it shorts even with the cable in.

I removed the jack INR2 thinking to replace it but it's fine, it's the circuit shorting somewhere else.

Where does that leave me - solder bridge under one of the chips, IC3A or IC3B?

Have you checked the pot for that channel?

[guest]

if you still have the jack out, leave it out till you find the short. basically there are 2 traces which go to that jack - one to the tip and one to the switch. see if you can locate them and find where they go. if they run next to each other someplace, drag something sharp and pointy between them to remove any flakes of solder or copper that might be there.

[brycecake]

Where does that leave me - solder bridge under one of the chips, IC3A or IC3B?

Not likely. Almost certainly the short is before the IC, since the other channels work fine....
Looking at the schematic, the most direct path to shorting those is going to be your potentiometer for that channel. And looking at the diagram for your pot, the pin out for the legs is
A2 A1 B1 B2 B3 A3
As you can see, A1 and B1, and A3 and B3 are right next to each other. My heart tells me A3 and B3 are shorted together, but check all the pot legs for channel two, in the way guest mentions above. I have had many projects held back by invisible hairs of solder shorting things.

[finnurbjarna]

I have the pot out now (oh joy of desoldering stereo pots) and it seems to measure fine.

I will clean everything up as well as possible, double check the traces and try again...

[brycecake]

I have the pot out now (oh joy of desoldering stereo pots) and it seems to measure fine.

Oh geez I'm sorry... I didn't really think you needed to take it out, just look very closely around it. I hope i didn't cause you too much distress in desoldering that, that sounds like a nightmare!

[finnurbjarna]

I have the pot out now (oh joy of desoldering stereo pots) and it seems to measure fine.

Oh geez I'm sorry... I didn't really think you needed to take it out, just look very closely around it. I hope i didn't cause you too much distress in desoldering that, that sounds like a nightmare!

Not at all - I was only joking.

I noticed a little trace on the board that looks a lot like solder, even using the 25x loupe, and which is probably the culprit. It certainly matches brycecake's hunch. However, I cannot get it off with the de-soldering braid for love nor money. That couldn't be a fault on the board, could it? More likely damage inflicted by me by exposing a trace or some such whilst de-soldering the pot?

I hope the picture shows it, right at the top under the offset diode between the first and second unsoldered pot pads:

[guest]

its hard to tell, but that could be it. if braid doesnt wick it up, then try using an xacto knife to cut it out.

[finnurbjarna]

I had a similar issue due to a faulty pot. The R jack on my unit would take precedence over the L jack on one of the channels.

Drop a line to Befaco, they would give you directions for debugging. I ended up sending it to them with a happy ending kit, but maybe you won't have to.

I feel like I may end up sending it back to them - I've hit a brick wall with this.

Thanks, everyone, for trying to help me out with this!

[synth.void]

I had a similar issue due to a faulty pot. The R jack on my unit would take precedence over the L jack on one of the channels.

Drop a line to Befaco, they would give you directions for debugging. I ended up sending it to them with a happy ending kit, but maybe you won't have to.

I feel like I may end up sending it back to them - I've hit a brick wall with this.

Thanks, everyone, for trying to help me out with this!

There's a "happy ending" kit that you can buy separately - they would finish it for you. In my case, they even refunded the price of the happy ending as it wasn't my fault that it didn't work.

[electrosandwich]

Sorry to necro this thread - but if you still haven't fixed your issue with the module, I just thought I'd share my experience. I know basically nothing about soldering - this was only the third module I've ever soldered in my life (and I never soldered anything before that). I just got done building two of these - and I ended up having the EXACT same problem you had on the same channel (two). Everything was summing to mono in only the left side on channel two. I was able to fix it by looking at the schematic - I re-soldered the pot on channel two, and then I also re-soldered the connection on "R17" - which I saw in the schematic was the resistor between the right input of channel two and the rest of the board. Unfortunately, I didn't scientifically test this, so I don't know which of those things fixed the problem, but one (or both) of those actions fixed the problem for me, and now channel two correctly works in stereo as it should.

Also - just a question to the more experienced soldering folks out there. These befaco boards are REALLY dense, and some of the resistor holes are literally right next to each other. Like most beginning solderers, I think I tend to put too much solder on rather than not enough. This worried me that some of those resistor connectors that touch each other, because it seems impossible not to have the solder from one resistor touching the solder from another resistor. They are so close together, I can't see any way to not have this be the case. But...I just want ahead and soldered through, the manual from befaco didn't say anything about being careful not to have the solder touching on these resistors. And sure enough - both the modules I built seem to work just fine. Does it just not matter for resistors if the solder connects? Is that only an issue for stuff like ICs or whatever?

[finnurbjarna]

I admitted myself defeated in the end and sent it to Befaco under the Happy Ending scheme.

The problem was diagnosed as a short under the 2nd pot, which I thought I had checked and eliminated.

I do agree, though, that they are very dense boards and not easy to troubleshoot, but I suppose that is the trade-off for the hp-savings we all love.

[Phitar]

Does it just not matter for resistors if the solder connects? Is that only an issue for stuff like ICs or whatever?

Yes it matters no matter what the component is.
If they aren't intended to be connected electrically you don't want a solder bridge between the pads.

If they connect electrically on the schematic then it isn't an issue and is probably the reason the pads on the PCB were almost on top of each other or overlapped to begin with.

Make sense?

[electrosandwich]

Makes sense - I shall trust the PCB!

[redlester]

Am building one of these right now, have just got to the stage of inserting the three IC's into their sockets. I just want to check something.

The sockets have the curved indentation at one end for polarity, which I have installed to obviously. But the IC's themselves don't have any visual indication of polarity other than a circle towards one side. Based on what I can just about make out from the install video on the Thonk site, the ends with the circles should match up with the curved indentations in the sockets - is that correct?

The only other module I have built was the Radio Music, which had just one IC but the polarity indication was far more obvious than it is on these.

[pjbulls]

Correct, the notch and the dot both indicate the pin 1 end:


(from here)

On some smaller packages the only indication is a slanted edge on one side (also the side of pin 1).

[redlester]

Correct, the notch and the dot both indicate the pin 1 end:


(from here)

On some smaller packages the only indication is a slanted edge on one side (also the side of pin 1).

Thanks, that's really helpful.

Well I've finished my build now, but unfortunately I have a similar, but different, issue to the OP.

Channels 1, 3, 4 and Aux all work correctly.
The right input of channel 2 is completely mute, and the left input of channel 2 does NOT sum to mono, it outputs only on the left.

Have had it apart multiple times, checked the soldering everywhere but can't see any problems, am going cross-eyed I've looked at it for so long. I have a multimeter but am not sure what I need to check for, specific to the fault I am experiencing.

If I can't fix it I will either have to settle for simply never using channel 2 or using the 'Happy Ending' service. Looking at the latter I see that for countries outside of the EU the cost is almost doubled. I am in the UK.

The above would take the total amount I've paid for the module to more than if I had just bought a ready-constructed one, so if anyone has any ideas on the above - particularly in relation to what I can check with the multimeter - I would be eternally grateful. It would still be extremely useful as a three input+Aux mixer, but obviously would prefer to have it working correctly.

[redlester]

Thinking about the above the morning after, if Ch2 is not summing to mono then is it true to say the problem is unlikely to be the R input? If the left isn’t summing to mono then am thinking for someone who knows more about electronics than me that must point to something to do with the Ch2 potentiometer or something down the line from that? I feel as if my next move should be to attempt to remove, inspect and reinstate the pot, but that’s a daunting amount of de-soldering for a novice like me.

Before I do anything, one potentially daft question; I have not soldered the two lugs either side of each pot. I have assumed that these are only for locating purposes, and have only soldered the six small pins on each. Are the lugs meant to be soldered as well? I assumed not seeing as some of them are well nigh impossible to reach with a soldering iron due to the components being so densely packed, but am now thinking this is wrong and that they need to be soldered.

[pjbulls]

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.