Problem with first group of 4 MEC switches on Le-Mec RH PCB

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Thanks and well done for such a beautifully engineered product (and excellent video guide). I’ve got to step 37 (Testing MEC LEDs). Everything works as expected apart from the left most 4 MEC switches on the Le-Mec RH PCB (SW17 to 20).

They light up (and register in MIOS studio) but also light up both RGB LEDs above in the same column (and register associated switch events plus the encoder press in MIOS studio).

Debug logs in MIOS studio when pressing a single button (SW17) running seq_l:

As well as the additional Matias switch and encoder press inputs from the same row being detected, there are also two more unidentified events with ‘No event assigned’.

[232840.726] MBNG_MATRIX_NotifyToggle(1, 25, 0)
[232840.726] MBNG_DIN_NotifyToggle(2025, 0)
[232840.726] [EVENT] id=BUTTON:2025 hw_id=BUTTON:2025 bank=0 fwd_id=LED:2025 type=NoteOn value=0 label=
[232840.727] MBNG_DOUT_NotifyReceivedValue(2025, 127)
[232840.727] MBNG_MATRIX_DOUT_NotifyReceivedValue(2, 25, 127)
[232840.852] MBNG_MATRIX_NotifyToggle(1, 9, 0)
[232840.852] MBNG_DIN_NotifyToggle(2009, 0)
[232840.852] [EVENT] id=BUTTON:2009 hw_id=BUTTON:2009 bank=0 fwd_id=LED:2010 type=NoteOn value=0 label=
[232840.853] MBNG_DOUT_NotifyReceivedValue(2010, 127)
[232840.853] MBNG_MATRIX_DOUT_NotifyReceivedValue(2, 10, 127)
[232840.857] MBNG_MATRIX_NotifyToggle(1, 41, 0)
[232840.857] MBNG_DIN_NotifyToggle(2041, 0)
[232840.857] No event assigned to BUTTON hw_id=2041
[232840.862] MBNG_MATRIX_NotifyToggle(1, 57, 0)
[232840.862] MBNG_DIN_NotifyToggle(2057, 0)
[232840.862] No event assigned to BUTTON hw_id=2057
[232840.866] MBNG_MATRIX_NotifyToggle(1, 17, 0)
[232840.866] MBNG_DIN_NotifyToggle(2017, 0)
[232840.866] [EVENT] id=BUTTON:2017 hw_id=BUTTON:2017 bank=0 fwd_id=LED:2018 type=NoteOn value=0 label=
[232840.866] MBNG_DOUT_NotifyReceivedValue(2018, 127)
[232840.866] MBNG_MATRIX_DOUT_NotifyReceivedValue(2, 18, 127)
[232840.871] MBNG_MATRIX_NotifyToggle(1, 33, 0)
[232840.871] MBNG_DIN_NotifyToggle(2033, 0)
[232840.871] No event assigned to BUTTON hw_id=2033
[232840.875] MBNG_MATRIX_NotifyToggle(1, 49, 0)
[232840.875] MBNG_DIN_NotifyToggle(2049, 0)
[232840.875] No event assigned to BUTTON hw_id=2049
[232840.880] MBNG_MATRIX_NotifyToggle(1, 1, 0)
[232840.880] MBNG_DIN_NotifyToggle(2001, 0)
[232840.880] [EVENT] id=BUTTON:2001 hw_id=BUTTON:2001 bank=0 fwd_id=LED:2009 type=NoteOn value=0 label=
[232840.880] MBNG_DOUT_NotifyReceivedValue(2009, 127)
[232840.880] MBNG_MATRIX_DOUT_NotifyReceivedValue(2, 9, 127)

Any idea where to start?

Feels like the problem should be a big enough clue for me to work it out but I’ve had a poke around and can’t find anything yet.

The offending Le-Mec board:

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Hi John, looks pretty good so far! Thanks for posting a clear error description and photos.

From what I understand, the whole DOUT column is triggering on each press of a MEC switch. What is the behaviour with the other switches in the column?

Can you check the soldering of R6?

Best,
Andy

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Correct. The other switches in the column work fine (I haven’t soldered the Matias switches obviously but testing using a wire jumper sends the correct messages to MIOS studio and the corresponding LEDs light up correctly, same for the encoder press).

The soldering on R6 did look wonky but it was connected, I reflowed to make sure and same issue.

An additional clue I forgot to record is pressing the other 4 Mec switches on this board makes the first corresponding Mec LEDs flash on very faintly (e.g. pressing SW21 lights up SW7 faintly and SW22 lights up SW18). This doesn’t happen with the Le-Mec LH. The buttons register in MIOS studio correctly though.

Thanks for such a quick response!

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For the second video, slight ghosting is fairly normal. You’re not likely to see it when the caps go on.

Do you have a cold joint on D28, pin 3? Generally resolder these components, as I suspect that the issue is on that sink row (four switches at a time are sunk on one transistor). If the transistor is slow to turn off (the Schottky helps in this regard), then at a different phase of the matrix scan, other DINs might register as on and MIOS will interpret that as a switch event and trigger the LEDs.

Other ideas:
Some things to check:
For SW19, do you measure conductivity where the PCB silkscreen is drawn? The switch internals provide a connection here.
Are all transistors the correct type and not confused with diodes? I think that’s the case but I can’t read the labels. Did a PNP transistor end up somewhere?
Are all LEDs soldered in with the correct polarity? What is the forward voltage of each LED? It can happen that there is one with a particularly low Vf, which I think I remember can cause issues.

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Thanks Andy! All sorted. T4 was bad, replaced it and all working (thankfully I think there are 2 spares in your mouser BOM). Would have taken me ages to find that without your helpful pointer (though now it seems obvious, I wish I’d worked it out myself!).

Interestingly, it also stopped the previous LED ghosting (which was quite bright, not captured well enough in the video). I guess that makes sense if the transistor was blown and allowing current through the wrong way.

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Awesome work!