SFX-6: cannot get MIDI OUT to work (photos/annotations included, and my understanding about the circuits)

Elektron Gear Monomachine
#1

Hello friends,

Lately I’ve got a SFX-6 to play with. Everything is functioning well except that nothing gets transmitted through MIDI-OUT. I have verified it is not a settings issue, because there’s nothing coming through even when I attempt sysex file send. Then I decided to do some simple diagnostics with a meter. Found MIDI-OUT pin 4 shorted to GND… which should’ve been 5V->R220->PIN4 according to common midi pinout.
I opened the chasis and removed the attached capacitor but then I realized the difference – in the common pinout, PIN4 connects to pull down R220 then 5V but here PIN4 is straightly TX->inverter, and the other end PIN5 is grounded. Not sure why they chose to implement it like this…

So I conclude that, either the inverter (74HC4049) is failing – not likely, because MIDI-IN is working fine, or, the micro controller TX is malfunctioning…

Here I’m attaching my notes on this issue:

MNM_MIDI
MNM_MIDI.jpg2685×1521 1.18 MB

Any ideas?

#2

Keep in mind that the 74HC4049 packs six independent level shifters into a single package. It’s entirely possible that the one that’s used for the OUT is burned out while the five others are still happily doing their thing. :slight_smile:

I’d replace it.

#3

Hey t thanks for your prompt reply! You’re right, can’t say the 74 chip is “totally functional” unless I probe it further. In the photo, the through hole annotated ‘2A’ is MCU TX and there’s actually a debug port on the left (not shown in the pic). Thinking about extending it with a jump wire and measure the voltage.

BTW, the circled capacitor is actually conductive (functioning as if it is a 0.02Ohm resistor… or is it really a voltage-sensitive resistor for protection?):open_mouth: If I later want to restore the connection, can I just short it? Sorry it’s a noob question :grinning:

#4

If I’m not mistaken, the 220Ω resistors should be there according to the MIDI specs. Regardless, I wouldn’t just take out parts that the manufacturer has put in; they probably have a reason for having it there.

#5

I too think there should be a 220Ω resistor, but I measured 430Ω at the other PIN – when connected, they should form a closed loop. I’m shorting the connection in ‘lab condition’ and doing diagnostics with an Arduino. The debug port inside the machine is awesome… Will get back to you when I obtain data – we’ll be able to see if TX is functional.

#6

O.k. got the results. TX is working, I get 3.3V on TX vs. GND, when I press a key, the meter displays slight disturbances – the MCU is pulling it down to transmit. However, at the inverter output, the voltage is always measured 0.15V vs. GND…

Adding 4049 to my next batch of chip purchase :slight_smile:
Will report back when I have progress.

#7

Please do. Sounds like you found the issue right there. :slight_smile:

#8

Can’t wait till 4049 arrive, I hacked in a 74HC04. But to my surprise it goes the same – MIDI IN working, MIDI OUT not. Measuring the output of 74HC04 I get 0.15V, but I can see disturbance in the reading when I press keys. Rolling up an Arduino monitor now.

#9

Hooking up Arduino software serial monitor at 31250baud/s.
Disconnect external MIDI-OUT connection to PC.
Attach monitor to MIDI-OUT pin.
A lot of garbage gets through, but when I push the joystick, I can see patterns like increasing number, interleaved with garbage (0s and 1s).
Connect external MIDI-OUT, keep monitoring on the pin.
Nothing comes through. A constant 0.15V.
Will dismount the parts and insert a standard midi-out tomorrow.

#10

hi @v-yadli, was the ‘unkown capacitor’ indeed a capacitor or a resistor instead? you would usually have a resistor for each pin on the output (and so for the thru port) as @t mentioned. i would probably also check that MIDI messages are being generated on the TX line (skipping the inverter)

#11

Update: @guga well it’s not a capacitor after all, there’re four plates below when I remove it, so I think it is a filter for EMR purpose, not a big deal if I short it in a “safehouse”.

I’ve also tried to measure the signal on the TX port directly using my arduino analog input pin. Weird, I get different signal when I repeatedly press a key (during each key press, the arduino oscilloscope gets triggered and captures recent signal into buffer).
I will attach the signal later.