Midi In not working on vintage synth

The synth is a Sequential Multitrak - Midi Out transmits as expected (both notes and sysex) but it appears the Midi In is “dead”; midi notes from PC (via DAW or MidiOx) aren’t played and sysex isn’t received (but transmitted ok).
I’ve checked the MIDI IN socket - it seems fine. Then I check the tracing to the first components for continuity and is fine. So, I’m excluding mechanical failure of the socket or surrounding area.
Any idea?

Before you go too far, is everything on the right channel?

MIDI IN runs to an Opto-Isolator, pins 1 & 2, with output on pin 4 going to a 68A50 Communications interface with a 270 ohm pull-up. My copy of the schematic is poor, but it looks like pin 6 is +5v and pin 5 goes to ground. If you have a scope, I’d put it on pin 4 and see if you have any activity.

It’s possible that you took some transient that killed the opto-isolator, but it should have protected the 68A50’s Rx line.

I’ve tried different Midi Channels and Modes - and all the various permutation with Program Change On/Off, Wheel change On/Off and other hidden Midi switches. Interestingly the Midi info about this synth is very scarce…the User Manual for example doesn’t tell how to receive/send Sysex dumps (but plenty on Tape transfers) nor how to trigger the hidden Midi switches (I had to serch online and sometimes info is unreliable). All in all, very poor Midi implementation (and this synth was produced a couple of years after the SixTrak!).
I haven’t yet done the checks you are suggesting and I’m also inclined to think there is something wrong with the Opto-Isolator (Midi Out doensn’t use it…am I right?).
Then the power supply is home-made and a bit suspect - it gives a bit less than required and I’m waiting to receive a new one (hopefully in a few days). For example some parameters don’t reach the full range; I thought about the potentiometer but maybe the voltage is just too low and not enough to take full control of this synth and to turn on the optoisolator? (maybe I’m talking garbage…I’m not an expert nor a professional).

I think Sequential was even at the front of MIDI… I ran across GliGli’s page where he talks about developing new firmware, and he references backing up patches over MIDI. I wonder if that’s something he implemented

You are correct about the opto-isolator. It is just on the input line. Presumably any device you connect will also have an opto-isolator on its input.

A home-made power supply? Oh, this is interesting… Someone must have had some issues with it, so I think you may be on the right track. The schematics just show five lines coming from a 15v transformer, and then off to a rectifier and a few regulators. Low AC might give you some problems with the +15v line, (used for the DACs,) but the +5v line needed for the CPU and opto-isolator should be fine. I’d still check the output of the four regulators with a scope to make sure there isn’t any significant ripple.

I wasted six months on a power supply with intermittent ripple. The ripple made everything sound awful. I thought the power supply was poorly designed! Turned out to be an issue with the mains. Low voltage in, low voltage out…

About the GliGli - I’m a bit confused about what the mod does - there is another mod listed on Tauntek and I was wrongly assuming it was the same. The GliGli seems a performance enhancer while the one in the Tauntek blog is specifically developed to improve the Midi of the Multitrak.

I’ve received the new PSU - unfortunately the DIN doesn’t fit the multitrak - it needs a 240 degrees rather than the normal MIDI 5 pins at 180 degrees (so I’ve ordered a 240 degrees DIN plug). Then in the rush of testing directly on the pins I managed to short the 2 fuses (measuring AC is tricky and not something I do normally). Anyhow, another hiccup, I have to find a replacement for the 2 blown fuses: 5a 240v?! but the Output is 16vac 2.5A 40VA - then why 2 fuses? (don’t know much about these center tap transformers).

I’m changed the DIN plug (from 180 to 240 degrees) of the new PSU and it works…(for anyone desperate for a psu replacement I suggest this “Colorado” also mentioned in the Tauntek blog) but the problems persists. I believe the optoisolator (MIDI IN) needs to be checked.

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Update - in fact MIDI In was “working” all along…the problem was the connection between the midi plug and the socket in the synth (overused socket and poor quality midi plug). A simple continuity test with the plug inserted is what I should had done from the start: more in detail; I should had plugged the cable and check continuity from the end of the cable to the Midi In socket (I did check the socket’s contact for continuity with the board - that was ok).

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