Finally a few free moments to work on the MSTE again
In preparation for recapping the PSU I've measured the various voltages and they all seem to be within spec. How "clean" and stable the voltage rails are is another question which I have no idea.
Next I've started to check the IC sockets and have so far removed and re-inserted all the DIL type ICs. The PLCC chips are another story as I need some more time for that, given that I don't have a dedicated PLCC extractor tool, but very carefully pry them out.
It may very well be that there is some contact problem(s) somewhere, but at the same time things seem to point towards the opto-isolator (U-302). I say this because I finally got the computer to boot, so I tried out Notator again, but as before no MIDI input
. But then I realized (I might have mentioned this before) that the missing keyboard response (no click while the TOS 2.06 memory test is running), and no response when pressing the keys as well as missing mouse response had to do with having the MIDI IN cable connected from the synth.
Furthermore I attached my old MIDI input-selector box (which I also added a "soft THRU"/"hard THRU" switch to, utilizing the non-standard MIDI connection so I could play other synths without running a sequencer program) and the "MIDI activity" LED I added lights up constantly!
Now, this is a DX-7 I attached, which is know to output an "active sensing" message, but this should flash the LED, not light it constantly. To me, this indicates a short between the MIDI IN and OUT jacks. An estimated guess tells me the opto-isolator is broken.
I decided to try out my MIDI switchbox before soldering new MIDI cables for testing.
Another discovery: I mentioned that the MSTE schematic says that U-302 is a Sharp PC-900, and that the one actually used was another type. Well, I misread the markings. It actually says H11L3 (not HIL3 as previously quoted), and that IC is still available for purchase without going to eBay etc. So that's good news. So what do you think? Does it sound like a broken opto-isolator could be the reason for my problems? I could desolder it, put in a socket and try out a new H11L3.
Well, that's a little update. I'm not giving up quite yet