[sdiy] CASIO MG-500 MIDI guitar synth almost repaired but...

[Jean-Pierre Desrochers]

I'm working on a CASIO MG-500 guitar synth that needs repair.
User's manual here:
http://www.arcenson.com/public/Casio_MG-500_MG-510_user_man.pdf

and a very badly scanned service manual here:
http://www.arcenson.com/public/Casio_MG-500_MG-510_service_man_and_parts_list.pdf

At first there was no MIDI out and the 'tune' leds didn't lite.
So I checked on the net for any clues about this guitar
and its 'common' failures..
I found out that most of the SMT electrolytic caps on its boards
were prone to get bad and spill bad electrolyte on the PCB surrounding 
traces
and eventually corrod them until they get cut open.
So I did a replacement on all of them and YES they mostly
were bad with corroded pads. OK.
Now I got MIDI back out and the inner tuner is working well. Fine.

Now here is the remaining bug..

The first time I plugged the guitar MIDI out to my
PC MIDI port (using MIDI-OX MIDI command viewer)
to check the MID generated commands I noticed
an incoming MIDI signal called 'Status:FE ....Event: Active Sensing'
that keep looping all the time out of the guitar MIDI output..(!!!)

When I played each strings I could see MIDI signal
responding to my pick strucks 'through' the annoying 'FE - Active 
Sensing' messages
that still keeps going and going..
See the MIDI-OX log snapshot here:
http://www.arcenson.com/public/MG-500_MIDI_log.JPG

First I thought there was some kind of noise coming
from the HEX pickup that would be amplified by the analog stages
and keep triggering such message.
There are 6 gain tuning pots at the back of the guitar
that need to be set for good strings sensitivity for each of them.
I tried tweeking each of them but the Active Sensing message kept going
and filling all MIDI-OX log lines.
In the schematic there are 6 tests points to verify the analog
amplification/filtering outputs for each string.
They showed a nice filtered waveform when each string are played.
Right next to these tests points there are comparator opamps
for envelope and Peak detection that I didn't check yet
but I don't think they are causing this MIDI message always showing.
Maybe they are ..

Please note that when strings are plucked there MIDI messages show
'inserted' through the Active Sensing invading messages
so I think I can tell the CPU and digital stuff work and read
clearly the 6 analog signals..

I'm stuck at this point so far..

Any help welcomed.

J-Pierre