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

[Pete Hartman]

Active sensing is totally normal, expected and yes, there are tons of
them.  Set MIDI-OX to ignore them, most keyboards will do the same.

On Fri, May 30, 2014 at 8:44 AM, Jean-Pierre Desrochers
<jpdesroc at oricom.ca> wrote:
> 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
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