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

[Jean-Pierre Desrochers]

Thanks for all your precious replies... !!!!

I'll go tweek that beast tonite.

JP

Le 2014-05-30 11:00, legion at helpwantedproductions.com a écrit :
> Hi,
> 
> This might be a stupid question but are the caps really SMT and not 
> DIP? I
> have heard of the "re-capping" procedure which needs to be done on 
> every
> original Casio MG or PG guitar but was always under the impression it 
> was
> older through hole ceramic caps.
> 
> Any verification or clarification appreciated!
> 
> Thanks
> 
> 
> David
>> 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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