[sdiy] Juno 106 repair, and maybe mods

[Metrophage]

My 19 month old boy Martin loves my Juno, it appears! He goes nuts
playing it for an hour at a time. But with the workout he provides, the
Juno has developed a problem with the key-scanning. At first I thought
it was the old voice-chip issue, but testing yields six good voices.
Every eighth key is silent, so I am poking around today in the address
and latch ICs on the CPU board. I *hope* this is where the problem is,
because to check out the keyboard PCB I'll need to remove the keys and
it might never play the same again. I did test the diodes which show on
the bottom of the key PCB, which appear ok. I hope those old 40Hxxx
logic ICs are compatible with 74HCxxx!

I know that the Juno is famous for being an oft-used synth which is
hardly ever modified. A couple of things which might be simple which I
am looking into now are extended LFO range, and bring the envelope to a
switch. This way I could do tweakier VCF modulation, maybe EG
controlled PWM. Trying to find a center-detent pot for the pitchbend
lever so I can get rid of the spring in there, My most insane ideas are
in contemplating the programmable interval timer which the DCO
waveforms come from. How much effort would it take to change the
programming of these so that I can get different frequency ranges, such
as a high-frequency driver for a wavetable board (or six).  

Some of the answer to this likely lies in the firmware ROM. On my 106s
the firmware is apparently burned into the MCU, I don't have a logic
analyser or anything clever to read it out of there. I saw on the PCB
though that there are pads for EPROMS near both MCUs which aren't used.
It appears that one version of the MCU has the firmware built-in, and
another version uses an external EPROM. What I wonder is whether or not
these MCUs will load the code from an external ROM if one is present.
Lastly, I have never seen the firmware ripped anyplace, so I don't know
how likely any of this is. Even then, I'd need to sufficiently
understand the code and modify it to do something else which is useful.
Maybe not likely but I never know until I try.

Anybody here ever mod their Juno 106?
CJ


		
__________________________________ 
Yahoo! Music Unlimited 
Access over 1 million songs. Try it free.
http://music.yahoo.com/unlimited/