hey!!!
i got verrry lucky. few weeks ago i bought jx3p with pg200 in perfect working order for about 60 euros from a local gipsy [in serbia]. it’s from the first series i think, withouth midi thru. at first i didn’t want to buy it because i have juno 60 which is considered to be big brother of jx3p, so i bought it anyway for singer of my band. for a few days i played with it and when finally singer came to me to give me money and pick his new synth, aaargh! - i wanted to keep it! well, at least, it’s not far from me - it’s still within my band 
and here are some tricks my friend found on internet:
1 Ok, I’ve stolen my musicians copy of Sound On Sound, and here are the ‘features’
that are hidden inside a Roland JX3P.
Have fun with it,
Anthony.
-8<- Snip -8<-
—Hidden Bank Access.
16 extra slots for user tones.
Hit Tape Memory.
Hit key 5 and hold it.
While still holding key 5, hit Tape Memory again.
To reset to normal operation, hit Tape Memory twice.
—Hidden Bank Tape Interface.
For saving/loading tones in the hidden bank.
Before hitting Save, Verify, or Load, go through the
Hidden Bank access steps described above.
—Poly II Mode.
This differs from standard Poly I mode in that only the
last note or notes played receive their natural release
length, and is much more suitable where certain types of
staccato playing to slow fade effects are required.
For use with original 32 presets:
Hit Tape Memory.
Hit key 3 and hold.
While holding key 3, hit Tape Memory again.
For use with the hidden bank:
Hit Tape Memory.
Hit key 6 and hold.
While holding key 6, hit Tape Memory again.
—Sequencer Lights.
The LEDs light up as sequences play - cute.
Hit and hold Rest button.
While holding Rest, hit Start/Stop.
—Manual Sequencer Play.
While in overdub mode, hit Tape Memory to advance
sequences manually.
—Modulating With Noise.
Hit Group B button and key 10.
Hit Group D button.
The following two modes were part of an EPROM update - if they
work you’ve got it…
—Dynamics When Played From A Touch-Sensitive Module.
With JX3P off, hit and hold key 5.
Turn on power.
Keyboard will cease to function (Local operation off).
Synth will now act as a touch sensitive keyboard.
Alternatively:
Hit Tape Memory.
Hit and hold key 7.
Hit Tape Memory again.
To reset the JX3P, turn it off, then on…
—For MIDI Channel 3 Operation When In Dynamics Mode.
Turn on dynamics operation as detailed above - via
either method.
Hit Tape Memory.
—To Turn Your JX3P Into A Tekkno Rave Machine.
With JX3P off, hit and hold *T*ape Memory.
Turn on power, then hit *R*est.
Then press keys 3, 0, and 3 in sequence in a squelchy manner.
Now kill yourself.
2 >How do you set the MIDI channel on the JX-3P? Or… how do I get it out
>of OMNI mode?
connect Midi in to Midi out and play more than 6 voices (or six). This trick
is mentioned a couple of times on this list.
I noticed a curious (and useful) occurence last night, due to
reconfiguring Cubase when I rearranged my room yesterday…the local
off/poly on commands I was sending to my CZ1000 were sent to my JX3P
and it stopped responding as Omni! So to set a JX3P to Omni-Off just
send a Poly-On (controller 127, data >= 1) message. At least it ends
the need to play around with MIDI cables to get the same effect 
3 The sequencer is a standard step sequencer, except that the 16th note you
are on within the bar is displayed by a led on the 1-16 buttons. So, as
you add notes, you see it advance one. The sequencer functions are
exactly as they look: 16th rest, hold-previous-note-16th(you use these
successively of course). I remember that one key let’s you go backwards
in the steps but I can’t remember which key ‘cause I’m not in front of my
jx3p now… I can’t remember the max notes either. You can use the
Roland CV to trigger the clock (each gate sets off the next 16th note in
the sequence, like the SH101’s clock in from the same cv voltage). For
arpeggiations where you want to tweak sounds as you play, this is the way
to go. For more melody-specific sequencing, you eventually have to give
up real-time tweakability for the midi-in. bummer, but it is sort of a
challenge that helps me compose between simple/complex chord prog and
melodies. Of course, you can manually switch from internal seq to
midi-in while playing.
“Thin”-sounding the jx3p is—in the sense of sort of sheering metal. Sort
of weak on the mid-range frequencies for some reason probably best
explained by Mr. Smartypants808. He once said something about the
filter… It is thin in a cold-analog way, like the Akai AK80/AK60, not
like the Korg Poly 800. That Korg can be ‘wimpy-thin’ if you don’t
really get in there and tweak everything a little off-sync/tune. If you
tweak that guy right, it sounds like the badass little ‘Commode’-64 ‘puter.
4 JX-3P test Modes:
If you hold down the 1 button while powering up you are actually entering a
test mode. The first 6 LEDs come on to show which of the 6 voices you are
playing. If 1 voice was bad the service person could quickly determine
which voice needs repair. Also, there was a way to enter test mode with
rotating (hold down 1) or numbered (hold down 2, I think) voice assignment.
:D