From: zowie@daedalus.stanford.edu (Craig "Powderkeg" DeForest)
Subject: hardware hack: _very_ portable Yamaha keyboard
Organization: Stanford Center for Space Science and Astrophysics
Distribution: alt
Date: 10 Nov 92 04:51:19

Well, here's an OK hardware hack:

This year, I've modified my Yamaha PSR-500 keyboard for use in the Stanford 
University Marching Band.  It's the only regularly-used keyboard I know of, 
in *any* marching band.  Has anyone heard of another?

The PSR-500 is just a five-octave 'board with velocity keys, 100 voices,
and some sequencing built in.  Speakers on board, with a couple-of-watt 
internal amp.

The main difficulties were suspension and volume.  I thought about
carrying the 'board on a high-stepper, a stand usually used by drum
players, to hold an instrument about two feet in front of the wearer.
I ended up carrying the instrument sideways -- the `front' of the
'board is up, the `top' is away from me.  The board generally hangs 
diagonally, high notes slightly down and low notes up, and I play with
one hand at a time on the field.  This is OK since, for maximum punch, you
want just one or two voices at a time.

Upon opening the case -- a tedious job, with about twenty screws on
the bottom of the body -- I was delighted to discover huge quantities
of empty space inside!  As with most consumer 'boards, Yamaha wanted
the body to look impressively large, so the customer would think he'd
really gotten something -- even though two VLSI chips took care of
most board functions.

I punched two holes in opposide sides of the bottom (lid) of the case,
near the left/right edges, and routed 1" nylon shock webbing through
them.  Clips from REI, sewn on the ends (outside the 'board) made
suitable attachment points for a shoulder strap, more of the same 1"
nylon shock webbing.  I'm considering using a normal guitar strap, but
the normal clips just aren't very tough (guitars are light!) and I'm
not sure I like 'em.

Volume was a little harder.  The Yamaha uses 9-12 VDC power, so I
bought a 120W car stereo amp, on special at Radio Shack.  It wouldn't
*quite* fit into the case, so I cut off about 1/2 of the heatsink --
the ends of all the flanges, and one entire flange that had to fit
around a plastic strut in the case.  I attached the whole thing to the
keyboard's cover via four long pieces of threaded rod and two cross
members of plexiglas -- imagine stacking the cover, the amplifier, and
the plexiglas, then passing the threaded rod through aligned holes in
the cover and the plexiglas to hold it in place.

I was (as it turns out, unnecessarily) worried about thermal
conduction, so I drilled about 60 1/4" holes through the cover, next
to the (shortened) heat sink of the amplifier.

The puny barrel-type DC connector Yamaha but on the board was clearly
not up to the current the amp would suck up -- so I stuck a Molex
connector through the back of the chassis, near the original D.C.
input.  Routed it through a SPST switch to the amplifier, with smaller
leads (unswitched) to the battery posts (there's a place to put a
bunch of D-cells inside.)  A large capacitor (100,000 uF) across the
power switch helps to remove those `I'm punching the percussion at 
full volume' blues...

A quad-pole, single-throw switch allowed me to switch all four speaker
wires between the original and new amplifier, and another SPST rocker
switch served to control the power amp independently of the keyboard.
I ran lines (with 100-ohm resistors in series) from the output of the
original amp, to the low-impedance input of the power amp.
Oh -- and stuck in a huge 100,000 uF capacitor, across the power switch.
I desoldered the LED power light from the amplifier's board, and routed it to 
an LED on the front panel of the instrument, right next to the power switch.

New speaker cones went in as well: 5-1/4", 4-ohm cones sold (again at
Radio Shack) as automotive speakers.  Because of the long traverse of
the speakers, I had to build offsets for them -- so they wouldn't
strike the plastic grille on the front of the body.  Thanks again,
Yamaha, for the ton o' room!

I had (and have) a serious problem with rattle/buzz.  The Yamaha
engineers used the rear half of the chassis not only to hold the
electronics, but as a resonant chamber for the speakers.  (I partially
spoiled this by drilling holes through the back of the body -- but you
can't have everything.  Bass output suffered; I may plug 'em up
again...)  They designed it structurally for about 4 watts of output;
running > 80 watts through it causes everything to shake.  I
took everything out, and reassembled, putting dabs of silicone caulk
everywhere.  Helped a lot, though I still have some trouble with the
back of the case rattling -- it *is* only 2mm plastic sheeting.  I'm 
not sure how to eliminate the buzz entirely at high volume, except to 
epoxy the plastic cover into the case.  But that's a once-only fix...

Assembling the whole thing, one gets an unremarkable looking board:
the only clues to someone playing it are the extra two switches and an
LED.  As long as the speaker selector switch is down, the keyboard
acts normal -- but switch on the amp and flip the selector switch up,
and you need earphones!

I get power on the field from a 20 Amp-hour motorcycle battery, cost around 
$40.  With this much charge, I can (theoretically) play at full volume for
about two hours continuously.  In practice, playing music intermittently 
with the band at rallies, football games, etc. for a whole day does not 
drain the charge.  The battery fits in a camera bag that I happened to have
that's just the right size.  I cut off the strap and sewed more of the 1"
shock webbing on the bag, to make it a boxy fanny pack for the battery,
which weighs about 12-15 pounds.  

The final problem, the icing on the cake, is the question of carrying
music.  I'm a very visual person, and it helps (even with songs I know
very well) to have the music in front of me.  So I purchased a
straight-stem brass lyre-style music holder (for, I think, a trumpet
or maybe a melophone[sp?]), and bent, cut, and welded the stem to make
a rigid frame -- I bent the stem 90 degrees to one side of the lyre, then
put a diagonal strut on the back.  The whole little affair gets held on
to the left end of the 'board with two wood screws and some large washers.
So I have a regular band-instrument lyre to carry my tunes around.

Admittedly, my whole setup weighs quite a lot.  The fanny pack with the battery
doesn't bother me much -- but carrying the keyboard around all day can be 
pretty tiresome.  It weighs about 15-20 pounds, all told.  (Of course, I could
have cut that down substantially by sawing off more of the heatsink on the
amplifier, and/or throwing out the chassis of the amplifier.)  That's less than
a bass drum or brass tuba, but more than a fiberglass tuba or any other
instrument in the LSJUMB.

As for loudness -- no problem.  It's hard, with a full band, to pick out the
sound of the board -- generally, I play piccolo or high trumpet parts with
the most grating, obnoxious voice I can find.  But that's because there are
~100 other brass instruments.  From limited band events, and sparsely populated
practices, I can compete for loudness with a whole section of, say alto 
saxes, or with four or five trombones; just not with the whole ensemble.  
That's loud enough for me -- I do stand out in the quieter parts of most
pieces, and have been known to do the occasional synth-drum solo... 

--
Craig DeForest -- astrophysicist for hire.   DoD#314159; PhD#271828