how to fix a weak and jumpy QS display

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Posted by sav on September 10, 1999 at 01:29:33:

oh, about a year ago, bought a used QS, pretty good condition, but some minor flaws. think i might've
put in a post here asking how to fix it, but i don't remember when. anyways, here the flaws, and how
i quickly (and cheaply) fixed them. i have no idea how applicable this stuff is for newer QS's, but i
can't imagine the guts being too different.

needless to say, if you choose to do this operation yourself, be sensible-- use ES protection, mark
all wires & connectors before you unplug them, etc.... and take all of this with a grain of salt. i'm not
a professional technician, just a guy trying to save some bucks. perhaps you have a better way.


symptoms: the display sucks. it's very dim, tooling around with the Global Parameter doesn't help.
perhaps it is unevenly lit too. if actual digits are crazy or don't show up, then you probably have
a different problem, like a broken LCD. sorry, no help here. search the posts.

-- turn the synth upside down, remove the 13 large black Phillips head screws. do not remove the
smaller ones, for they seem to hold the keyboard action assembly together, as well as the circuit
boards. the keys are not going to fall out or anything.

--holding the synth together with your hands, turn it rightside up (keys up). take top off, but not too far.
it is connected to the bottom by (2) grey ribbon cables, and one pair of wires (which appear to attach to
the key sensing assembly). you can disconnect the 2-wire cable in the middle. there is enough slack on
the ribbon cables to be able to flip the top over and work on the electronics without disconnecting them.
putting a towel down on the keys, and resting the semi-connected top on the towel seemed to work
for me.

--while you're at it, clean the poor thing! i found a spider's nest and some really weird hair in my QS case.
i'm not asking, and the QS isn't telling.


-- in the middle of the synth, where the display is, is a tent-shaped piece of white plastic. this is the
back of the display. apparently, the LCD display is semi-transparent, and a tiny (0.5"x5.0") circuit board
with 6 small incandescent bulbs shines through the display. remove the board by separating the
white prongs that hold it in place. when i opened my QS up, only one of six of
these lights was still working. i had to use a flashlight at gigs to see what program i was using!
this display board disconnects from the "mother"board of the synth by gently pulling on the orange
and yellow wires at its connector on the MB. be careful to support everything and not break anything!

-- i measured the voltage and current going to the bulbs: 60mA across 5V. looking up these in a
Newark Electronics (18004639275; no, i don't work for them, but they have good service, and will
sell to anyone with a credit card) catalog, they are Type 2200 (part number 50N8156), T-1 /34, 2-wire
terminal base. i bought a pack of 10 of them, came in about 3 days, total cost: $10.44. not too bad.
Radio Shack does not stock these bulbs, i called.

-- replace the old bulbs with the new ones! this will require a soldering iron and a good touch. the
original lighting board is probably assembled by machine, with pretty tight solder lines, so
removing the old bulbs and putting in new ones is probably tight work. i chose to make another one
from scratch rather than risk screwing up the old one. you can buy one-sided copper circuit boards
from Radio Shack or Newark for pretty cheap. or make it out of cardboard, if you want. use your

--reinstall the new board with new lights and enjoy the blaring visual display! (may want to
reassemble the synth first, going in opposite order from above)


this isn't really a true "fix", but a semi-permanent solution. symptoms of the problem:
doing nothing, the display would go berserk, as if one were spinning the Quad knobs back and
forth. this really sucks during the middle of a tune, as my beautiful organ patch suddenly sounds
like a wet fart, then a marimba, then a flute...

--open up the synth as above-- one cannot clean the potentiometers (pots) when the synth is
assembled. at least i haven't figured out how to remove the rubbery knobs from the shafts of the
pots without destroying both.

-- on the right side is the board which holds the Quad knobs. get yourself some good
contact cleaner (111 Trichloro, Freon, etc)-- i used stuff called SPRA KLEEN 10-8666-16 from
GC ELectronics, Rockford Illinois 61101. spray the crap out of the Quad pots from the backside.
there's a little hole in the back for just this purpose. be careful about dripping solvent-- it can
probably eat plastic if you let some sit for too long, but clean the pots well. i did this, and haven't
had a "jumpy" problem again. could probably replace the pots entirely, but that seems unnecessary.

and that's all, folks. any other good home-repair tips or suggestions?

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