[sdiy] U-20 questions

[WeAreAs1 at aol.com]

In a message dated 6/22/05 11:26:14 PM, sounddoctorin at imt.net writes:

<< Tony Kalomiris wrote:


> Hi Tim,

>

> I have a U-20 here with 'dead' keys problem too, except it doesn't 

> respond even over MIDI.

> Only some notes in each octave (I forget which) work.I removed every 

> key, spring - pain in the arse !

> I washed the keys, but didn't touch the carbon pads. >>

Just to be perfectly clear, are you referring to the MIDI In or the MIDI Out? 
 If you're talking about the MIDI Out, then, yeah, the problem probably does 
lie somewhere in the keyboard or key scanning circuit.  That unit has one of 
those Roland Key Scanning Gate Arrays, and they can go bad -- but only very 
rarely, so don't jump to that first, especially if you're inexperienced with 
repairing surface-mount IC's.

Also, do you have the same dead keys no matter what sound you call up?  Try 
lots of different sounds, just to make sure -- not just two or three adjacent 
sounds.  The reason I ask is because very occasionally, one of the Mask Wave 
ROM IC's can go bad, and you might lose just some of the sounds, or some of the 
key ranges -- but the specific keys that go south would not always be the same 
ones for every patch.  I have seen this happen on U-20's before.  The fix was 
to replace the Mask Wave ROM IC -- about $35.00 from Roland.  If this is the 
problem, then you would have the same dead notes whether you played them from 
the keyboard OR from MIDI (by controlling the U-20 via an external MIDI 
keyboard plugged into the U-20's MIDI Input).  If you try this test with an external 
MIDI controller, and the bad notes do work with the external controller, then 
the Wave ROMs are definitely NOT the problem, and you should concentrate on 
the keyboard (first) and the key scanning stuff (second).  If the problem 
exists both from the U-20 keyboard and from an external MIDI keyboard, then your 
problem *definitely* has nothing to do with the U-20's keyboard or key contacts, 
and the Mask ROMs start looking rather suspect.  

I don't have the schematic handy, so I am not sure whether the MIDI IN is 
routed directly into that Gate Array IC (which is possible, but still unlikely, 
even for an oddball unit like the U-20...).  Bob W. <sounddoctorin at imt.net> has 
the schem, and can let us know if this is or isn't the case.  If the U-20 
MIDI In does go through the Gate Array, and the trouble shows up both with the 
U-20 keys and with MIDI Input, then the Gate Array could be the problem .  
Again, you really don't want it to be the Gate Array, because you're probably not 
qualified to repair that yourself (without destroying things in the process).  
BTW, sometimes Roland puts this Gate Array IC on a separate circuit board, and 
not on the Main Board (like, for intance, on the D-50) -- again, Bob can 
quickly verify this for you.  If the trouble turns out to be related to the Gate 
Array, I would first VERY CAREFULLY reflow the solder on all the pins of the 
Gate Array IC.  Again, not an easy task for someone without surface-mount 
experience, but definitely easier than actually removing and replacing the chip.  
This sort of reflow is best done with liquid solder flux, and NOT with fresh 
solder.  Use a magnifying glass to check your work.  Did I mention that it must 
be done VERY CAREFULLY?  Just making sure.

On the subject of things that you probably are qualified to repair yourself, 
take a close look at all of the diodes on the keyboard contact board.  Use a 
magnifying glass if you've got one.  Look for cracked or broken diodes, or 
diodes that may have bad solder connections on the board.  If you have a meter 
that can check diodes, test each one of them (you don't need to remove them from 
the PCB to test them).  You may have a diode or diodes that are internally 
bad, and the only way to see this is with your meter.  You can replace those 
diodes with any standard small-signal silicon diode, such as 1N4148 or 1N914  -- 
widely available everywhere.  Also look carefully at the solder connections for 
the connectors for the keyboard scanning lines both at the keyboard end as 
well as at the Main Board end.  Go ahead and touch up all of these connections 
with fresh solder -- it'll only take a minute or two, and it may instantly nail 
the problem.

Michael Bacich