ASR-88 Keyboard
If only one key is bad (or if groups of keys are bad but not in groups of eight or every eighth key), the
problem could be any of the above. The first thing to do is remove the key and see if there is anything
obviously wrong with the bubble:
Look for damage to the bubble itself.
If the bubble is damaged, the circuit board must be removed so the strip can be replaced.
Check that the bubble strip is seated flat against the circuit board.
If the strip is improperly seated, use an appropriate tool (a straightened paper clip works well; don’t
use a sharp tool as it can puncture the rubber strip) to force the nubs on the strip into the holes on the
circuit board. The strip should lay flat against the circuit board.
Check that the bubble isn’t backward. If installed correctly, the deeper of the two bubble
contacts should be at the rear of the keyboard.
If the strip is in backward, remove the circuit board, pull the strip off the circuit board, turn it around
and reinstall it.
Remove any foreign material caught between the bubble and the key.
See if the plunger on the key makes proper contact with the top of the bubble.
If the piunger on the key forces the bubble down unevenly (with one side of the bubble being much
higher than the other side), loosen the screws that hold the circuit board in place and slide the circuit
board over slightly to better align the key and the bubble, then re-tighten the screws,
Check that both diodes for that key are inserted properly (the banded end of all diodes should
face the same way).
If a diode is in backward, the circuit board must be removed and the diode must be unsoldered,
reversed and re-soldered.
Check that both diodes for that key are working properly.
Select the “diode check” setting on an ohmmeter and test the diodes. The diode should conduct
when the negative (black) lead of the ohmrneter is on the cathode (banded) end of the diode and the
positive (red) lead of the ohmmeter is on the anode (unbanded) end of the diode. The diode should
not conduct when the negative (black) lead of the ohmmeter is on the anode (unbanded) end of the
diode and the positive (red) lead of the ohmmeter is on the cathode (banded) end of the diode. If a
diode is bad, replace it with a lN914B diode.
If there is no obvious problem, remove the circuit board:
l Examine the circuit board for short circuits.
These are usually caused by solder bridging. Touch up any shorts with a soldering iron and/or razor
knife.
l Look for open traces.
These usually occur at the break-away points along the edge of the board and near the connectors.
Solder a wire jumper in place to fix any broken connections.
l Remove the bubble strip and clean both the circuit board contacts and the bubble contacts
with alcohol and a cotton swab.
Allow them to air dry before putting the bubbles back on the circuit board.
If after all this the keyboard still doesn’t work properly, replace the entire keyboard assembly.
ASR Service Manual
21
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