Service manual

ASR-88 Kevboard
ASR-88 KEYBOARD
The ASR-88 uses the same keyboard as the KT-88 that has “bubble” switches, the ASR-88 keyboard is
only different in the adapter board. Instead of mechanical switches, this keyboard has a molded rubber
bubble under each key. The keyboard circuit board has conductive carbon contacts printed on it (which
appear as small black strips under each key). Each rubber bubble also has small conductive carbon dots
printed on the inside. The bubbles are made in strips which attach to the circuit board using small nubs.
The nubs on a strip are pushed through holes in the circuit board, in order to hold the strip in place.
As a key is pressed, it forces the bubble down until the carbon dots on the bubble hit the carbon contacts
on the circuit board. This completes the circuit. The circuit has two contacts per key, a back contact and
a front contact. The back contact closes first when a key is pressed, then the front contact closes. We
measure the amount of time between when the back contact closes and when the front contact closes.
This tells us how fast the key was hit, making the keyboard “velocity-sensitive”. In other words, we can
tell how hard the musician is playing and can adjust the volume and brightness of the sound in response
to the playing style. Each bubble switch also has a diode in series with it for proper circuit operation.
Key Response Problems
Although bubble switches are more reliable than mechanical switches, there are still many things that
can go wrong with this keyboard. If the bubble switches don’t switch in the proper order (first the back
contact, then the front contact) or if the switches don’t make clean contact, several problems can occur.
These include:
l Keys that don’t sound at all
l Erratic keys that “chatter” as they are played, held or released
l Keys that sound much louder than other keys
l Keys that sound much quieter that other keys
These problems can be caused by several things, including:
l Open or shorted traces on the circuit boards
l Bad or dirty carbon contacts on the circuit boards
l Bad or dirty carbon contacts on the bubbles
l Tom or otherwise damaged bubbles
l Bubble strips that are installed backward
l Interference between the key and the bubble (such as foreign material trapped between the key and
the bubble)
l Improper alignment between the key and the bubble
l Bad diodes
Usually failures will fall into two categories; either one key is bad, or a group of keys is bad. If a group
of keys is bad, all the keys may be grouped together (usually a group of eight) or they may be spread
across the keyboard (usually every eighth key).
If keys fail in a group of eight or every eighth key fails, the problem is most likely an open or shorted
trace on the circuit board or a problem with the keyboard processor board (that is mounted to the bottom
of the keyboard).
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ASR Service Manual