Midi Resistor (R5); Midi Pullup; Rf Shields; Lcd Contrast Adjustments - Alesis HR-16 Service Manual

7.7 MIDI Pullup

•
Revision A.
Some early units were prone to sending false or incorrect
data through the MIDI out. This was traced to insufficient pull up
resistance in the 8031's TXD port. This can be solved by installing
a 4.7K resistor between R6 and R7 as shown in diagram 13.

7.8 RF Shields

•
Revision A.
In an effort to further reduce the R.F. emissions of the HR-16, a self sticking R.F. shield was
introduced. Care should be taken when installing the shield, as wrinkles in the shield may easily
Diagram 14
HR-16/HR16B Service Manual 1.00
Diagram 12
cause shorts to the main PC board. The tab of the shield should
make a firm mechanical contact to ground when the board is in
place (for a short time revision CA main PC boards were
manufactured with the solder mask covering the ground plane that
will need to be scraped off in order to achieve a proper connection)
If this is not done correctly, this may actually create more R.F. noise
than before (the shield starts acting as a capacitivly coupled
antenna). Also note that some PC board revisions have the DAC
adjust trimpot on the bottom of the board for external access. On
these units, either cutting a out a circle around the trimpot, or the
addition of electrical tape will insulate the trimpot from the shield.

7.9 LCD Contrast Adjustments

•
All Revisions.
Normally the LCD contrast is adjusted at the factory.
However, in the event an older LCD is replaced, or the contrast is
not enough to suit the user, it may be necessary to change the
hardware to achieve a suitable contrast level. Diagram 14A shows
the changes needed to convert older revisions so that the trimpot
will function in a more appropriate range. Diagram 14B shows the
latest configuration used on older revisions to achieve a nominal
contrast level. In revision AQ boards, the only way to change
contrast is by adding (or removing, if one is present) a jumper
across D21 (located by J12).
7.6 10K MIDI Resistor (R5)
•
All Revisions.
Since the HR-16 was released,
we have found that a 10K resistor
provides a better signal threshold
level to the opto isolator than the
original 47K specified in the design. If
the older value of 47K is found, then a
12K or 13K resistor soldered in
parallel with R5 will achieve the
desired threshold level.
11
Diagram 13