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. 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 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). Diagram 13 Diagram 14 HR-16/HR16B Service Manual 1.00 11 Diagram 12