HR-16/HR16B Service Manual 1.00
12
7.10 DAC Drift
•
All Revisions.
It has been found that some PCM-54's
linearity tend to drift with time and temperature. In
order to improve the linearity 1 leg of R60, one leg
of C47, and pin 27 of the PCM-54 should be lifted
and soldered together (see diagram 15). This
should only be necessary if the user complains of
noise after the unit warms up.
7.11 Old Keypad PC Boards
•
Revision
A
The original HR-16 came with standard PCB
type contacts (tinned copper traces), but these
were found to oxidize over time. This was
especially evident when the unit was used in a
moisture rich environment (such as near the
ocean). We have since switched to using PC
Boards with carbon contacts. We recommend
replacing the older type PC Boards with the newer
variety. In the event that the newer board is unavailable (emergency rushes, etc.) it is possible to
clean the old keypad contacts with a pencil eraser. While this will restore operation to the unit, such
a measure is strictly temporary, as the contacts will eventually reoxidize.
7.12 Lifting R41
•
Revision
A
It was found that the feed through below R41 (located near the battery) would occasionally
short directly to the resistor above it. If this happens then battery backup will not function. The
simplest solution is to lift one or both leads of the resistor slightly with a soldering iron (pushing them
up through from the bottom of the board is probably the easiest). This should prevent any physical
contact of the resistor to the feed through.
7.13 Reset Capacitor
•
Revision A & CA
The blue monolithic block RAM capacitors used throughout the board were found to be
unreliable when used in timing critical applications due to excess leakage current. C20 is a prime
example of just such an application. If C20 leaks too much, it acts as a voltage divider with R24, and
never exceeds the threshold voltage required to finish the reset process. A large percentage of
"locked up" units can be repaired by replacing this capacitor with either a ceramic disk or film
(WIMA) type of capacitor. This replacement is recommended regardless of whether the capacitor
seems to be failing or not.
7.14 Sram Capacitor
•
Revision
A
The blue monolithic block RAM capacitors used throughout the board were found to be
unreliable when used in timing critical applications due to excess leakage current. In this case the
capacitor causes excess battery drain, and is the primary cause of battery failure. These capacitors
should be replaced with either a ceramic disk or film (WIMA) type of capacitor, regardless of
whether the capacitor seems to be failing or not.
Diagram 15
