E-8. Relay Board General Operation
The internal 640N PCB labeled “620-relay
board” replaces the range switch S1 on the main
board. IC-1 is actually a ribbon cable header
that goes to the top panel and connects to the
range push buttons (covered by silicone rubber
boots) and their corresponding panel mount
LEDS. IC-2 is a latch that will latch its output to
the corresponding range push button input. Its
output turns on the front panel LED (acts as an
range mode indicator LED) and a corresponding
relay that connects to the main PCB to select the
range. RL1 is for the 20 Ohm range, RL2 is for
the 200 Ohm range, RL3 is for the 2K Ohm
range, and RL4 is for the 200K Ohm range.
For 630 versions with the Diode Test (Forward
Voltage) option, when the diode test is selected
the relay board selects the 200 Ohm test current,
and disconnects diode D203.
Summary of Contents for 640N
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