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6.3.1 Circuit Boards
6.3.2 Main Board
6. Check to see if there is excessive current draw in some or
all of the circuitry.
7. Be sure that correct input signals are sent to the unit.
8. Check to see that the crystal oscillator is operating and at
proper frequency.
9. Examine the reset circuitry to be sure that it is working
properly.
10. Check lithium battery for 3 VDC output when main power
is removed.
The following is a general description of the 3700 Sampler elec-
tronic circuitry. While reading this description, refer to the pic-
torial views of the circuit boards (Figures 6-6 and 6-7).
The 3700 Sampler is a microprocessor-based device that executes
a software program stored in ROM. The program is a series of
instructions that tell the microprocessor what to do in order to
accomplish the various functions that the sampler must perform.
In the following discussion of the hardware, the software is dis-
cussed only as is necessary to describe the operation of the cir-
cuitry. The A 12 VDC connector on the side of the control box is
used to connect to an external 12 V source. A 3.75 Amp PTC
device provides circuit protection.
The 3700 Sampler contains two printed circuit boards: the main
board (Figure 6-6) and the pump control board (Figure 6-7). The
display module (attached to the main circuit board) also contains a
small circuit board. This module is fully assembled when pur-
chased. It does not contain any user-serviceable parts. Unless the
display module is being replaced, do not disturb it.
P4 supplies power to the circuitry pin 1 is ground and pin 2 is 12
volts. A 5-volt switching regulator (IC U11) obtains a 5-volt rail
from the 12-volt supply.
All of the ICs used in the 3700 Samplers are powered from the
5-volt rail. IC U11 is also used as a low-voltage detector, con-
stantly monitoring the 12-volt supply. IC U17.
A voltage converter provides a 5-volt supply, which some compo-
nents require. IC U8 is the microprocessor, IC U5 is the ROM,
and IC U7 is the RAM. These three ICs make up the brains of the
3700 Sampler. The microprocessor executes the program stored
in ROM. While executing, information (program settings, sample
data, etc.) is retrieved from and stored in RAM. A 4.608 MHz
crystal oscillator (Y1) sets the microprocessor's execution speed.
The 3700 Sampler keeps track of time with a real-time clock (IC
U3). Both the RAM and IC U3 are battery-backed with a lithium
battery (B1). IC U1 is a switch that selects between battery and
system power.
Three devices allow the microprocessor to communicate with the
outside world: the 2-line, 20-characters-per-line LCD, IC U9, and
IC U18. The LCD facilitates communication with the user and is
connected through P3. IC U13, transistor Q2, thermistor R15,
3700 Portable Samplers
Section 6 Servicing
6-9
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