Troubleshooting
2-109
2.11.28 ACA switch test
Test 502.1 — AC amps switch
Type
Pass/Fail
Fault message
AC amps switch
Description
The 7.2mA ohms source is switched in using the /OHMA line at U223 and U220. U232 pin 2
to 3 connects the REFBUF to U233 pin 3. The 7.2mA current divides between Q211 and Q212,
then flows through Q213, R322, CR217, K201, R334 and R342. This point is the HI input ter-
minal to the DMM. The HI to AMPS jumper routes the current to the AMPS line. The 200µA
range switch U234 pin 7 to 6 is closed. 7.2mA through 1000
Ω
would normally generate a large
voltage at U225 pin 5; however the voltage is clamped at two diode drops (approximately 1.2V)
by CR218. U234 pin 2 to 3 is closed, which routes the 1.2V to the ACA line.
In the AC section, the REL3 and DCF control lines are set to route the ACA signal through U413
pin 6 to 7 and U428 pin 10 to 11 respectively. The ACLOW control line turns Q422 on, which
routes the 1.2V ACA signal to the AC volts input buffer to the AMP IN line. AMP IN is tied to
the inverting and noninverting paths of the FWR. NETOUT from U417 in the noninverting path
is monitored by the ZERO CROSSING amp for either polarity of input signal and generates the
COMP- signal that is tied to comparator U408. The comparator selects the path that AMP IN
signal will follow through the FWR with switches in U410. The plus AMP IN path is through
R452, Q412, Q411, U417, R453, U410, Q404, and U419 to pin 12 of U414 MUX. The output
of the MUX is tied to the A/D through buffer U403, ACV/A input to U222, and the A/D buffer
in X1 gain configuration.
NOTE: INPUT HI and AMPS must be externally jumpered to run this test.
Drawing reference
Analog Board; 2002-100
Components
U410, U411, U413, U414, U419, and associated components.
Bit patterns
Bit pattern*
Register
—U400—
01111100
—U811—
00001101
—U224—
00010011
—U432—
00000001
—U810—
00000011
—U206—
01110000
—U203—
10001110
—U411—
10011011
—U809—
11100111
—U207—
11001111
—U221—
01110111
—U406—
00001111
AD_STB
MUX_STB
R1_STB
R2_STB
*Bits associated with register IC terminals as follows:
Q
Q
Q
Q
Q
Q
Q
Q
87654321
87654321
87654321
87654321
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4.