Table 8-4 Troubleshooting Procedures - Honeywell 7866 Operation And Maintenance Manual

Troubleshooting
7866 Digital Controller Test
At the control-unit terminal board, disconnect the sensing unit leads connected to terminals 9, 12, 13, and
14, and connect a dummy load of 30 ohms, 25 watts across 13 and 14. Voltage measured across this load
should be between 24 volts dc and 28 volts dc.
Use a six-volt battery, the current meter and an adjustable resistance (up to 20K ohms) in series across
terminals 12 (+) and 13 (–). Connect (–) battery terminal to terminal 13. Pass about 350 uA between
terminals 12 and 13. The control unit should display 0.0 % for Range 1. Refer to Section 5 for the proper
Calibration procedure, if necessary.
Leaving the above circuit connected, set the meter to its 20 mA range. Now adjust the current flow and
observe the mA reading. The current flow required to obtain a 5-volt reading across terminals 12 and 13
must not exceed 20 mA. The control unit should display 100.0 % for Range 1. Refer to Section 5 for the
proper Calibration procedure, if necessary.
If either of these values are not achievable, then the controller's analog input card is suspect. Before
drawing that conclusion, check the obvious. Wiring and bad connections often are the cause.
Sensing Unit Test
Remove the upper housing. The entire cell assembly can be left in the lower housing or removed as
desired. Connect the 26 Vdc to 32 Vdc power supply (1 ampere) to terminals 13 (–) and 14 (+) and wait
one half hour for warm-up. Connect a current meter in series with 2500 ohms between terminals 12 (+) and
13 (–).
Pass the "zero" gas through the sensing unit at a rate of 500 to 1000 cc per minute. (Provide a reference gas
flow if required.) The current reading on the test meter should be 200 uA to 500 uA. If not, adjust coarse
zero jumpers per the Coarse Zero Adjustment (R11 and R16).
Remove the "zero" gas and pass a full-scale gas through the sensing unit at a rate of 500 to 1000 cc per
minute and note the current reading on the test meter. The current must increase by at least 1 mA to 2 mA
for the change from "zero" gas to full-scale "span" gas. The change may be as much as 20 mA depending
on range.
If any of these conditions cannot be attained, probably the circuit card or the thermistors are defective.
Symptom
1. Blank display
2. Cannot adjust for zero
signal with "zero" gas.
70 7866 Triple Range Digital H

Table 8-4 Troubleshooting Procedures

Possible Cause
1a. No line voltage.
2a. ZERO adjuster in 7866 Digital
Controller is out of adjusting
range.
2b. Open signal circuit. (Causes
below-zero signal.)
2c. Detector unbalance is too
large.
and CO Analyzer/Indicator - Operation and Maintenance
2 2
Recommended Checks
1a. Check for correct voltage at terminals L1/H
and L2/N.
1b. Replace the display.
2a. Adjust coarse ZERO in sensing unit in
accordance with the Coarse Zero Adjustment.
2b. Check interconnecting wiring between 7866
Digital Controller and sensing unit for
continuity.
2c. With "zero" gas flowing, connect a high-
impedance voltmeter (on 5 V range) first
between pins A and E, then between pins A
and C.
If (VAE – VAC)/(VAE + VAC) is greater than
± –0.035, replace or clean the pair of
detectors in the cell block.
9/06