Teledyne Analytical Instruments
23
Thermal Conductivity Analyzer Model 212R
tom must be evident with zero gas flowing. If this symptom is displayed only
when the sample is flowing, the problem is related to one (or more) of the five
(5) points outlined in section 7.3.
To check the temperature control circuit, use the following procedures:
1)
With the power switch ON, open the control panel door and
check to see if the fan is running. If it is not, check the left hand
fuse. This fuse protects the entire temperature control circuit
(see schematic). If the fuse will not hold, a short circuit is
indicated. Disconnect the printed circuit card and replace the
fuse. If the fuse holds (as indicated by the fan running) the
printed circuit card will have to be replaced.
NOTE:
Unless competent electronic technicians are available, TAI
recommends that a replacement printed circuit card be ordered
and the existing board returned to the factory for repair. Repair
charges (out of warrantee) will be based on time and material.
A schematic of the proportional temperature control card is
included among the drawings at the rear of the manual.
2)
If the fuse will not hold with the board removed, a short is indi
cated in the fan, the heater resistors, or the interconnecting
wiring. With the board removed, check wiring and components
for short circuits.
3)
If the fan is running, connect an AC volt meter across anyone of
the heater resistors on the fan assembly (the meter should be set
to read 50 VAC). In a properly operating circuit, the meter
should read approximately 47 volts (heater control full on) as the
control panel is open and the thermistor should be demanding
maximum heater voltage. If no voltage is present, either the
circuit card or the thermistor probe is faulty. Isolate the problem
by first disconnecting the circuit board from its socket and with
an ohmmeter check between terminals 5 and 6 of terminal strip
TS2. If a reading is obtained (disregard actual resistance) the
thermistor is intact and a new circuit card will be necessary. If
no reading is obtained, a new probe assembly will be required.
4)
If approximately 47 VAC is measured across the heater resis-
tors, a runaway, or correct, circuit is indicated. To determine
which is the case, hold a soldering iron in close proximity (1/2")
to the end of the thermistor probe and observe the meter.