HP 5334B Service Manual page 179

NOTE
When reassembling the oven mass, the heater transistor screws must be
tightened to a torque of 0.6 newton-meters (5 in.-lbs.) (See paragraph 8-
385(c)).
f. Tilt oven oscillator assembly back and remove foam insulator between oven controller assembly and
the oven mass. Be careful not to break the two black thermistor wires attached to the oven controller
assembly.
g. Remove eight screws (four each side) securing the covers to the oven mass assembly.
h. Use two of the screws from each cover (removed in step g) to secure the boards to the oven mass for
troubleshooting.
8-399. Go to paragraph 8-414, Option 010 Oscillator Circuit Troubleshooting. When reassembling unit, reverse
the above procedure.
8-400. OPTION 010 OVEN CONTROLLER TROUBLESHOOTING
8-401. General
8-402. The oven controller section consists of three major circuits and a 10V voltage reference for increased
stability of sensitive circuits. Figure 8-9 shows the major circuits and active components involved in their opera-
a
tion.
8-403. The temperature sense circuit monitors the temperature of the oven mass and reduces the power drawn
by the oven heater transistors when the oven mass has reached operating temperature. After power-cut-back,
this circuit monitors the oven mass temperature and controls the power in the heaters to maintain the constant
temperature. The thermistor (RT1) has a negative temperature coefficient. At room temperature the thermistor
resistance is approximately 100Kn, while at operating temperature (=82OC) the resistance is approximately
9ICIZ. Shorting the thermistor to oven common makes the oven mass appear too hot to the temperature sense cir-
cuit. This in turn causes the temperature sense circuit to shut off power to the oven heaters. This technique is
used in the troubleshooting procedure.
8-404. The warm-up current limit circuit controls the maximum current the oven may draw during warm-up
(380 to 490
mA
with 20V dc oven input). This circuit is only active during the warm-up phase of the oven circuit
operation.
8-405. Normal Operation
8-406. When the oven is tested under the normal conditions ( ~ 2 5 ° C ambient temperature) it will initially draw
380 to 490 mA. After 5 to 10 minutes the oven current will start to drop. Over the next 10 to 15 minutes the oven
current will fall to the 60 to 150 mA range where it will stabilize. The oven circuit should not oscillate.
HP 5334B
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Service Manual
8-59