Oven Controller Troubleshooting; General; Figure 8-14. Oven Controller Block Diagram - HP 10811A Operating & Service Manual

Quartz crystal oscillator

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d. Oven heater transistors Q7 and Q8: The replacement transistors for Q7 and Q8 have formed

leads for easy installation (Part Number 10811-80001). Holding screws for Q7 and Q8 must

also be torqued to a specific force of 44 newton-metres (5 in.-Ibs.). There are several

available Pozidriv

8-69. OVEN CONTROLLER TROUBLESHOOTING

8-71.

The oven controller section consists of three major circuits and a 10V voltage reference for

increased stability of sensitive circuits.

ponents involved in their operation.

8-72.

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 100K

ohms, while at operating temperature (-82°C) the resistance is approximately 9K ohms. Shorting

the thermistor to oven common makes the oven mass appear too hot to the temperature sense

circuit. This in turn causes the temperature sense circuit to shut off power to the oven heaters.

This technique is used in the troubleshooting procedure.

8-73.

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.

torquing screwdrivers.

Figure 8-14

shows the major circuits and active com-

Model 10811A/B

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10811b