Tait T855 - T859 Fan Control Circuitry

D4.12 T858/859 Fault Finding M850-00
31/09/98 Copyright TEL
4.6.5 T859 Fan Control Circuitry
Key transmitter.
Does fan run at
room temperature?
Spray "freeze" on R116
or short R113.
Does the low temperature
inhibit circuit stop the fan?
Spray "freeze" on R116
OR short R113.
Check IC3 pin 14 >11V.
Check Q21 & Q20.
N
N
Y
Y
Check Q20 & Q21 collector
voltages. Check Q17 & Q18.
Check IC3 pin 14 <1V.
Check Q19.
Check Tx-key is low (at ground).
Do not key
transmitter.
Does high temperature control
circuit activate fan?
Heat R72 with tip
of soldering iron.
Check IC3 pin 8 >11V.
Check Q21 & Q20.
Fan control OK.
Heat R72 to >70°C with
tip of soldering iron.
N
Y