Tektronix 494AP service manual O o o o o O a a o a o o a a a o o o a o, Determiningcomponents 107

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Fan Drfve Circuit

The fan drive circuit provides a temperature-con-trolled current drive to the fan motor. if,e circuit pro-

thr€e-phase_drive

1y:?: "culent of approximatety 240 Hz operating freguency. The actuar ctrive circuit operatesas a ring counter.

TransistorsOl03g and e1044 form a vottag€regu_ by th€rmistor RT2O4S. The

!:tlT- _c_ontrottedvalue of RT2045variesinverselywith tf," int"in"i temperatureof the anatyzer. The therinistorand ,".Gioi nzo+2 fix the turn-onvoltageat the emitterof e1044 ai approximatety -13 V. The voltagegoes mor€ positiveas'tireandyzt,Jr warms up.

The ring counterconsistsof three stages. Because of circuitimbalances,whenthe anrtyzer-is-nrstpoweredup one-ofthe stagesbeginsto con'ductbeforethe oth- 9r1 Tfie stag€s consastof e1025 .nJ-etOzO, ,ittRlGlt/C1092 and RtOzZClOle as- ine'trequency-

e2025 anO-'-btOtg,

99t^"ITjli.nS_-components;with R1033/C1033and R2Ot9/Ct01gas the frequency_

$t^"Tjl'lg,components; and e2030*itt R20141C2012and R2016/C2O18 "nOite-OZOZO,rrequency-

Theory of Operation- 4gtA/4g4Ap Servlce,Vot. 1

Assume that the stage with e1025 anct e1020
beginsto conduct first. The collectorvottageof e1025
is near-1TV, whichfixes that pointas the most nega-
rlng consisting		ot	itrOgz, R1029, niOig,
!y"-^rl _a_
R2036,R2034,
and RiO36.Sincethe emi*er vottageof
the threecontroltransistors(OfO2O,el01g, anOOiOeO;
is the same, the voltagedivisionaroundihe resistive
ring is such that el0lg	and e2020 remain cut ofi.
{he1 the capacitivecharge that hotds etO2O in con-
duction bleeds off, the transistor cuts off and th€ next
stage can begin to conduct. Operationof the other two
s.!ag,es is prevented	until		the RC cornbination
discharges. The fan

motor inductanceworks in con- junctionwith the RC componentsto regulatethe switch_ ing of the stages.

This ring-counteraction builds up slowly until the circuit producesa three-phasedrive signatof approxi- mat€fy 240Hz. Th€ inductanceof the m-otorcoilsiound off the othenrvisesharp comers of the drive signal; so, the currentwaveform p2O2O

al pins 1, 2, and 3 looks s-imilarto the output of a half-waverectiher. The fan drive signals are phased approximately120 degrees apart.

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