Tektronix 494AP service manual O a o o o o o o o a o o o o o, O o o o A o o o o o

Theory of Operaton - 494A/494ApServtce,Vot. 1

u$36. PrN2

u603A

PIN 3

u6036, Pl{ 7

Figure7-34, Primary.egutatorInputand output waveforms.

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Ramp Generator. The ramp generatorcircuit is a gated sawtooth generator that consists of T6044. Q5023,Q6034,Q5032,and relatedcomponents. The negative excursion of the rectangularshaped signal from U6059 is coupled across T6044 to force e6094 into conduction.Thisforuvard-biasese5032. lts coll€c- tor movestoward+17 V to chargeCsoggto this value. 06034 loses drive (since the pulse coupled across T6044has died away)and the two transistorscut off. 05023 acts as a constant-cunentdrain to linearly dischargeC5038. This signalis coupledacrossdivider R5036/R6032,then apptiedthroughC6Og9to the input of comparatorU6036,which is part of the primaryregu- lator.

Prlmary Regulator. The primary regulatorcircuit consistsof comparatorU6036and U6046,photocoupler U6043,and relatedcomponents.The circuitvariesthe duty cycle of the driving signal for the inverter. The +17 y is dividedby R6038and R6097to approximatety +4.8V and apptiedto the invertinginputof U6036. Thi +5 V referenceis apptiedthrough R6022 to the non- invertinginput of u6036,where it is combinedwith the ramp signalfrom the ramp generatorstage. The non- invertinginput receivesa sawtooth signal of approxi- mately500 mV peak-to-peaksuperimposedon a +S V dc level.This is comparedwith the +4.9 V on the other input, so the comparatorswitcheswith each sawtooth cycle. Note in Figure 7-34 that as the level at pin g (which correspondsto the +I7 V supply variations)

rises and falls, the duty cyeleof th€ output waveform variesaccordingly.

The output signal of U6036is appliedto opticaliso- latoru6043,which drivesthe inputof u6069,

lnverter Loglc. This stageconsistsof steeringflip- flop U6063Band dual quad input NAND gate U6069. The flip-flopis connectedso it toggl€s to enablefirst one gate then the other. The sguare-wavgoutput from the multivibratordrives the clock inputof U60638. The signalalso enableseach gate to ready it for the other signalsthat arrive later. The output state of U60638 determineswhether the upper or lower section of U6069will be readyfor the enablingsignal.

Assumethat the Q outputof U60638is holdingpin

2of U6069high. This meansthat the complementout- put of the latchis holdingthe oppositesideof the gated pair disabled. When the output of U6043moveshigh, U6043controlsthe duty cycle of the inverter,the upper sectionof U6069 producesa low state. This causes currentto flow throughhalfthe primaryandQ6078only. On the opposite cycle of the multivibratorsignal,the latchis reset,so the lower halfof U6069is enabledand Q6077is now in the conductionpath.

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7-104

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