Tektronix 494AP service manual 58O

Theory of Operaton - 494A/494ApServicg Vot.1

U1016Adelaysthe markerdot to allow retracewhite gating DOT INV low to set up th€ disptay. DOT tNV

afiects th€ readoutdeflectionoutputs in the fottowing ways.

1.The horizontal output is connectedeither to ground,for a center-scr€endot, or MAX DOTCON- TROL, for a max span pointer. MAX DOT CON- TROL is proportional to the center-frequency readout offset from the center of the frequency range.

2.The U10258 output goes tow during the dot intervalto cause 0301I to insert an offsetcurrent Into th6 vertical output, to shift th€ dot position down on the screEn.

3.R/O OFF is gat€d low to switch the d€flection amplifierinputs from the TraceModeto the Readout Mode, using the marker dot horizontaland vertical signals.

When the r€trac€ one-shot tim€s out aft€r about

5.9ps, it'sQ line triggers the unblankingone-shot u10168,which sets R/o UNBLANKhigh for 5ps, via DISPLAYMKR DOT through U1G}7A. This refreshes the dot. DISPLAYMKRDOTalsoholdsDOTINWL low throughCRl013, until the dot markeris drawn. Rl015

and G1011slow the rise of DOT INTVLto preventa 'diode

spurioussignalthroughthe AND'gate.

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