MODEL 8559A

SERVICE

MARKER ASSEMBLY A8, CIRCUIT DESCRIPTION

The Marker Assembly A8 comprises the Marker Generator, the DPM and Second LO Drivers, the ALT IF and SIG IDENT circuits, the Auto Scan Time Drivers, and the Scan Attenuators.

Marker Generator (F)

The Marker Generator is basically a zero voltage detector. The four summed resistor voltages at pin 5 of U14 equal OV only when the sweep voltage and the tune voltage correspond to the same frequency. The marker is then displayed at that frequency. If the input of U14b (pin 5) is at OV, the outputs (pins 7 and 1) should be at OV. The anodes of CR7 and CRlO should therefore be at OV also. Resistor R45 pulls their cathodes down to about

-0 . N This turns on U6c, which normally has its emitter held to about +0.7V (+ 1.2V in fullband). As the emitter voltage of U6c increases, it turns on Q1. This pulls the video shift line down, shifting the signal and noise at the Log Amplifier Assembly A14 about one division toward the bottom of the screen. The output of the Log Amplifier Assembly A14 is permitted to be pulled low by the log shift resistor (A14R119) at its output.

DPM Driver (D)

The DPM Driver is an inverting operational amplifier circuit. The appropriate combination of input, offset, and feedback resistors is selected by U5 for the chosen frequency band (see Figure 8-30). Input control lines H2, H3, and P M carry the encoded band information. A truth table on the schematic, Figure 8-33, shows the levels of these lines during each band.

tR offset

R Feedback

R ~ n p u t

R

 

R

 

R

SELECTS

offset

Input and Feedback

Band

 

R Offset

R Feedback

 

 

 

 

 

 

 

 

 

 

 

 

FIGURE 8-30, DPM DRIVER GAIN SELECTION. SIMPLIFIED DIAGRAM