SERVICE

MODEL8559A

Resistor R92 is an adjustable current limiter that makes possible sweep width adjustment by changing the gain of the stage. Delay compensation for FM coil swept spans is provided by U14, C14, R96*, and potentiometer R83, the delay compensation adjustment.

YTO Main Coil Fixed Driver (F)

This driver supplies current to the YTO main coil to set the start frequency of the first LO WIG-Tuned Oscillator Assembly A6) at approximately 3 GHz. Resistor R8 adjusts this frequency by changing the reference voltage at U3 and, therefore, the drive to Q5. MOSFET Q5 buffers the operational amplifier's output and supplies current drive to the YTO main coil.

Alternate IF Driver (YTO) (C)

A voltage divider, R18 and R19, form a nominal + 5V source that supplies U7 and establishes pull-up voltages on the H2, H3, and PM lines. When alternate IF is selected, current to the YTO main coil changes, shifting the sweep-center frequencies by +15 MHz/N, where N is the harmonic number associated with the selected band. Four-to-ten-line decoder U4 decodes front panel band information and activates the appropriate section of U1. This selects the resistor that is paralleled with R9 in the YTO Main Coil Fixed Driver. Altering the effective resistance of R9 changes the current drive to the YTO main coil by changing the gain of the YTO Main Coil Driver circuit.

YTO Main Coil Filter (E)

When FM coil spans (<1 MHz/div) are selected, A16Q1 connects A16C22 (both located on the motherboard) - across the main coil of the YTO to filter noise and line related signals. During wide spans (>1 MHz/div), the charge on A16C22 is maintained by US, Q1, 4 3 , and associated circuitry. Diodes CR3 and VR1 protect the filter from excessive back EMF (electromotive force) generated by the YTO.

Voltage Regulators (H) (I) (K)

Precision, temperature compensated, Zener diode VR2 provides the reference for the voltage regulators. The output of the + 14.5V supply is fed back through R39 to bias VR2, while VR3 ensures that VR2 initially turns on. Transistor 4 4 is a series pass element driven by U6 and Q5, while R35, R40, and R41 sample the output voltage and provide adjustment.

The - 12V supply tracks the + 14.5V supply and consists of a pass element, 47, driven by U8.

The - 10V regulator supplies the voltage to the TUNING control, and is heavily filtered by C3 and R33. Transistor 4 6 is the series pass element driven by U7 and resistor R29 adjusts the output voltage level.

FREQUENCY CONTROL ASSEMBLY A7, TROUBLESHOOTING

The Frequency Control Assembly A7 is the principal cause of excessive residual FM of the YIG-Tuned Oscilla- tor's output. The following are a series of tests to help isolate the source of FM to a function block on the Frequency Control Assembly. Components most likely to be the source of the FM in each block are also listed. Be sure to check the following power supply voltages, for correct level and excessive ripple, before proceeding: the + 14.5V Regulator (block H), the - 10V Regulator (block I), the - 12V Regulator (block K), and the + 15V and - 12.6V Power Supplies (block J).