SERVICE

MODEL 8559A

Second Xtal Pole (G)

The operation of the Second Xtal Pole is identical with the First Xtal Pole.

Second LC Pole (F)

Operation of the Second LC Pole is the same as the First LC Pole, except that R56* performs the same function as PIN diode CR5.

Output Buffer Amplifier (H)

The Output Buffer Amplifier is a complementary pair of transistors in which Q9 acts as a source follower with its output current boosted by Q10. The current through input FET Q9 is established by R53:

I,,, = Vb,(Q10)/R53

Which becomes:

I,,, = .7/196 or about 3 mA.

The total current through Q9 and Q10 is set by R54. The input signal path is selected by either CR15 (during crystal mode) or CR16 (during LC mode).

BANDWIDTH FILTERS No. 1 and No. 2 ASSEMBLIES A11and A13, TROUBLESHOOTING

Observe front panel switch positions in relation to the problem to isolate the area of the failure.

Check for leaky diodes and capacitors. Loading of the signal path can alter either a pole's gain or bandpass shape or both.

Isolate crystal poles from LC poles to prevent interaction of failure symptoms. Isolation of the crystal poles from the circuit is best achieved by removing CR8 and CRl5 (blocks D and G). Isolation of the LC poles is best achieved by removing CR9 and CR16 (blocks C and F).