TM 11-6625-2781-14&P-2
Q1 and Q2 are common-emitter cascaded amplifiers
required. If the signal is low or missing proceed to step 1 and if
employing collector to base feedback. CR1 and CR2
necessary, step 4 If the trouble is found and remedied in step
determine quiescent collector voltage. C2 and C7 bypass the
3 and/or step 2 repeat steps to verify proper mixer operation.
zener diodes to provide a signal feedback path. L2 and L5 are
used to peak stage gain and for parasitic suppression. C8
MIXER DIODE REPLACEMENT
adjusts the overall gain of Q1/Q2 to 14 dB. The 230 MH tuned
The mixer diode, A9A2 CR 3, 4, 5 and 6 can be replaced if a
trap consisting of L8 and C10 in the Q collector circuit
certain amount of care is exercised. The following tips should
prevents possible residual responses due to harmonic mixing
be followed when replacing the diodes to avoid damage to the
of the first and second local oscillator signals. The 100 MHz
new diodes. Refer to Figure 8-29, Service Sheet 4, for
tuned trap consisting of C13 and L10 in the Q1 base circuit
component identification and location.
prevents low frequency residual responses due to high
frequency signals at the input.
1. Remove the A9A2 1st Mixer Assy, HP Part No. 08553-
6002 from the A9 200 MHz IF Assembly.
TEST PROCEDURE
2. Remove the bottom cover of the mixer assy and remove
Disconnect the first mixer assembly and connect a 200 MHz, -
CR3, CR4, CR5, and CR6 using a 231/2 watt soldering
17 dBm signal from the HP 608F to the input of the IF
iron. Note the polarity and positioning of each diode as it is
amplifier. Connect the HP 8405A to TP B (input to the 200
removed.
MHz filter). A reading of approximately -3 dBm should be
observed. If the correct signal level is present proceed to step
3. Clean the vacated holes in the printed circuit board using a
4. If not, check Q1/Q2 and associated components. The
solder puller.
signal level at TP D should be approximately -10 dBm.
4. The replacement diodes are contained in a matched set of
4. 200 MHz BANDPASS FILTER
four, HP Part No. 10514-8454. Pre-form each diode to fit
the printed circuit board spacing; trim off any excess lead
The 200 MHz Bandpass Filter is a passive filter' consisting of
length.
three coils wound on a common form; separate input and
output coils, and switch capacitors. Three of the capacitors,
5. Again, using a 23/2 watt soldering iron and heat sink,
C3, C5 and C6 are adjusted for maximum flatness at 200
apply heat and solder as quickly as possible to secure
MHz 2 MHz.
each diode in place.
TEST PROCEDURE
6. Remove excess rosin from the printed circuit board and
replace the mixer cover.
With the -3 dBm signal specified in step 3 applied to the input
of the bandpass filter, connect the HP 8405A to the output of
7. Return the mixer assembly to the A9 board. Install the A9
the bandpass filter TP C (this output is available at the center
assembly in the 8553B.
conductor of a feedthrough capacitor that is not used for
coupling purposes). The reading observed should be
8. Check the operation of the 1st mixer per step 2.
approximately -6 dBm. If the correct signal is obtained the IF
assembly is serviceable. If the signal observed is not correct,
3. 200 MHz IF AMPLIFIER
repair or replace the 200 MHz Bandpass Filter.
Simplified Test Procedure Tree
