HEWLETT PACKARD (HP) 78100A
Service Manual

Models 78100A. 78101A

Section V Maintenance

a. Disconnect the 10020A Probe from the Spectrum Analyzer RF Plug-in and connect it to the 8901A Modulation Analyzer. b. Disconnect the AFC wire attached to AFC pin on HF board. c. Adjust C30 (Figure 5-9) to bring L.O. frequency to within + l o 0 Hz of that specified in Table 5-6. d. Reconnect AFC wire (9-1-3) to AFC Pin and remove bottom cover of receiver allowing access to the LF board. e. Remove power from receiver and connect TPl on LF board to ground with a clip lead. (See Figure 5-7 for location of TP l .) f. Disconnect subcarrier wire (9-7) from �SUB� pin on HF board (see Figure 5-16). g. Adjust LF Board R93 (Figure 5-7) to obtain a L.O. frequency that is greater than 5 kHz but less than 10 kHz below that frequency specified in Table 5-6. Adjustment closer to 5 kHz better protects against crosstalk. h. Disconnect LF board TPI from ground and connect to LF board terminal E with a clip lead. less than 10 kHz above that frequency specified in Table 5-6. Adjustment closer to 5 kHz better protects against crosstalk. j. Disconnect subcarrier wire from terminal E (+12V supply) and reconnect it to pin SUB.
i. The L.O. frequency should be greater than 7 kHz but

5-64. Capacitors Z l C l and C2, and Z2C1 and C2 are located at the ends of RF resonators Z1 and 22, which are transmission lines fabricated as part of the H F board. They are adjusted in two stages:
1. The image frequency is rejected by adjusting Z1C2 and

Z2C2 to resonate in series at the image frequency. The impedance at series resonance is very low and bypasses or shunts the incoming image frequency t o ground. 2. The resonators are then tuned for parallel resonance at the carrier frequency (by Z1C 1 and Z2C l), which provides the required passband and RF amplification. An interstage adjustment with C8 tunes the collector of Q1 for a pass band at the carrier frequency. a. Disconnect P2 of the HF Board (Figure 5-9) from chassis cover P1. Connect P2 to the remote antenna connector, 53, on inside of receiver rear panel (see Figure 5-16). b. Set 8656A frequency for the Carrier Frequency of the channel under test (Table 5-6). Adjust amplitude for -40 dBm. c. Use Cable B to connect the 8656A Signal Generator to the receiver rear panel input BNC antenna connector as shown in test setup in Figure 5-16. d. Connect 10020A Probe with 250 ohm tip (C- Figure 5-16) between the Spectrum Analyzer RF input and TP4 on HF board. Attach probe ground to RF ground pin on HF board. e. Remove power cord from receiver and then remove the protective covers from input resonator capacitors Z l C l , ZlC2, Z2C1 and Z2C2. Now adjust the tuning slugs on these capacitors CCW until the top of the screw is flush with the threaded shoulder. f. Reinstall power cord and apply power to receiver. g. Turn ZEROlPER DIVISION/O-1250 MHz Switch on 8554L to �Per Division� position. h. With the 8554L Frequency tuned to 22 MHz on the dial, observe minus 21.82 MHz, 0 Hz and +21.82 MHz on the spectrum analyzer (Figure 5-18). The +21.82 MHz (IF Signal) should be centered on the display. i. Tune the 8554L to 21.82 MHz with a BANDWIDTH of 300 kHz and SCAN WIDTH of 1 MHz/Division.
Note: Maintain 8656A output amplitude low enough to

k. Remove 10020A Probe (Cable A, Figure 5-16) from the HF board.

5-62. R F Amplifier Gain and Tuning.

5-63. The radio-frequency (RF) amplifier filters the input from the antenna to reject spurious signals before they can reach the mixer stage, to prevent their being converted into spurious intermediate frequencies (IF). The spurious signal primarily rejected is the image frequency. It is found at the same frequency separation (21.82 MHz) below the L.O. frequency that the carrier is found above the L.O. frequency, and thus would produce an invalid signal at the proper IF if it were mixed with the L.O. output. The RF amplifier adjustments also maximize the gain at the carrier frequency to insure satisfactory transmission range and signal quality.

078 100-2

5-25