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Models 78100A, 78101A Section V
Maintenance
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I d. If both supplies are bad, check U4, Q 1 and -1 2V regu- d. At maximum mixer output, the Spectrum Analyzer
lator passive components. should read at least -32 dBm. If not, measure the local
oscillator (LO) output at TPl. It should be greater than
PROCEDURE 12: Low power from Transmitter. -12 dBm at the fourth harmonic of the crystal oscillator
(refer to Table 5-6 for a value that is four times the crystal
With clip leads, attach an 8.4 volt battery or power supply frequency). If the L.O. power output is less than -12 dBm,
to the battery terminals, with a current meter in series with attempt to peak the power using C21 and C17. If either
! one lead. adjustment causes more than 3 dBm increase in power out
of the associated amplifier stage, check the components in
a. If power is (1.25 mW and the current drawn is 6.5 mA that stage. Failures of Q3 or Q4 result in abnormal emitter
I +I mA, the second (diode) frequency doubler (Q22 and cir- voltages. Note that the bias on Q4 is altered if the crystal
cuitry) may be faulty. Use the Spectrum Analyzer and a oscillator is inoperative.
coil to look for the 200 MHz at Q22 output.
e. If the L.O. power is OK, RF amplifier power or mixer
b. If the current drawn is (4 mA, check the first fre- power may be deficient. To detect faulty transistors,
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I quency doubler, Q21. It is likely that the tuning capacitors measure collector voltages of Q1 and Q2, and the source
C25, C28, C30, C31 and C33 are faulty or misadjusted. voltage of Q6. To detect the failure of passive components
These capacitors are easily broken. Check C32, CR7 and in the first RF amplifier stage, try to increase gain by ad-
L5 also. justing ZlC1. Failures in either stage would make C8 ad-
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justments effective in increasing gain.
c. If the current drawn is between 4 mA and 5.5 mA
(after tuning), check that R55 has a tuning effect. If it Note: Adjustments for this test using Z2C1 are not valid.
does, check Q22. If R55 has no affect, check Q21 or CR8.
f. If RF gain (checked in step e) is good, check the
d. If harmonics are not 30 db below, check C29, C32,
IF gain. Install Telemetry transmitter into Test Box
'1 andL5. 0078063030, and set attenuator on test box to 0 dB. Con-
nect test box to receiver rear panel ANTENNA connector.
PROCEDURE 13: Receiver HF Board.
Attach 10002A Resistive Divider Probe, 50: 1 ratio, with
10035A pincer jaw tip 5060-0417, found in 10035A Probe
Excessive power loss causes RANGElBATT Light to come
Tip Kit, to the input of the 1740A Scope.
on; fault is not in antenna or in receiver internal antenna
cable.
Observe signal at TP5 of HF Board (Figure 5-11). It
a. Connect receiver internal antenna cable to rear panel should be possible to attenuate with the test box by at least
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antenna connector 53. 80 dB before the signal at TP5 drops to less than 0.5V pp.
If not, try to isolate the IF stages by peaking them: if C60
b. Set 8656A Signal Generator frequency for the Carrier increases IF gain, check U2; if C51 only can increase gain,
Frequency of the channel under test (Table 5-6). Adjust check U1.
amplitude for -40 dBm. Set the Spectrum Analyzer Display
section as follows: g. Check U3 for loading of TP5.
Scan Width 1 MHz/Div PROCEDURE 14: Receiver. Transmitter is off, but ELEC-
Bandwidth 300 kHz/Div TRODE light is on.
Frequency 21.82 MHz
a. On the Receiver LF Board, check voltage on the posi-
I c. Connect the 8656A Signal Generator to the receiver tive end of C9. It should be noise voltage (subcarrier noise
ANTENNA connector. Make sure the transmitter is turned determined in troubleshooting tree) pp, less one volt. If
off. Connect the 10020A, 250-ohm resistive divider test not, check the Threshold Lo Comparator circuit (short
probe to the Spectrum Analyzer input. Measure the mixer time-constant pp detector).
output (TP4) at 21.82 MHz. Fine tune the 8656A Signal
Generator to obtain maximum mixer output. As the genera- b. Check voltage on positive end of C7. It should be noise
tor is tuned, the mixer output should dip sharply at exactly voltage pp, less one volt. If not, check the Threshold Hi
i 21.82 MHz. If the frequency of the mixer output differs Comparator circuit (long timeconstant pp detector).
from the frequency of the dip by more than 1 MHz, check
mixer Q6. c. Check CR10.
078 100-2 5-1 5 |
ACEC
