6-8 Maintenance 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
6.2.6 No Transmission
If no signal is transmitted, after confirming the supply voltages described in
6.2.2, a single tone approximately 1kHz at 20mV rms should be applied to
the microphone input. The voltages to be expected in the transmit mode at
various points with this input applied are shown in the circuit diagram.
6.2.7 Unlocked Synthesizer
Should the synthesizer lose lock (indicated by UL being shown on the
channel display and Unlock indicators H1 or H2 on the RF Mixer and Dual
Synthesizer PCB, being lit) the following procedure should locate the cause
of the failure:
• Verify the correct voltage on Z2 as shown on the circuit diagram
04-02450.
• Verify that the Enable, Data and Clock pulses on J3 pins 3, 4 and 5
respectively are the correct levels (0 or +5V).
• Verify the correct output level from IC6 and IC8 as shown on the circuit
diagram.
• Verify the VCO control voltages at TP8 and TP9 are within the range
shown on the circuit diagram.
• Verify that the VCO frequencies and levels (V6 and V14 emitters) are
as shown on the circuit diagram.
6.2.8 Typical PA Voltages
In order to optimise the amplifier performance the PA amplifier transistors
are matched in pairs identified by a coloured dot. Measurements taken in
this area will depend upon the matched pair of transistors fitted and the
frequency of transmission.
The tables below are a guide to the peak-to-peak voltages to be expected at
specified points in the PA circuit. They are given for full power output when
driven with a two-tone input. For the tests the supply voltage is 13.6V and
the output is terminated into a 50Ω load.
Frequency Battery V18/V19 V16/V17 V12
(MHz)
Current
(Amps)
C
Vpp
B
Vpp
C
Vpp
E
Vpp
C
Vpp
B
Vpp
2 9.7 27 2.4 13.2 0.4 3.2 0.9
4 9.8 27 2.4 13.2 0.5 3.2 0.9
6 9.2 27 1.9 12.0 0.4 2.7 0.9
8 9.0 32 2.5 13.5 0.5 2.7 0.9
16 10.4 24 2.8 14.0 0.7 5.0 1.5
18 8.9 26 2.3 11.5 0.7 6.3 1.3
24 8.9 22 2.4 12.5 0.8 7.5 1.8
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