SERVICING
4.3.2 REFERENCE OSCILLATOR (U806)
Check the signal at U806, pin 8. With 800 MHz models, it should be 17.500 MHz, and with UHF and 900 MHz models, 14.850 MHz. The level should be approximately 400 mV rms. If this signal is not present, replace the reference oscillator because it is not serviceable.
| Mode | Shift 1 | Shift 2 | |
(Pin 4) | (Pin 3) | ||
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UHF MODELS |
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Rx Mode | L | L | |
H | L | ||
L | L | ||
H | L | ||
Tx Mode | L | H | |
H | H | ||
L | H | ||
H | H | ||
800/900 MHz MODELS |
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Normal Mode | H | L | |
L | H | ||
4.3.3 VCO MODULE
The VCO is a separate assembly that is covered by a metal shield and soldered directly to the RF PC board. This module cannot be repaired. One reason for this is that the center frequency is set by laser tuning ceramic resonator L101 (UHF) or L907 (800/900 MHz) and cannot be readjusted if it changes as a result of changing a part. In addition, the VCO has a ceramic substrate that can easily be damaged by excessive heat. It is also recommended that modules that have been removed using a standard soldering iron not be reused.
Perform the following checks on the VCO module to determine if it is defective.
Supply Voltage - The supply voltage at pin 15 should be 5.0 VDC (UHF) or 7.0 VDC (800/900 MHz).
Output Level - The output level on pins 1 or 2 can be measured with an RF voltmeter or some other type of
Control Voltage - Check the DC voltage on pin 7 with a channel near the middle of the band selected. If the VCO is locked on frequency, it should be a steady voltage between 1.1 and 5.2 volts. If it is not locked on frequency, it should be near the lower (1.1 V) or upper (5.2 V) end of its range.
Frequency Shift Inputs - The pin shift signals on pins 4 and 3 should be as follows (L = 0V, H = 5V):
Frequency - If the VCO is locked on frequency, in the receive mode the output frequency should be the receive frequency – 52.950 MHz (800 MHz models) or the receive frequency – 45 MHz (UHF and 900 MHz models). In the transmit mode it should be the transmit frequency. If it is not locked on frequency, it should be near the low end of its operating frequency range when the control voltage is low and near the high end of its range when the control voltage is high.
4.3.4 SYNTHESIZER IC (U804)
Since U804 does not have output pins from the various counters, their operation cannot be checked. However, the following signals can be checked to verify proper operation of U804.
Ref Osc In (pin 8) - Check the reference oscillator output frequency and level as described in Section 4.3.2.
VCO RF In (pin 6) - Check the VCO output as described in the preceding section.
VCO Control Voltage (pins 13, 14) - If the VCO frequency is too low, the signal on these pins should be high with very narrow
Lock Detect Output (pin 18) - This output should be high when the synthesizer is locked on frequency and low when it is unlocked.
| February 2001 |
Part No. |