Pa Functional Testing - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Troubleshooting Procedures: Power Amplifier Procedures
If, for diagnostic reasons, a chip component needs to be removed to facilitate testing, such as a
series capacitor removed to allow for signal insertion, then the component(s) returned to the circuit
should be new parts. The application of a soldering iron to many chip components will tend to cause
leaching which could lead to failure.
If the harmonic filter is damaged and needs to be replaced, then removal and replacement requires
the use of a hot-air source capable of reflowing the solder beneath the filter hybrid. When replacing
it, add small amounts of fresh solder paste to the silver regions beneath the ceramic to assure
adequate electrical ground contact. Save the original input and output connectors (J-straps); these
are not included with the replacement kit. No tuning is required. The harmonic filter may be ordered
separately, but if the PA kit is ordered, a filter kit comes with the PA kit.
After a PA board is replaced, or if any power control circuitry components are replaced, readjust the
power according to instructions in the ASTRO Digital Spectra and Digital Spectra Plus Mobile Radios
Basic Service Manual (6881076C20).
NOTE: Due to high operating frequencies, you must use specified Motorola parts when component
replacement is necessary. Substitute components may not work. It is also critical that you use
great care when replacing parts. Excessive solder or flux, longer than original leads on coax
connectors, misorientation of parts, and other commonly benign imperfections may cause the
radio's performance to degrade.

4.5.2.2.2 PA Functional Testing

Test the PA assembly for proper operation as follows:
1. Disassemble the PA assembly from the radio, leaving the power cable connected to the rear
connector. Replace the PA shield and cover. Disconnect the coax connectors and the ribbon
cable. Connect a power meter to the antenna port using minimum cable length.
- When setting or measuring RF power at UHF, follow these guidelines to avoid
measurement errors due to cable losses or non-50-ohm connector VSWR:
- All cables should be very short and have Teflon dielectric.
- Attenuators and 50-ohm loads should have at least 25 dB return loss.
- Mini UHF to 'N' adapter, P/N 5880367B21, should be used at the antenna connector. All
other connectors should be 'N' type. No other adapters, barrel connectors, etc. should be
used.
- Maximum input level to the PA is 50 mW. Too much input power could result in damage
to the LLA stage.
2. Apply the input power and DC voltages indicated in
amplifier assembly. To make the DC connections, use small spring – clips or make a test
adapter similar to that shown in
3. Apply the required input power via an adapter cable. For this application, non N-type
connectors are acceptable.
4. With the applied control voltage initially at 0 V, slowly increase the voltage until power out
equals 46 Watts. Power should rise smoothly with control voltage once the tum-on threshold
is reached. Control voltage should not exceed 10.0 V.
5. If 10.0 V does not produce 46 Watts, then a failure exists in the power amplifier circuit.
6. Refer to the voltage chart
are not within the limits shown in the chart, then a failure exists in the PA assembly.
7. If the voltages in the chart are correct, verify that the injection is at least 30 mW. (See the
VCO troubleshooting section.)
6881076C25-E
Figure 4-9 on page
(Table 4-20 on page 4-59). Measure the indicated voltages. If they
Table 4-19 on page 4-58
4-58.
September 5, 2008
4-57
to the power