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Replacing the driver FETs Q4 and Q5
- To remove the FETs it is necessary first to remove the PCB from the chassis. Simply undo all of the
bolts on the PCB. The PCB will then lift clear.
- Unsolder the FETs gate and drain leads. This will almost always break the FET. This then leaves the
large Source tab in the middle of the FET. Using a high temperature on the soldering iron, heat and
remove the FET.
-
Clean the solder pads on the PCB with solder wick, and solder the new FETs in place.
Adjustments
Following PCB repair, it may be necessary to realign or recheck adjustable components on the relevant
PCBs.
This section describes the methods employed to ensure that the reinstated PCB's conform to the original
specification.
Front panel
The contrast of the LCD display can be adjusted on the control head PCB
Master oscillator frequency adjustment
The master oscillator has no physical adjustment. It is adjusted in the factory using a software package and
should require no further adjustment.
To check the transceivers frequency use the “Tune” function, note that when using a frequency counter to
measure the RF that the tune function emits a carrier 1.6kHz above the Suppressed Carrier Frequency
(SCF) (displayed frequency) e.g. a channel programmed to 3000.00 kHz will measure as 3001.60kHz.
Microprocessor/ RF Audio PCB
There are three adjustable components on this PCB which under normal circumstances should be left
untouched.
VCO frequency adjustment
Program the transceiver to 30MHz and using an oscilloscope measure the wave form on TP8. The
oscilloscope should show a stable sine wave of frequency of 75MHz. A moving waveform indicates that the
VCO is attempting to lock and that the core of L25 needs to be adjusted. Using a non metallic trimming
tool, adjust the core of L25 until a stable waveform is seen on the oscilloscope. Then adjust the core of L25
the other way until the waveform becomes unstable. Now adjust the core to be in the centre of the range
between the upper and lower unstable waveform positions.
