Pye/Philips  PFX
Conversion  to  4m,  2m  &  70cm

Adjusting the VCO

These notes assume you have a 'E' band version for conversion to the 4m amateur band, a 'A/B' band version for conversion to 2m or a 'TP/WP/UP' version for conversion to 70cm. Assuming you have fitted a new PROM for the band of your choice, your first alignment task is to get the synthesiser VCO (voltage-controlled oscillator) into lock.

See the PMR Picture Gallery for pictures of the PFX pcb.

Always use a proper non-metallic trimming tool for adjusting RF coils and trimmers.

The transmitter and receiver VCOs are housed in a screened module on the opposite side of the main chassis from the logic board. The VCO module is the second one down from the top, the largest visible, with three holes for trimming tool access (though you may have to remove some tape to see the holes). The left hand hole gives access to the core of L6, the receive VCO inductor, the middle and right hand holes are for L4 and L2 respectively, the inductors for the two transmit VCOs.

PFX PROMs are normally programmed to use TX VCO #1, so adjustment of L4 is not usually required. TP4, the test point for the synthesiser control loop voltage, is accessible on the other side of the set through a small hole on the left hand side of the logic board, at the top left-hand end of the PROM socket.

It is easiest to perform the following adjustments with the set standing upright. The antenna or a suitable RF load should be connected to the set along with the battery or PSU. Ensure that none of the connections to the front panel are touching chassis, then switch the set on and select a mid-band simplex channel. Connect the positive lead of a voltmeter to TP4, with the negative lead connected to chassis (0V). Key the transmitter and adjust L2 for 4.5V on TP4. If 4.5V cannot be achieved, adjust for maximum voltage on TP4. Now release the PTT and adjust L6 in the same way.

Receiver Alignment

The alignment adjustments are all performed at the rear of the set. See the PMR Picture Gallery for pictures of the PFX pcb.

The receiver section has a line of seven tuning coils (L1-L7) in the RF amplifier and mixer circuits, with a further L1 in the IF amplifier circuit located to the right of L1-L7, at the bottom of the pcb. L1 (IF) is the FM quadrature detector coil, and should not normally require adjustment. The round shaped pot to the upper right of L1 (IF) is RV7, the squelch preset. RV7 should be set to open the squelch before making your adjustments.

Receiver alignment differs slightly for VHF and UHF sets. For VHF sets tune L1-L5 and L7 for maximum FM quieting, with L6 (the top coil ie closest to the VCO assembly) tuned for minimum distortion. If you have a UHF set, tune L1-L4 and L6-L7 for maximum quieting, with L5 (the smaller elongated can third down from the VCO assembly) tuned for minimum distortion.

Ideally you should use a signal generator to provide your alignment signal source, reducing the injected signal strength as you bring up the sensitivity of the receiver. If you do not have access to such an instrument you can use a strong signal source for your initial adjustments, but always complete your alignment with a weak signal eg a distant repeater.

When you are satisfied with receiver sensitivity, reset squelch pot RV7.

Transmitter Alignment

See the PMR Picture Gallery for pictures of the PFX pcb.

The transmitter output stage uses a monolithic power amplifier and no adjustments are required. Two types of PA module are used: standard low power types are marked with a reference number containing the sequence '700/'; high power types contain the sequence '710/'. Actual power output can be set with RV5, a square shaped pot located on the right hand side of the pcb near the PA module.

Nominal output is 1.5W for standard low power sets and 4.0W (UHF) or 5.0W (VHF) for high power sets. Current consumption should not exceed 750mA (VHF) or 900mA (UHF) for low power sets, or 2A (VHF) or 2.2A (UHF) for high power sets.

Deviation & Frequency Adjustment

The synthesiser reference oscillator trimmer is accessible through a hole in the screened module above the VCO module ie in the top module. Adjust this trimmer to adjust TX/RX frequency

Transmit deviation can be set with RV1, the second from right 2k2 pot in the row of four located between the reference oscillator module and the front panel.