Pye/Philips  PF85
Conversion  to  70cm

Changing the Crystals

The PF85 has up to three receive and transmit channels and you will have to fit two HC-18/U miniature crystals (RX & TX) for each 70cm channel you wish to use. You can gain access to the crystal board by removing the rear panel of the set. The receive crystals are at the left-hand side of the pcb, with channel #1 at the bottom, channel #2 in the middle and channel #3 at the top. The transmit crystal are on the right-hand side of the pcb, with channels #1 - #3 in sequence from left to right. Your first conversion task is to calculate the crystal frequency needed to get the set working on the 70cm channel of your choice.

For 'T' band sets, use this formula (all frequencies in MHz):

RX crystal frequency = (RX frequency + 21.4) / 9

TX crystal frequency = (TX frequency) / 9

For 'U' band sets, use this formula (all frequencies in MHz):

RX crystal frequency = (RX frequency - 21.4) / 9

TX crystal frequency = (TX frequency) / 9

In practical terms, receive crystals for 'T' band sets, with upper side local oscillator injection, usually work satisfactorily in 'U' band sets.

The frequency adjustment coils are on the crystal board next to the crystals. The receive coils are in the following sequence: channel #1 coil is at the bottom-left, adjacent to the channel #1 and channel #2 crystals, channel #2 coil is at the top-left next to the channel #3 crystal, and the channel #3 coil is to the right of coil #2. The adjustment coils for the transmit crystals are immediately above the corresponding crystals.

Receiver Alignment

See the PMR Picture Gallery for pictures of the PF85 pcb.

The first job is to adjust the receive local oscillator crystal(s) for the correct receive frequency. This can be performed with a frequency counter fed via a pick-up coil held near the frequency adjustment coils.

If you have a three-channel PF85 set and you are using a range of frequencies, select a frequency in the middle of the range. The squelch pot, RV7, is located in the bottom right-hand corner of the main pcb. RV7 should be set to open the squelch before making the following adjustments.

There are a total of eight coils to be adjusted: L7 and L8 on the crystal board, and L7, L6, L4, L3, L2 and L1 on the main receiver pcb. L7 - L1 are in a column with L7 at the top and L1 at the bottom.

Two coils are at IF (21.4MHz), and do not normally require adjustment: L5 (the smaller elongated can third down from the top in the column of coils) and the FM demodulator coil L1 (on its own to the right of the column, immediately below RV7).

Great care must be taken when adjusting the ferrite and plastic cores used in the receiver coils, as they are are easily damaged. Always use a proper non-metallic trimming tool.

The eight relevant coils should be adjusted for maximum FM quieting. 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 PF85 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 main receiver pcb near the PA module. RV5 should be turned fully clockwise (for maximum power) before aligning the transmitter.

If you have a multi-channel PF85 set and you are using a range of frequencies, select a frequency in the middle of the range. All of the tuning adjustments below involve the low-power multiplier stages, and the coils concerned, L9 - L13, are located to the right of the transmit crystals. L9 is at the top followed by L10, L11, L12 with L13 at the bottom, near the PA module. A test point, TP1, located between L13 and the top of the PA module.gives access to the RF output voltage during tuning adjustments.

Great care must be taken when adjusting the ferrite and plastic cores used in the transmitter coils, as they are are easily damaged. Always use a proper non-metallic trimming tool.

Connect a diode probe to TP1 and key the transmitter. Adjust all the coils in sequence for maximum reading, starting with L9, then repeat the sequence. The actual transmit power can now be set using RV5. Nominal output is 1.5W for standard low power sets and 4.0W for high power sets. Current consumption should not exceed 900mA for low power sets, or 2.2A for high power sets.

Transmitter Deviation and Frequency Adjustment

Each transmit channel has its own deviation adjustment pot located on the crystal board below the transmitter crystals. RV1 (channel #1) is located to the left, below the channel #1 crystal, RV2 (channel #2) is in the bottom right-hand corner of the pcb, and RV3 (channel #3) is below the channel #3 crystal.

The transmit crystal frequency adjustment coils are above the relevant transmit crystals.