Pye/Philips  M294
Conversion  to  4m & 2m

Changing the Crystals

The M294 has up to six receive and transmit channels and you will have to fit two HC-25/U crystals (RX & TX) for each 2m or 4m channel you wish to use. The crystals are housed in a compartment with a hinged lid on the left-hand side of the pcb. On multi-channel sets the lid of the crystal compartment is usually marked with the crystal channel numbers and RX/TX positions.

Your first conversion task is to calculate the crystal frequency needed to get the set working on the 2m or 4m channel of your choice.

For 2m operation, use this formula (all frequencies in MHz):

RX crystal frequency = (RX frequency + 10.7) / 3

TX crystal frequency = (TX frequency) / 16

For 4m operation, use this formula (all frequencies in MHz):

RX crystal frequency = (RX frequency + 10.7) / 2

TX crystal frequency = (TX frequency) / 16

The receive and transmit crystal frequency adjustment coils are acessible through the holes in the lid of the crystal compartment.

Receiver Alignment

See the PMR Picture Gallery for pictures of the M294 pcb.

Fit a suitable RF load to the BNC cable socket, a 3-8 ohm speaker to the speaker wires (brown/blue), a microphone to the mic connector (pin 1 = mic O/P, pin 2 = mic ground, pin 5 = 10V O/P, pin 3 = 10V PTT I/P) and a 13.8V, 8A PSU to the power leads (large red/black wires), and switch on.

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.

The next task is to retune the receiver local oscillator multiplier stages. If you have a multi-channel M294 set and you are using a range of frequencies, select a frequency in the middle of the range. The tuning coils concerned, L15-L17, are situated between the crystal compartment and the on-off push switch. TP1 is to the right of the crystal compartment beside L16 while TP7 can be found behind the on/off switch.

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

Switch your multimeter to the ranges indicated below and perform the following:

1. Tune L15 for maximum volts then L16 for minimum volts on TP1 (2.5V range)
2. Tune L16 and L17 for maximum volts on TP7 (10V range)
3. Retune L15, L16 and L17 for maximum volts on TP7 (10V range)

Having got the multipliers stages working properly, turn your attention to the signal frequency stages of the receiver. There are five tuned circuits to adjust: L1 & L2 in the large aluminium screening cans, and L3-L5 in the smaller screening cans. The two smaller screening cans next to the on/off switch house L6 and L7, and as these are at IF (10.7MHz) they should not require adjustment in a normal conversion.

The pot between L2 and L3 is RV4, the squelch preset. RV4 should be set to open the squelch before making your adjustments.

Receiver alignment consists of adjusting L1-L5 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 RV4.

Transmitter Alignment

See the PMR Picture Gallery for pictures of the M294 pcb.

The first job is to retune the low power multiplier stages. If you have a multi-channel M294 set and you are using a range of frequencies, select a frequency in the middle of the range.

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

Ensure a suitable 50 ohm load is fitted to the antenna connector, switch your multimeter to its 10V range, key the transmitter and perform the following:

1. Tune C119 for maximum and L26 for minimum volts on TP2
2. Tune L27 for maximum and L28 for minimum volts on TP3
3. Tune L29 for maximum and L30 for minimum volts on TP4
4. Tune L31 for maximum, L32 for minimum and L34 for maximum volts on TP5

Now connect a diode probe to C162 and adjust L26-L32, L34 and L35 for maximum RF volts.

Having got the multipliers stages working properly, turn your attention to the PA stages of the transmitter. You should now see some RF volts at the transmitter output, if not turn the transmit power pot RV7 (near rear panel) fully clockwise.

There are six trimmer capacitors to adjust in the PA stages: C158, C163, C172, C177, C183 & C185. These should be adjusted in sequence for maximum RF power output. Note that C172/C177 and C183/C185 are interdependant and may require a number of tuning 'cycles'.

To preserve your transistors while tuning up the PA stages, take care to keep transmit periods to no more than one minute, with one minute 'cooling-down' intervals in between.

Once you have completed the transmitter retuning, the actual RF power output can be set to the required level with RV7. M294 sets are usually capable of at least 25W, but if you like to have long transmit overs it would be wise to use a maximum of 10-15W to give the PA transistors an easier life.

Transmitter Deviation and Frequency Adjustment

The transmitter deviation control is RV6, located to the right-hand side of the crystal compartment. Do not confuse this with the microphone gain control, RV5, located on the left-hand edge of the pcb just behind the crystal compartment.

Most M294 sets are already adjusted to the 2.5kHz maximum deviation required for 12.5kHz-spaced channels. However if you wish to adjust the deviation in your set, tune C119 (to the left of L4) for maximum deviation then set the required maximum deviation level using RV6.

The transmit crystal frequency adjustment coils are acessible through the holes in the lid of the crystal compartment.