Pye/Philips M296
Conversion to 70cm
| Changing the Crystals |
| Receiver Alignment |
| Transmitter Alignment |
| Deviation & Frequency Adjustment |
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Changing the Crystals
The M296 has up to six receive and transmit channels and you will have to fit two
HC-25/U crystals (RX & TX) for each 70cm 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 70cm channel of your choice.
For T1 band sets, use this formula (all frequencies in MHz):
RX crystal frequency = (RX frequency + 21.4) / 8
TX crystal frequency = (TX frequency) / 32
For U0 band sets, use this formula (all frequencies in MHz):
RX crystal frequency = (RX frequency - 21.4) / 8
TX crystal frequency = (TX frequency) / 32
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
M296 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 M296 set and you are using a range of frequencies, select a frequency in the middle of the range. The tuning coils concerned, L15-L21, are situated between the crystal compartment and the on-off push switch. TP1 and TP2 are in the same area while TP3 is next to L4.
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.
Switch your multimeter to the ranges indicated below and perform the following:
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 screening block, and L3-L5 in smaller screening cans. The smaller screening can next to the on/off switch houses L7, and as this is at IF (21.4MHz) it should not require adjustment in a normal conversion.
The pot at the top right of the pcb is RV3, the squelch preset. RV3 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 RV3.
Transmitter Alignment
See the PMR Picture Gallery for pictures of the
M296 pcb.
The first job is to retune the low power multiplier stages. If you have a multi-channel M296 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 and plastic cores used in the transmitter coils, 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 the ranges indicated below, key the transmitter and perform the following:
Having got the multipliers stages working properly, turn your attention to the PA stages of the transmitter.
You should now be seeing some RF volts at the transmitter output, if not turn the transmit power pot RV6 fully anti-clockwise. RV6 is vertically-mounted and accessible through a hole in the front of the small screened compartment in the mid-rear of the pcb.
If RF volts are present at the transmitter output, tune L43 and C165 for maximium RF output. If no RF power is present, tune L43 and C165 first for maximum supply current (10A range ammeter), then retune them for maximum RF output. Re-adjust L30-L40 and L42 for maximum RF output. Tune C170, C183, C184, C193, C194 and, where fitted (on the 25W PA stage), C306, for maximum RF power output. Note that C183/C184 and C193/C194 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 RV6.
If you make adjustments to RV6 it is wise to check the tuning of C193, C194 and,
where fitted, C306.
M296 sets without the add-on PA stage give about 6W maximimum output.
M296 sets with the extra PA stage 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 transistor an easier life.
Transmitter Deviation and Frequency Adjustment
Transmit deviation can be set with RV5, the vertically-mounted pot to the right of the crystal compartment. Do not confuse this with the microphone gain control, RV4, located on the left-hand edge of the pcb just behind the crystal compartment.
The transmit crystal frequency adjustment coils are acessible through
the holes in the lid of the crystal compartment.