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I have come across a company that sells a great conversion board for the mx290/296 take a look. http://garex.co.uk/pmr/tedmod.htm

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PMR Conversion Details
PMR equipment has long been used on the amateur bands - It's very well built, simple to convert - and they're very easy to use.  They have minimal front panel controls (no fiddly menus here), the receivers are sensitive and good at rejecting intemod - and the transmitters are built to run at full-pelt and not complain.  The following information is loosely based on material that I've found on the internet, along with my own experiences and mods.

If you wish to convert a Pye/Philips M290-series PMR to 2m, 4m, or 70cm - you can obtain the units from Atlanta Communications.  They offer basic units ready for conversion (supplying all the technical information), units with a programmed EPROM (you just need to tune-up the radio), or a fully converted unit ready for use.  For Xtals (M294/296), try Quartslab.  Need CTCSS encode/decode for your M290-series radios ?  Try East London Communications.

Remember, the serial numbers on these radios actually mean something!  Do not take a trader's word that it is a "70cm UHF rig", it could be a '293 VHF AM set.

   
   

These 2 units (M294+M296) were recently obtained via eBay, and will be used for UI-View on 144.800 and EchoLink on 434.475.  I also have 2x MX296 70cm units fully converted for another project...


Pye M294 VHF to 2m/4m
The M294's are quite simple radios as they are crystal controlled.  They're ideal for dedicated aprs/packet and for ragchews on just a few frequencies....

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
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
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 accessible through the holes in the lid of the crystal compartment.


Pye M296 UHF to 70cm
The MX296's are quite simple radios as they are crystal controlled.  They're ideal for dedicated aprs/packet and for ragchews on just a few frequencies....

The M296 also came with an optional extra PA circuit, boosting the output to 25w.  The picture above shows my M296 with the CD4442 PA board - If you do not have this fitted, then your radio is the more popular 6w version.

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 accessible through the holes in the lid of the crystal compartment.

Receiver Alignment
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 multi-meter to the ranges indicated below and perform the following:

1. Tune L15 and L16 for maximum volts then tune L17 for minimum volts on TP1 (2.5V range)
2. Tune L18 and L19 for minimum volts on TP2 (10V range)
3. Tune L20 and L21 for maximum volts on TP3 (2.5V range)
4. Retune L18, L19, L20 and L21 for maximum volts on TP3 (2.5V 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 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
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:

1. Tune L30 and L31 for maximum volts on TP4 (10V range)
2. Tune L32 and L33 for maximum then L34 for minimum volts on TP5 (2.5V range)
3. Tune L35 for maximum and L36 for minimum volts on TP6 (2.5V range)
4. Tune L36 and L37 for maximum then L39 for minimum volts on TP7 (2.5V range)
5. Tune L39 and L40 for maximum then L42 for minimum volts on TP8 (10V range)

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.

Pye MX294 VHF to 2m/4m
The MX294's are great units to convert - once you've got an EPROM sorted with the channels you want, getting the VCO, RX, TX and Deviation set are very simple and should take no more than 20 minutes...

VCO Alignment
The first step to converting an MX294 is to program your EPROM for the channels you require.  Next step is to adjust the VCO into lock... LED2 is the synthesiser 'in-lock' indicator and aids VCO adjustments - it's located on the left-hand side of the main board near the HEF4750 chip.  Whilst aligning, you should monitor the VCO control loop voltage using a multi-meter connected to TP3.

The VCO is in its own screened compartment fitted with a aluminium top panel secured by four screws - this panel MUST remain fastened in place during your adjustments.  The two holes in the top of the panel allow access to trimmers C8 (RX) and C11 (TX). C11 is the trimmer nearest to the PROM socket. I tend to label these using a black marker pen so that I don't forget.

First, you'll need to connect an RF load to the aerial socket, select a simplex channel (145.2-300 is ideal) and switch on. TP3 is where you need to take readings from (black meter lead to the chassis).  The RX trimmer needs to be adjusted until LED2 lights - then keep adjusting it to achieve a voltage of of 6.5v ( 0.5v) on TP3 and making sure that LED2 remains lit.  Place the radio into transmit and adjust the TX trimmer in the same way for 6.5v ( 0.5v) on TP3.

Receiver Alignment
The receiver section is on the front right-hand side of the PBC (opposite the PROM socket).  Attach an RF load to the BNC socket, an 8ohm speaker the speaker wires (brown+blue), a microphone to the mic connector:

Mic Connector pinouts:

  • Pin 1 : Mic Audio

  • Pin 2 : Gnd

  • Pin 3: PTT

  • Pin 5 : 10v

Finally, attach a power-supply and switch the unit on...  There are nine RX tuning coils in aluminium screening cans: L1-L4 and L6-L7 at signal frequency, L8 at local oscillator frequency and L9 and L12 (L11 in older versions) at IF (10.7 or 21.4MHz). L1-L4 and L6-L7 are in the larger screening cans with L1 and L2 at the rear and L3, L4, L6 and L7 in line towards the front panel, L7 being nearest to the front. L8, L9 and, where fitted, L11, are in smaller screening cans just in front of L7 near the main IF filter (FL1) - L8 is on the left, next to the VCO compartment, L9 is on the right and, where fitted, L11 is at the front. In most sets L11 is a fixed value RF choke, not an adjustable coil, but there is a further adjustable coil, L12, in the centre of the RX PCB section. As they are at IF, L9 and L11/L12 do not normally require adjustment.

The pot next to L7 is RV1, the squelch preset, and this should be set to open the squelch before making your adjustments. Receiver alignment consists of adjusting L1-L4, L6, L7 and L8 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 (such as a distant repeater).

Take care when adjusting the ferrite cores used in the receiver coils, as they are are easily damaged. Always use a proper non-metallic trimming tool. When you are satisfied with receiver sensitivity, reset squelch pot RV1.

Transmitter Alignment
The transmitter PA circuitry occupies the rear section of the PCB.  There are three trimmer capacitors in the transmitter PA stages, C126, C129 and C138, and these should be tuned for maximum RF output into a 50-ohm dummy load.  To preserve your PA transistors while tuning up, 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 RV4 (at the rear of the receiver section next to L2). A range of 6W (RV4 at minimum) to 25-35W (RV4 at maximum) is typical. Go back to the top of this page

Deviation & Frequency Adjustment
Transmit deviation can be set with RV6, the 22k pot located next to TR29 at the top right corner of the VCO screened compartment. Check the transmit output frequency with a counter fed via a pickup loop or a power attenuator. If the set is off-channel, adjust the synthesiser reference frequency trimmer (behind IC11), to correct both TX and RX frequencies.


Pye MX296 UHF to 70cm
The MX296's are great units to convert - once you've got an EPROM sorted with the channels you require, getting the VCO, RX, TX and Deviation set are very simple and should take no more than 20 minutes...

Adjusting the VCO
Once you have fitted a new PROM/EPROM for the 70cm band, your first alignment task is to get the synthesiser VCO (voltage-controlled oscillator) into lock. A synthesiser 'in-lock' LED, marked 'LED2', is provided to aid VCO adjustments - this is located on the left-hand side of the main pcb near the PROM socket. During your adjustment you can monitor the VCO control loop voltage using a multimeter connected to TP2, located near the top-left end of the VCO compartment between IC5 and RV6.

The VCO used in MX-series sets is in its own screened compartment fitted with a aluminium top panel secured by four screws. Make sure that this top is securely in position before making VCO adjustments. There are two holes in the top of the panel giving access to L1 and L7, the RX and TX VCO trimmers respectively. L1 is the trimmer nearest to the PROM socket.

To perform the adjustment, fit a suitable RF load to the aerial socket, select a mid-band simplex channel, connect power to the set and switch on. Connect the positive lead of your multi-meter to TP2, with the negative lead connected to chassis (0V). Adjust the RX VCO trimmer until LED2 lights, then adjust the trimmer to achieve a voltage on TP2 of 6.5V 0.5V, checking that LED2 remains lit. Now key the transmitter and adjust the TX VCO trimmer in the same way for 6.5V 0.5V on TP2.

Receiver Alignment
The receiver circuitry occupies the front right-hand side of the pcb ie the opposite side to the PROM socket. 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.

There are nine RX tuning coils in aluminium screening cans: L1-L5 at signal frequency, L9 and L10 at local oscillator frequency and L7 and L8 at IF (21.4MHz). L1 and L2 are in a twin screening can at the rear of the RX pcb, L3-L5 in a triple can in front of RV4, L9 and L10 in a twin can next to the VCO compartment, and L7 in a large single screening can next to L9/L10. L8 is in a smaller screening can next to RV2. As they are at IF, L7 and L8 do not normally require adjustment.

RV1, the squelch preset, is at the front of the RX pcb, and this should be set to open the squelch before making your adjustments.  Receiver alignment consists of adjusting L1-L5, L9 and L10 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 such as a distant repeater.

Great care must be taken when adjusting the plastic cores used in the receiver coils, as they are are easily damaged. Always use a proper non-metallic trimming tool.  When you are satisfied with receiver sensitivity, reset squelch pot RV1.

Transmitter Alignment
The transmitter PA circuitry occupies the rear section of the PCB. There are four trimmer capacitors in the transmitter PA stages, TC1-TC4, which should be tuned for maximum RF output into a 50-ohm dummy load.  To preserve your PA transistors while tuning up, 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 can be set to the required level with RV4 (at the rear of the receiver section next to L3). A range of 3W (RV4 at minimum) to 10W (RV4 at maximum) is typical.

Deviation & Frequency Adjustment
Transmit deviation can be set with RV5, the rearmost of the two pots located on the synthesiser pcb behind the VCO compartment.  The synthesiser reference frequency trimmer is behind IC11 and takes the form of either a discrete coil in a screening can or a trimmer accessed through the opening in a larger screened module. Check the transmit output frequency with a counter fed via a pickup loop or a power attenuator. If the set is off-channel, adjust the synthesiser reference frequency trimmer to correct both TX and RX frequencies.