Pye/Philips  MX294
Conversion  to  4m & 2m

Adjusting the VCO

Assuming you have fitted a new PROM/EPROM for the 2m or 4m band, your first alignment task is to get the synthesiser VCO (voltage-controlled oscillator) into lock.

See the PMR Picture Gallery for pictures of the MX294 pcb.

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 HEF4750 IC. During your adjustment you can monitor the VCO control loop voltage using a multimeter connected to TP3, located to the right of the same IC.

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 C8 and C11, the RX and TX VCO trimmers respectively. C11 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 multimeter to TP3, 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 TP3 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 TP3.

Receiver Alignment

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

See the PMR Picture Gallery for pictures of the MX294 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.

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 eg a distant repeater.

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.

When you are satisfied with receiver sensitivity, reset squelch pot RV1.

Transmitter Alignment

The transmitter PA circuitry occupies the rear section of the pcb.

See the PMR Picture Gallery for pictures of the MX294 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.

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 tranmit 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.