Pye/Philips  MX296
Conversion  to  70cm

Adjusting the VCO

Assuming 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.

See the PMR Picture Gallery for pictures of the MX296 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 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 multimeter 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.

See the PMR Picture Gallery for pictures of the MX296 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-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 eg 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.

See the PMR Picture Gallery for pictures of the MX296 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 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 to correct both TX and RX frequencies.