Burndept/Dymar  2000
Conversion  to  2m

Component Changes

The component changes described in this section are required if you are converting a Dymar/Burndept 2080 mid-band (AM) set to operation on the 2m amateur band.

See the PMR Picture Gallery for pictures of the D2000 pcb.

In the synthesiser circuit, C227 (TX VCO tuned circuit) should be changed to 15p from 39p, and C248 (output tuned circuit) should be changed to 12p from 18p. In order to frequency modulate the transmitter you will also need to feed a sample of TX audio to the VCO control loop. The audio can be picked up from TP21 via a potential divider eg a series 100K resistor followed by a 10K pot connected to earth. The audio sample must be fed from the pot wiper into the synthesiser box via screened cable and applied to the cathode of varicap diode D202 via a 100K resistor in series with a 4.7nF capacitor.

In the transmitter RF stages, you will need to change the following capacitors: C85 (22p) to 12p; C86 (82p) to 47p; C92 (22p) to 15p; C93 (82p) to 47p; C94 (22p) to 12p; C100 (150p) to 47p; C148 (150p) to 47p; C108 (18p) to 10p. Also in the transmitter stages, remove C156 (27p) and R107 (3.3k).

One turn must be removed from the following coils in the transmitter PA stages: L14, L16, L17, L20, L21, L22, L23 and L24. There is no need to remove the transmitter coils from the pcb to perform these modifications. Just snip out one complete turn, push the two free ends together and solder them, taking care to ensure that there is no solder bridge to adjacent turns.

In the receiver, D3 and D4 should be removed and TP11 linked to TP8. A 0.1uF capacitor connected from TP8 to earth will provide NBFM de-emphasis, improving demodulated AF quality.

Adjusting the VCO

The Burndept/Dymar 2000-series sets use two 2716 or 2732 EPROMs, EPROM 'A' (IC204) & EPROM 'B' (IC 205), both fitted in the screened synthesiser box. Assuming you have fitted a new EPROM set for the 2m 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 D2000 pcb.

L202 and L201, the RX and TX VCO coils respectively, are located at the rear of the synthesiser PCB. TP205, the test point for the synthesiser control loop voltage, is just behind L202. D206, the red 'synthesiser shut down' LED, is at the front of the synthesiser to the right of EPROM 'B', IC205.

Fit a suitable RF load to the aerial socket, select a mid-band simplex channel on the thumbwheel switches and connect the positive lead of a voltmeter to TP205, with the negative lead connected to chassis (0V). Make sure that none of the tracks on the synthesiser PCB are touching the metal chassis, then apply power to the set and switch on.

Always use a proper non-metallic trimming tool for adjusting RF coils and trimmers.

Adjust L202 so that D206 is extinguished, then adjust L202 to achieve a voltage of 6.0V ± 0.5V, checking that D206 remains out. Now key the transmitter and adjust L201 in the same way for 6.0V ± 0.5V on TP205.

The synthesiser output frequency is give by these formulae (all frequencies in MHz):

RX frequency = (RX operating frequency - 10.7) / 2

TX frequency = (TX operating frequency) / 2

Check the synthesiser output with a frequency counter. If the set is off-channel, adjust C212, the synthesiser reference frequency trimmer (located next to the crystal), to correct it.

Receiver Alignment

The receiver section is on the left hand side of the main pcb.

See the PMR Picture Gallery for pictures of the D2000 pcb.

There are six RX tuning coils in aluminium screening cans: L1-L5 at signal frequency and L7 at LO frequency. RF transformers T1-T6 are at IF (10.7MHz), and do not normally require adjustment. Adjust the front panel squelch control to open the squelch before making your adjustments.

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.

Receiver alignment consists of adjusting L1-L5 and L7 for maximum sensitivity. As the receiver is already tuned to around 140MHz, very little adjustment is needed to get optimum performance on the 2m band. 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.

Transmitter Alignment

The transmitter PA circuitry occupies the right hand section of the main pcb.

See the PMR Picture Gallery for pictures of the D2000 pcb.

Connect a RF probe between TP16 (near RV5) and chassis, and adjust the cores of L9, L10 and L12 for maximum voltage (1.5-3.5V typical reading). Now tune C95, C96, C98, C103, C104, C105 and L12 for maximum RF output into a 50-ohm dummy load. You will probably need several tuning 'cycles' to achieve optimum performance. If the output power exceeds 15W, use VR5 to reduce it.

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.

A spectrum analyser or frequency counter fed with a sample of RF output will help to prevent mis-tuning and/or spurious outputs. A 2m receiver will also help to alert you if the transmitter becomes unstable while you are tuning the RF stages.

Deviation & Frequency Adjustment

Transmit deviation can be set with the AF sample pot fitted to TP21.

Check the tranmit output frequency with a counter fed via a pickup loop or a power attenuator. If the set is off-channel, adjust C212, the synthesiser reference frequency trimmer (next to the crystal in the synthesiser screened box), to correct RX/TX frequencies.