Dawson Red PLL

[projuicer]

found one in my box of "stuff that might come in useful sometime."

Planning on using it as a round the house/street rig. Can someone point me at tuning instructions please?

[Albert H]

Is that the one with the whacking great big 74HCT4059 on the board? If it is, just look up the pin-out of the 4059 and you can work it out for yourself!

[projuicer]

Thats a start, thanks Albert.

I remember something about needing to achieve a particular volatage at a test point somewhere on the board?

[Shedbuilt]

I believe this is the schematic http://www.hllye.com/uploadfile/201402/ ... 617196.jpg

PLL take off is at f/2, then divided by 2 by the 4024, and the 6.4MHz crystal is divided by 256; giving 25KHz loop reference. This means that the 4059 needs to divide f/4 down to 25KHz, so I think the switches should give 100KHz increments. I believe switches 1 to 4 on the left should be increments of 100KHz (100K, 200K, 400K, 800K respectively). Switches 1 to 4 on the right, should be increments of 1M in the same way, and the same order. Switches 5 to 8 on the right should be increments of 10M - again in the same order. Switch 8 on the left should be on for 100M+, and off below. Switches 5, 6, and 7 on the left, should always be off. Voltage could be tested at the junction of R18, R19, and R20 (immediately to the right of the 4046).....

[teckniqs]

I've got them on a hard drive somewhere but not used those boards for years, thankfully.

....If you get really stuck I'll try and dig them out.

[FM King]

Shit old boards from a shit old builder.

[Albert H]

It's actually quite funny - that PLL circuit was one that I came up with back in about 1979. The 74HCT4059 was a really new part, and I liked the idea of being able to programme the divider using BCD rotary switches which would directly represent the frequency of the output!

The actual exciter circuit that was originally used had a Colpitts 1/2f oscillator feeding a bipolar buffer stage with almost equal emitter and collector resistors, with a capacitor from the emitter to the clock input of the 74HC4024 (chosen because it would work at up to 70MHz). The collector fed through a capacitor into a push - pull doubler stage using a couple of BF494s. This was high pass filtered and there was almost 400mW at the filter output. This was plenty to drive an MRF237 or SD1127 to about 5 Watts. The whole exciter - including the logic - fitted on a 10cm X 7.5cm PCB with a vertical screen to separate the RF from the digits. The PLL gave an out-of-lock relay output to disable the PA until it was ready to go. The modulation input was buffered through a very low - noise dual op-amp, with the other half used as part of the PLL loop filter.

The exciter was close to a "no-tune" design, but the VCO coil core had to be chosen and adjusted (ferrite for some frequencies and brass or aluminium for others). There was also a peaking trimmer in the doubler stage.

For such a simple design, the power output was relatively high and the spectral purity was exceptional. It was also possible to add a second push - pull doubler on a small sub - board to use the thing on Band III for links.

I had 120 of these boards made, and they all got used. The whole board used to cost around £20 in parts and take about an hour to build (once you'd made a few!). Stephen and I seriously considered using this circuit for the next NRG product (with some improvements).

The circuit that Dawson was selling was a very cut - down version of the PLL and had a fairly awful RF side of the board.