Three Digit Counter
A counter is probably the most useful addition to any rig! It has a single input channel that is normally fed from the rig’s Local Oscillator signal source. This much reduces the complexity (and hence cost), making it much easier to build. It shows just the important KiloHertz numbers of the input frequency – any MHz numbers overflow (normally) out of sight to the left! This means that it is suitable for direct conversion rigs, and for the superhets whose IF is an integer number of MHz – as in WE kits. It uses high efficiency small 0.3″ wide by 0.5″ high common cathode 7 segment LED displays with the least significant digit showing the units of KHz, and the top digit shows the hundreds of KHz. It uses low noise CMOS integrated circuits that count, store and decode the incoming signal for direct driving of the displays with all the advantages of low noise and only DC currents in the wires to the displays. For use with additive or subtractive superhets, it can count either up or down. It uses a crystal reference oscillator whose fundamental and harmonics are outside any amateur band. Input sensitivity is typically about 50 mV p-p at 10 MHz. The PCB size is 50 x 80 mm; supply required, 7 to 22 volts. The 21 segment resistors are mounted behind the display or end-on at the logic PCB. The kit can drive physically larger displays if required. The photo above shows the wiring between the displays and the three digit counter PCB when installed with a signal generator (an old retired kit!) that also has two extra counting chips to drive the extra MHz digits. The basic 3D counter cost is £26. Two extra displays with 2 counting chips are £5.
Mini AGC and CW kits
Although designed originally for the Stone/Stoke TCVR, these two units can be added to almost any receiver, and to a phone only transmitter. The Mini AGC kit adds an Automatic Gain Control function so that very loud signals are automatically reduced to a comfortable level; it has a preset to adjust the actual maximum output level so that it can work with high (LM386) or medium gain (LM380) audio output stages. There are also facilities, with scale and offset presets, to drive either your own moving coil voltmeter acting as an S meter or the supplied LED. There is also an RF voltmeter, with an output level preset, so that either of these indicators can function as a RF power output indicator. The PCB is a single sided 50 x 80mm and costs £20.
The Mini CW kit provides a narrow extra audio filter specifically for CW reception. It is selected by the provided toggle switch and has a peaked response centered on 725 Hz with sharp roll off above 1 KHz. It can be used with the above AGC kit too. For transmission, the sidetone oscillator also runs at 725 Hz to feed keyed audio into both the RX and to the transmitter’s speech amplifier, with level presets for both. Transmit & receive control is provided with the normal small delay after key up to cater for morse character gaps. There is also a centre off toggle to provide either Tuning – RF output without the T-R delay; or Netting, which just activates the sidetone oscillator without RF, so you can check that the other station is tuned with the same beat note as the sidetone oscillator. The single sided PCB is 50 x 80mm and costs £22.
Antenna Matching Unit
The Mk 6 AMU uses switched toroids for the variable inductor to avoid the hassles of winding large multi-turn coils! It uses the T matching circuit, because this a very versatile circuit able to match a wide range of impedances and over a large frequency range. Winding toroids is easy and the switches are surprisingly easy to use too! The design also has a resistive 50R matching bridge, a broadband RF transformer as well as the T configuration impedance matching parts. Power is limited to around 20W by the variable capacitors! When selected for tuning up, the resistive matching bridge will always present a safe load to the TX no matter what the load from matching parts and the antenna looks like! The bridge uses a high intensity LED for match indication but can also work with a conventional meter or even in conjunction with an S meter. When the bridge is out of circuit the LED responds to RF output voltage as applied to the matching circuit. The broadband RF transformer (the white toroid) allows the matching section to work with balanced or unbalanced feed lines, and isolates the matching parts and antenna/feeder from the incoming unbalanced RF feed from the transmitter. The T matching section (two variable capacitors and the switched inductors) can match load impedances from about 25 to 2000 Ohms over the frequency range 2 to 30 MHz. The main inductor comprises seven switch selected inductors wound on T50-2 toroids so that the inductance can be increased in steps from about 0.1 /uH in a near binary sequence up to a maximum of about 20 /uH. For any particular band, feeder and antenna combination, usually only one or two (occasionally three) adjacent inductors will be selected which makes adjustment very easy by flicking probably just two adjacent switches. The price is £37.
Extra Band Filters
There are two types – the Dual BPF for the receiver and Dual LPF for transmitters. Both PCBs are double sided 50 x 80mm. Both sets of filters can be used in either signal flow direction. See photo of three band Barle and Barwick. They both have relays switched by a toggle in the positive supply to select the filter for each band. The Dual BPF has twin TOKO style top coupled resonators similar to the rigs, for 20 and 80m normally (so assumes the base rig has a filter for 40m) plus an electronic switch to also select the appropriate LO signal. Each filter has a low impedance input winding with a choice of low or medium impedance on the other end (to suit SA602 mixers). The cost is £25. The filters of Dual LPF are of the double pi form cutting off from just above the chosen band. The parts allow for any band 20 to 80m and are suitable for 50R in and out. They are usually arranged for two lower bands with the filter in the TX fitted for the highest band. The Dual LPF filter costs £15. Or £40 for both filter PCBs.
Linear Amplifier kit
The High Power Linear has large heatsinks and uses IRF520 devices for about 15W out on 20m with 15v supplies while needing about 0.5W of drive for full output. 20m is intended to be the normal highest frequency band but on the lower bands, up to 30W is possible with about the same drive. The IRF520s really come into their own on higher supplies up to the maximum of 30v when 20W on 20m, or nearer 50W on 160m is possible (with care over heatsink temperature and that Walford Receivers are usually limited to 22v absolute max). The design includes two TR relays so that it is only activated during transmission; these relays and the bias circuits are activated when you ground your push to talk line. Following the amplifier there is a single double Pi harmonic filter for your band. The size is 100 x 180 x 35 mm over the topside of the PCB. The price is £38.
Walford Electronics Ltd. 