The A200 is a 'stick it in the boot and forget it' add-on linear amplifier. Not many amateurs realise that PMR equipment of days gone by could sometimes do with a few extra watts when trying to achieve the maximum in range. Improvements in base station receivers, coupled with the ever-increasing need for spectrum re-usage, has meant that these units are increasingly finding their way onto the amateur market. Inside these sturdy weatherproof boxes you will find a piece of mint-looking board with a 50W-plus linear amplifier, complete with automatic RF sensing and change-over switching. Just the job for 2m, 4m, or with some modifications, 6m.
Identification:
The A200 is easily identified as a solid die-cast unit with heat dissipation fins on its topside, and are always painted black. It always has three connections at the end, these being RF input, output, and a thick DC power lead. The only similar-looking unit around has the same outer case but with two thick d.c.leads coming out of the side, this is a type VR200 24V-to-12V converter, so don't be misled by appearances.
There have been two types of A200 in manufacture, an early model having a TNC RF input socket and an N- Type RF output socket, and you may sometimes find a later model, with one S0239 output socket and a flying co-ax lead for the RF input. Internally they are virtually the same, but you may find the later model arrangement is easier to install. The DC cable is actually a very heavy current AC cable, with brown, blue, and green/yellow leads - brown is used as the positive 13.8 V supply, blue as negative, and green/yellow as a switching lead, but please don't wire it up to your mains plug as the capacitors inside make a lovely exploding noise!
Selection:
On the side of the case, you will find a riveted plate with "Cat No. A200" marked, and below this is space for the aligned frequency of operation. Unfortunately this is often blank, but take a look at the section marked "Code", you will see something like "01 E0", which provides the frequency band information. The first two numbers are the market code, which is irrelevant for our purposes, the final letter/number combination gives the frequency range:
EO: 68-88MHz,
M1: 105-108 MHz,
BO: 132-156MHz, and
AO: 148-174 MHz.
The E0 is useful for 4m, and both the B0 and A0 models will tune to 2m. The E0 model will also, with the modifications described, operate very satisfactorily on the 6m band.
Circuitry:
Internally, a pair of MPX085P or BLW60 transistors are used, which have forward bias applied via a wirewound resistor and two forward-biased diodes. Input and output printed circuit inductors together with compression trimmers and a three-stage low-pass filter are used in the RF path, a further capacitor and plate resistor on the input form a gain control to ensure the amplifier is operating in the linear portion of its input/output curve. RF sensing circuitry detects input drive, and switches in the amplifier if DC power is supplied. The unit is extremely rugged both physically and electrically, and even incorporates an over-temperature cutout to stop the transistors overheating in use. I have never yet seen a faulty amplifier sold, but it would be wise to check inside to ensure there are no components that have obviously been removed.
Preliminaries:
Check that the internal pins are correctly linked, with pins 2 and 3 connected together, and pins 4 and 5 connected together. This will ensure that the RF sensing circuits will always be powered up, ready to switch DC to the amplifier itself via the internal DC relay when RF is detected. If you wish to use a small toggle switch for on/off switching however, make sure both of these links are removed and then use a switch between the green/yellow DC wire and 0V to connect the amplifier into circuit. When not switched in, RF input and outputs are simply linked for a 'straight through' connection. Connect up your DC supply, with positive to the brown wire, negative to the blue wire on the thick power lead, ensuring your power supply has the current capability required. As a guide, a 10-15A 13.8V supply will be required for maximum output with a 10W nominal drive power. Connect up your transceiver to the RF input connection, and a suitable load capable of handling 60-70 W to the RF output. If your A200 has a TNC input connector, you may find it more useful in practice, to remove this and replace it with a coax lead, terminated in a PL259 or BNC as appropriate to suit your transceiver.
Tuning:
Alignment to 2m or 4m is extremely simple. Initially set C7 for minimum capacitance, i.e. its vanes fully apart. This is used as an 'RF Gain' adjuster, to reduce the input drive power to the amplifier stages. Whilst transmitting 2W-15W of RF power into the unit, check that the relays click over, and simply tune C8 and C17, both for maximum power output, re-tuning as required until you cannot achieve any further increase. If you have an in-line SWR meter, you may like to insert this in the co-ax between your transceiver and the A200 amplifier, and re-adjust C8 slightly as required for minimum indicated SWR, this should coincide with maximum RF power output.
You will typically get around 50-60W output for 10-15W input, with less than this of course with reduced input drive. If you intend using the amplifier on AM (see later for SSB), then it is important now to adjust C7 until you reduce the output power to around 25 W. This will ensure the amplifier will handle the positive peaks of modulation presented to it from your transmitter.
SSB Modifications:
As the A200 is a linear amplifier, it will of course handle SSB quite well. However, as it was originally designed for remotely mounted AM/FM use, as soon as the RF power falls below a given level the amplifier relays cut out, and as a result, use on SSB causes a good deal of relay 'chattering' and broken transmissions. We may add a suitable 'hang time', i.e. a switch-off delay, by adding a small - electrolytic capacitor across C2 and C3. A value of 0.68μF will give - around three-quarters of a second delay which I have found is the most acceptable, 0.47μF gives around a half second, and 1μF around one second. Any capacitor of at least 10V working voltage will suffice. Fit the capacitor with its positive lead to the D2 cathode/R3/C2/C3 junction and with its negative lead to the PCB ground plane.
You may also find it useful to increase the RF switching sensitivity, to prevent losing the first syllable of speech following a pause in transmission. To do this, fit a small ceramic capacitor of around 4.7pF in parallel with C1 on the A200, this will increase the RF detection sensitivity to below 50mW. Both of these additional capacitors may easily be soldered to the component side of the PCB, hence removing the need to extract the main board from the chassis.
Following this, tune the amplifier as described with C7 at minimum capacity. Once this has been done, adjust C7 so that the output RF power decreases by approximately 10%, this will ensure you are not overdriving the amplifier and that it is operating in the linear portion of its transfer characteristic. After doing this, you may find that you need to slightly readjust C8foroptimum output power. As a guide, you will typically achieve around 45-50W PEP power output with 28 dB rel. PEP level 3rd-order IMD products. Don't try to squeeze the last drop of power out on SSB, this will degrade the linearity of your signal, resulting in audio distortion and splattering across a wide bandwidth.
