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RECEIVER ALIGNMENT:
The receiver alignment was fairly straightforward. Firstly the IF was aligned. This was done
by connecting the scope across the 10 Ohm dummy speaker load resistor and applying the
signal from a Philips PM5326 RF generator to the antenna connection.
The generator being set for an output on exactly 455kHz and a carrier modulation level of
30% and the volume control set at maximum. The receive crystal X1 was unplugged to
disable the converter. Enough level was provided so the recovered signal was visible just
prior to significant AGC activation and the IF transformers T1, T2 and T3 were peaked.
After this the receive crystal was plugged back in and the generator set for 27.085MHz and
the receiver aligned for maximum gain by adjusting L1 and L2. After that was done, the
generator was physically disconnected. A small antenna was attached to the generator
output and L1 and L2 adjusted again with the GW-21A's antenna extended with the unit
some meters from the generator. This is done in case the attachment of the generator had
caused some de-tuning effects, but it turnout out the slugs of L1 and L2 were in the correct
positions.
Signal is resolvable or audible out of the noise floor when the generator's variable Attenuator
was in the region of -70 to -80 dB. With noise and signal about equal to the ear, the
attenuator was on -75dB. The PM5326 generator, on 0dB applies 50mV rms into 75 Ohms
and with a high Z load applies about double that. This suggests that the receiver is able to
resolve a signal of about 17uV out of the noise floor.
Once the receivers were aligned it was time to try them out. In practice, on full volume on the
control, there is moderate audible noise. Nothing as severe as a super-regenerative radio
though. The squelch control works well, not like a typical squelch that suddenly kills the
noise, its effect is more gradual. I could hear intermittent transmissions of people speaking at
times, with American accents, making me wonder if that was some sporadic short wave
transmission on CH11 from overseas. In any case, the receiver appears very sensitive
indeed.
So far I have tried these radios with about 100m separation with very good results. I am
going to perform a maximum line of sight test on them soon.
BATTERIES:
The battery compartment is a little large for a typical 9V battery. So I modified some 6 AAA
cell holders and fitted them with a 9V battery power clip. It pays with these holders to tape
the batteries in. I use Scotch 27 fiberglass tape as it can be re-used a few times and it stops
the holders sliding around too. The photos show the relative size:
It was necessary to add some soft packing into the battery compartment so the batteries
would be a snug fit.
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