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Dear LF Group,
I bought a rather battered AR88LF at a rally a few
years ago - This version of the AR88 has an LF band covering 75 - 200kHz, so it
is potentially useable for amateur LF. I had not really tried it seriously, so
with all the pining for good olde days that has been going on lately, I dragged
it out of the shed to see what could be done with it.
After replacing the usual catalogue of leaky
capacitors, temporarily bypassing a broken IF transformer, and peaking up
the IF amplifier, the sensitivity was good - 0.3uV gave a good audible signal.
The input impedance on 136k is quite high, something of the order of 1kohm.
Connecting a 200pF capacitor in series with the sig gen to simulate the source
impedance of a long wire antenna only reduced the signal level by a few dB, so
it should be possible to connect the RX directly to such an antenna and get good
sensitivity without additional matching components (allthough this does not work
at my QTH - you get about 100V of 909kHz from Brookmans Park at the RX
input...)
The selectivity was poor however. The IF frequency
is 735kHz, so one can expect worse selectivity than other old RXs with 455kHz
IF. This meant that when tuned to 136kHz, large signals from DCF39 and the
Datatrack beacons just outside the band pass through the IF and intermodulate
together at the detector, producing lots of heterodyne whistles even with the
BFO switched off, as G3GVB mentioned. It has a single crystal filter, but this
did not seem to make much difference. However, this proved to be due to
mis-alignment - I think every receiver with one of these
single-crystal filters I have ever encountered has been wrongly
aligned. Partly it is due to ageing I'm sure, but also I expect a lot of
over-enthusiastic fiddling has gone on over the years! The main problem was
the 'phasing' trimmer, which was a long way from where it needed to be. After a
lot of re-tweaking, the -3dB bandwidth on the narrowest position was only
about 200Hz. This proved good enough to get rid of the worst of the whistles,
however, the skirt selectivity is still not good enough really - with the RX
tuned to 136.5kHz, the best I could get was about 40dB rejection of DCF39 on
138.83kHz, which still gives quite a loud whistle above the band
noise.
So I decided to graft on an audio filter from a
CR100 receiver, which happened to be lying around. I connected this between the
1st audio stage and the audio output stage; allthough really just a lash-up it
works quite well. I also increased the amount of BFO injection. On the AR88 the
BFO signal is injected into the final IF amplifier using just stray capacitance
between the wiring. I added an extra bit of wire to increased the rectified
voltage at the detector from 4V to about 15V. The idea of this was to make the
BFO signal much larger than the unwanted signals at the IF output, and so
hopefully reduce the amount of audible intermodulation products produced - It
seems to work. It does mean that the AGC has to be switched off, otherwise the
BFO signal causes a big reduction in gain. The best way to set the controls is
with the AF gain set to as high a level as possible without producing excessive
receiver noise, and then controlling the gain with the RF gain control. This
ensures the lowest possible signal levels at the detector, reducing the
likelihood of IF or detector being overloaded by strong out-of-band
signals.
The end result works quite well. This morning, when
the QRN was relatively low, it was possible to hear the Loran chatter above the
band noise, with only faint whistles from out of band stations, so sensitivity
ought to be adequate for most things. The warm-up drift is a few 100Hz, but
after it has been running for a few hours, the frequency only wobbles about by
several Hz, so in fact it is adequate for QRSS3 reception. When I get round to
doing a proper job, I will add a product detector (probably using the socket of
the not-very-useful noise limiter valve). Also, the CR100 audio filter is really
too narrow - with 100Hz bandwidth, it is quite hard to get the BFO pitch just
right so that the peak of the IF response coincides with the peak of the audio
response, and it has to be re-adjusted due to drift of the BFO. A filter with
200 - 400Hz bandwidth would be better.
Obviopusly, an AR88LF is not as good as a modern
receiver. But with some simple mods it is certainly usable. I think mostly the
same considerations apply to other old equipment, such as the HRO and CR100 when
used on LF.
Cheers, Jim Moritz
73 de M0BMU
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