Dear Bryan, LF Group,
Before hauling the AR88LF back to the shed, I
tried using it as a tunable front end for another receiver tuned to 735kHz, as
was suggested last week. I found a good place to tap off the IF signal was at
the cathode of the first IF amplifier valve (V5 - 6SG7). On my unit this has a
100ohm un-bypassed cathode resistor between pin 3 and ground. Signal was taken
from pin 3 to the second RX input through a 0.1uF DC blocking capacitor.
This gave a satisfactory signal level for both my IC718 HF rig (which has
somewhat reduced sensitivity at 735kHz) and RA1792 receiver. Obviously,
selectivity was greatly improved, and sensitivity was good, as before. The
AR88LF was operated with BFO and AGC switched off, and the selectivity switch
in position 3 or 4. Having the second receiver connected did not seem to
particularly alter the behaviour of the AR88 used by itself. The main
drawback of this combination as a 136kHz receiver is the relatively poor
frequency stability compared with a modern RX - but this would not be an issue
for aural CW reception.
Another possibility is to use only the RF stages to improve the
front-end selectivity. Pick off the signal at the input of the
mixer.
I do so to improve the large signal behaviour of my W &
G SPM-12 selective level meter. The meter has no input selectivity
at all, the signal is fed directly to the mixer.
I use the front-end of my Teletron LWF45 receiver (5 tuned and
ganged circuits) as a preselector before the SPM-12 with good
results.
73, Dick,
PA0SE
