Quote:
Originally Posted by g4aaw pete
I've added a cheapo counter, (the type typically costing >£10), to two receivers.
B40 - loose coupling to local oscillator - drove counter easily.
CR100 - had to use much tighter coupling to local oscillator to get enough drive.
Both these receivers use a separate valve for local oscillator.
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I first tried making an LO sniffer with a CR100, ISTR it consisted of a 2N3189 source follower feeding a BCY70 with a low collector load and a fair amount of current, the FET being fed by a couple of turns of insulated wire around a grid connection. This showed me just how critical even slight extra capacitance was to both calibration and stability, and also getting a good frequency response from a buffer. CR100 afficionados will recall that the wide tuning ratio means that very small trimmer capacitance changes makes a big change to HF-end calibration, the top 3 ranges being set by selected low-pF fixed capacitors as built.
Shifting this technique to the spookily-stable AR88 from the happy-to-wander CR100 showed even more starkly that a stable radio can be degraded by what is cheek-to-cheek with the original LO implementation. I'll plead guilty to a degree of perfectionism, but I always feel that add-ons should be a genuine enhancement!
That CR100 looks as though it is tuned to The Buzzer
