02-28-2020, 11:24 AM
Quote:but the 3rd step if adjustments had no effect,band four is very quiet with no static ,but will pull in a station at night.+1, the shortwave band in question, however, lacks sensitivity. Since the BC band pasess through the IF and that is OK...
Lack of sensitivity can be attributed to at least four obvious things, more, if nits are to be picked...
1 - The RF or mixer tube isn't up to snuff of there is low plate voltage...
2 - There is poor "Q" in the RF/osc coils the signals are lost in "wet" coils
3 - There is a ground connection to the chassis via a rivet that is failing.
4 - There is a leaking mica bypass cap, could be one associated with tuning or the oscillator.
5 - Propagation is poor.
6 - What ever antenna is poor or your location is, for shortwave.
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#2 can be very difficult to address. Gently warming the coils in question with a heat gun will drive off moisture...
#3 The plated chassis and the rivet react with time to set up a corrosion. Even though the connection can be measured at zero ohms, that is DC not RF. Burnish such a riveted connection with a wire brush, add small amount of ACTIVE water base flux. Apply heat from iron with large copper tip and rosin solder. If all is well solder will flow around rivet, solder-tab and chassis without forming a ball. Using a heat gun to warm the chassis helps. Alternate fix: Drill out rivet replace with hardware and serrated washers.
#4 A "special" heat gun with a silicone rubber nozzle can be used to find a leaking mica or freeze spray. In the RF the effect is either drifting of the signal or the signal fading. If the signal increases then the tuning needs work.
Any alignment adjustment that does not peak or null as expected is a problem area. Components and connections associated with that adjustment will have to be inspected.
Great so far!
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”