[Hallicrafters] RE: Black Beauties in SX-100

[GARDGORE at aol.com]

Capacitor technology today is far advanced over what was available 50 years 
ago. Modern capacitor characteristics are much different today and performance 
is far better than that of the earlier ones. The early original capacitors 
were actually small networks of capacitance, inductance and resistance even when 
they were new. The engineers took this into account when they designed the 
original circuit, specified values and decided on component arrangement and 
placement. It has been my experience that any Black Beauty that has seen 100 volts 
or more should be suspect but usually the ones in the selectivity circuits are 
OK. The sharpness of the receivers selectivity is determined by the 
frequency, number of tuned circuits and "Q" of the IF amplifier. Hallicrafters was 
perhaps the first to use a high "Q" 50KC IF in a dual conversion scheme for a 
bandwidth of 500 cycles or less in the sharpest position (depending on model) 
without resorting to the use of a crystal filter. The bandwidth is widened by 
switching in predetermined values of resistance and capacitance to "spoil" the "Q" 
and control the widening shape factor. Replacing these parts with components 
having different characteristics will alter the original performance. Some 
capacitors are in more critical positions than others. For example, screen and 
plate bypass caps in any amplifier circuit should always be suspect. You can 
measure tube pin voltages to ascertain the general health of the caps in your 
receiver. If a pin voltage is off by 20% or more I go looking. Coupling caps can 
be judged by checking to see if any control grids have positive voltage on 
them indicating a leak which will cause the tube to draw heavier current, run hot 
and distort. For old equipment that is in the operating position here I 
usually check tubes and transformers for heat level from time to time to try to 
catch a failure early if possible. I'm looking for a transformer that is running 
hotter than I think it should or a tube that is hotter than others of its 
type. I once found a leaking coupling cap in an RF stage because a 6SK7 was 
running much hotter than the others. These things will teach you something if you 
let them. Depends on what you call fun I guess...

Regards, 
Greg Gore; WA1KBQ


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