"Michael Black" wrote in message news:alpine.LNX.2.02.1406021612540.11397@darkstar. example.org...
On Sat, 31 May 2014, gareth wrote:

Sorting through the junk box (50 years + and growing!) I am amazed at
the number of ex-valve-consumer-radio 2-gang tuning condensers (not
capacitors in them thar days!) that I have in the junk box, which was the
germ of an idea for preselection without coil switching - ie, one twin gang
condenser giving two tuned circuits for each band in the HF range.



It was covered in "73", at least, two articles, the same author, in "73".
One was definitely in 1964, but I can't remember which. The first one
used one of those large air variables, the other used one of those mylar
insulated cariables as seen in small transistor radios.

A websearch says the small one was in "73" in December 1964, W6SFM was the
author of that and the earlier article. Both definitely have an
explanation of how it works, two coils with quite different inductance,
one coming into use while the other is mostly insignificant as the
variable capacitor changes value, and vice versa.

I ahve definitely seen the technique in a QST converter article, so I
suspect the idea may date quite far back, but not sure. That article
didn't give any real explanation of how the tuner worked, as if we were
supposed to know from past experience.

That said, it's only one tuned circuit. Good for a lot of things, but
unless you cascade a few, you won't get very good bandwidth.

Michael

Many of my two gang condensers were from early Superhetrodyne's where one gang tuned the local oscillator, and the other tried to tune the antenna input circuit. This resulted in major engineering headaches, trying to get the oscillator tracking the antenna........ or was it the other way around? Anyway, the gang used for the local oscillator was much smaller (in fully meshed capacitance) and with strangely shaped plates. In fact one of the outboard plates on each gang, was cut into multiple "fan" shaped segments, for adjusting resonance of it's stage in mid band. Little "padders" were quite often part of the assembly, for tracking on the open or closed ends of travel.
A hot radio repairman could achieve reasonable tracking in a few minutes with a Hickok Analyzer, and fiber screwdriver. Less than hot repairman were advised not to just "tighten up all the screws" Heehaw.
Some of the old collectables had multiple "gangs" with the same number and shape of plates in each gang. These were often extracted from (now priceless) "TRF" radios with several stages of tuned RF stages.... (OK, I know, TRF means that.. grin) They tried everything!

Old Chief Lynn, W7LTQ