Counting FM gangs

Eric Blacker

Hi all,

I noticed a post about five or four FM gangs and wondered if members
know how to tell the AM gangs from the FM gangs and how to identify
the local oscillator gang from the RF gangs.

I have heard some people count all the gangs on the shaft as FM gangs.

They aren't.

If you see two types of gangs; some with only a few plates and some
with many plates, the ones with many plates are AM gangs and don't
count as FM gangs.

Also the gang furthest away from where the co-ax from the antenna
connects to the front end is the local oscillator gang. The local
oscillator gang is also the one closest to the IF matching transformer
and ceramic IF filters. This is tuned to a difference frequency
10.7MHz above or below the RF depending on the design, so it doesn't
really count as an FM RF gang since it doesn't add to sensitivity or
help with overloading. It is driven by the local oscillator transistor
not by regenerated RF.

The shape of the local oscillator plates may be shaped slightly
different from the FM RF gang plates or have one more or one less plates.

A superhet AM radio requires at least two linked tuning capacitors,
one for RF frequency and the other for the local oscillator which
tracks at a difference frequency of 455kHz (usually).

So if you see five gangs probably two are for AM and should be
subtracted, leaving three FM gangs. Most people count the local
oscillator as an FM gang but strictly speaking it is not an RF
gang because it is always tuned about 10% off the RF.

The really good tuners have many FM RF gangs. This does add
sensitivity and helps prevent overloading. Better Kenwoods for
instance. Keep in mind that Kenwood makes killer ham gear as well as
consumer electronics. A friend of mine recently bought a $750 Kenwood
Home entertainment reciever and was just dumbfounded by the FM tuner
section's performance.

Some manufacturers removed gangs from all but the top models to yield
poorer specs than the top models.(to keep the high end specs better) I
suspect that is why a shaft has moving plates that have no fixed
plates associated with them. So in that case there may only be two
gangs dedicated to FM, one is the local oscillator, the other is the
RF. This is actually the minimum that is required for the tuner to
work. One FM RF gang and one local oscillator gang.

It seems to be a consensus that a tuner without at least four FM gangs
is junk and is not considered a high end tuner. That is one local
oscillator and three RF gangs. Some of the GESRs say five!

I hope this does not sound too hard nosed but, when people don't count
the gangs correctly it's like what others do on ebay by claiming an
amplifier is 170Watts because the UL tag says 170Watts max input
power. The amp in that case would be a 40WPC amp.

Best regards, Eric Blacker.

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