On Wed, 8 Feb 2017, Benderthe.evilrobot wrote:


wrote in message
...
On Tue, 7 Feb 2017 16:11:28 -0500, Michael Black wrote:

There are stories of Collins receivers, I forget which model, where the
capacitor feeding the mechanical filter can go bad, and the result is
a ruined mechanical filter, it can't handle the DC going through it.

SO that's the sort of thing knowledge that is out there for people coming
to an old receiver for the first time.

Michael


What is a " mechanical filter"?

Pretty much what the name says it is.

Usually a row of disks with a mechanical resonant frequency, AFAIK: the
transducers at each end were usually inductive, but I believe there were
piezo types.

At one time they were the most common type of IF selectivity in Ham radio and
other communications gear.

I think that's debateable.

For a long time IF transformers were "good enough" and a single crystal
filter was the step up. Those were common until the SSB age in the
mid-fifties, when mechanical filters became somewhat common, but
multi-crystal filters also came along at that point, and within a few
years the shift was to a crystal filter in the HF range.

Collins used mechanical filters in their receivers, but not all of them.
Some CB sets used them, helped in part because some company in Japan made
a cheaper mechanical filter, but that wsa sort of a blip, ceramic filters
ame along soon after and they were cheaper. Upper end equipment tended to
use mechanical filters right to the end, when Collins stopped making them
a few years ago, the implied move to software radios taking over.

Some ham SSB sets used them, and I had an RCA SSB Carphone that had a
250KHz mechanical filter.

But lots of other equipment used other things rather than mechanical
filters.

Michael