[R-390] Testing 390A mechanical filters

[John Kolb]

On Thu, 6 Mar 2003, Jerry Kincade wrote:

> Anybody know of a quick and dirty test to see used filters are still ok?
> Have accurate millivoltmeter, scope, HP sig gen, etc. etc. available. I'd
> just dive in and try some things but don't want to take a chance on frying a
> good one in the process.
> 73, Jerry W5KP

First step would be to measure the coil resistances. A F455N-160
filter (R-390A 16 kHz) here measures 41 ohms each coil, A
F455FA-21 measures 53 ohms. Don't use the X1 range of
a VOM as the low range puts more current through the measured
resistor. (and you can hardly measure 40-50 ohms on the low
range anyway) Each coil should read open from the coil to the
case or gnd terminal.

The next test would be to drive one coil with a signal
generator and put a scope or AC or RF voltmeter across
the other. Most Collins filters are spec'ed as 2 or 2.5 V
RMS max for rceommended operating conditions, so keep the
signal generator down around 1V RMS to be safe. You would
have to terminate the filter coils with the proper
resonating caps and load resistors to run accurate curves,
but tuning across the expected passband freq will tell
if the filter passes anything or not. Filter insersion
loss can be as low as 5 db high as 25 or 30 db,
so it helps to have a data sheet for the filter you are
testing.

The filter type number will tell you a little about the filter
F455FA-31 has a carrier or center freq of 455 kHz and is
31 times 100 hertz (3.1 kHz) wide at the nose, and is in
the "FA" case. This system works for most filters, but
not all. Specifically, it doesn't work for "Q" or "Z"
filters. "Z" filters (F500Z-10, etc) are generally
either LSB or USB, rather than centered on the listed freq,
and anywhere from 2 to 3.5 kHz wide.

A full data sheet will give passband wt the 3 or 6 db points
and also at the 60 db points, as well as impedance, resonating
caps, insersion, case dimensions. Full data sheets are
rather rare. I have a small number of them so may be able
to look up some filters.

PLUG: I have a number of filters for sale - e-mail for list.

John  KK6IL  jlkolb@cts.com