[R-390] Mech filter REPAIR

[2002tii]

Dave wrote:

>using a current filter available from one of the various electronic 
>suppliers.  As I recall, it wasn't perfect but was 
>relatively  inexpensive. <snip>  we need to explore a reasonable 
>cost substitution for a component that cannot directly be 
>substituted. $600 of filters in a $600--$800 radio seems a bit questionable.

Unfortunately, 455 kc (455 kHz for the younger crowd) IFs are rapidly 
becoming a thing of the past.  Murata makes several series of 455 kc 
ceramic resonator filters, some of which perform extremely well, but 
I have not found any of them available in the US except direct from 
the manufacturer in 100s.  Even these would likely require some 
impedance-matching to retrofit into a boatanchor, so the chances are 
slim that someone could make a ready-to-use replacement available for 
less than $100-200 each.  However, if the filters were available in 
small quantities, DIY might be a viable option.

The three desirable Murata series, in order of preference, are CFK, 
CFS, and (distant third) CFZM.  A specific filter is specified by the 
series designator, the center frequency in kc (455), and a letter 
indicating the bandwidth.  The bandwidth codes are:

Code      6dB BW (kc) [calculated; Murata gives half-BW]

A             35
B             30
C             26
D             20
E             16
E10         15
F              12
G              8
H              6
I                4
J               3

BWs A through D are generally too wide for a general-coverage 
receiver.  The filters we can use, then, are:

CFK455E, CFK455E10, CFK455F, CFK455G, CFK455H, CFK455I, and CFK455J;

CFS455E, CFS455E10, CFS455F, CFS455G, CFS455H, CFS455I, and CFV455J; or

CFZM455E, CFZM455E10, CFZM455F, CFZM455G, and CFZM455H (there is no 
CFZM455I or J).

If anyone knows where any of these can be purchased in small 
quantities, please let us know!  Some of the Japanese radio 
manufacturers (e.g., JRC) use(d) Murata filters (sometimes only as 
narrow-BW accessory filters), so some useful filters may be available 
from those manufacturers as spare parts.  I do not know generally 
whether they use filters from the three series mentioned above (which 
have the best out-of-band performance).

Best regards,

Don