[R-390] Filter Repair

[Bob Camp]

Hi

The skirt selectivity of the "low cost' filters isn't quite as good as it might be. Normally cascading filters is a problem. They interact in odd ways. In this case you need more loss than the filters have. That would let you separate a couple filters with an attenuator. 

One cheap / practical approach might be to combine a low cost mechanical with a bit wider ceramic filter. If the ceramic adds 40 db to the skirts, it's well worth it. 

More or less:

Ceramic filter 	9 db loss
Collins filter 		9 db loss
matching 		2 X 1 db loss

That would let you run about a 6 db pad. I suspect that it would need to be fiddled a bit to work. 

Bob

On Feb 27, 2013, at 2:51 PM, Garry Stoklas <jergar at sbcglobal.net> wrote:

> Even before this latest round of discussion on filter repair, I had thought 
> about doing what Dave Curry had done; take a new, modern Rockwell Collins 
> filter, and an impedence matching network and put the whole thing in the case of 
> a bad, unrepairable R-390A filter. Their "low cost" product line has filters of 
> .3, .5, 2.5, 5.8, 7.0 and 10.0 kHz @ 3 db bw. While these bandwidths don't match 
> up with the R-390A 2,4,8,16, they might well be better than the aging filters 
> that are failing. Their "low cost" filters are in a plastic case that is about 
> 1.25" long x .5" wide x .5" high including the pins so they and the matching 
> network would fit nicely inside a R-390A filter case. They have lots of other 
> badwidths that are closer or eqaul to the existing filters but they are not "low 
> cost".
> 
> This discussion got me thinking so I called Rockwell Collins this morning to 
> find out how much the filters are and where to get them. First, you can buy them 
> directly from Collins, they'll even accept a credit/debit card over the phone, 
> and they have no distributors. Second, the "low cost" line is still pretty 
> expensive - $105.34 in 1 to 4 quantity, $86.69 5 - 9, $70.85 10 - 24, and $60.16 
> 25 - 49. These "low cost" ones are normally a stock item. The closet non-low 
> cost on their filter list they have to a 4 kHz is 3.8 kHz that is also in the 
> same platic package as the low cost line. Out of curiosity I asked the price of 
> this filter and was flabbergasted by the 1 to 4 price of $329.05 each. No wonder 
> the Dave Curry filters were expensive and he has stopped making them. Also, many 
> of the other filters are in a metal package that are too big to fit inside a 
> R-390A filter.
> 
> I have a Kenwood R-1000 that has the tiny Murata filters that I have found to 
> not be all that good. I may buy one of the 2.5 or 5.8 filters to try in that 
> radio to see what they can do. Will let the board know the results if I do.
> 
> Garry Stoklas
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