These ceramic filters are a relatively new innovations in electronics, and have only been in use about 15 years. Early ceramic filters had much looser specifications and wider bandwidths - Delco car radios are notorious for this! Vast improvements can be made by swapping these older filters for newer ones. Even older receivers may not use them at all, having several "IF transformers" instead. If you really know what you are doing, you can modify these radios by finding an interstage coupling capacitor or two, and swapping it out with a ceramic filter. You will need to hook the center conductor of the filter to ground. However, watch the DC voltages in tube equipment!

The majority of these filters are manufactured by two manufacturers - Murata and Toko.

IF filters come in several varieties, or bandwidths. Almost without exception, what is installed in your tuner is a 230 kHz variety. Since FM stations are spaced every 200 kHz, this means that they are wider than they have to be for each station. The reason for this is that they are not precise, they can be a little high or low. If you use 280 kHz, you are guaranteed of covering the whole station.

So - by test selecting filters that are all centered the same, you would only need 200 kHz. Standard filter values, or bandwidths are: 280, 230, 180, 150, 100, and 90 kHz. So you should use 280 KHz filters? No, the way FM is actually transmitted, you only need just over half of the 200 kHz bandwidth for stereo reception. The rest of the channel is used for elevator music, paging, and telemetry. A good compromise is to use 150 KHz or 110 KHz ceramic filters, matching them carefully for center frequency. This will yield superb selectivity, and you will notice a significant increase in sensitivity as well. The reason for this is the "gain/bandwidth product". As the bandwidth goes down, the gain goes up. It isn't hard to understand the conceptual reason - instead of amplifying parts of adjacent channels and all the sub-carrier signals you don't need, you are focusing right on just the important portions of the station you are interested in. You aren't wasting energy and resources on noise. It is VERY important to match the filters. If they are not matched, your stereo reception will suffer.
The usual impedance for ceramic filters is 330 ohms. If the existing filters in your FM tuner have a red dot on the filter, make sure any replacement filters also has a red dot. The red dot indicates that the ceramic filter has been designed for 10.7 MHz centre IF frequency.

The best way for an amateur without the necessary equipment is trial and error. It is relatively easy if your stereo only has two of these filters. If it has three, the best thing to do is leave the first one alone and just change out the second two. Then you can go back and change the first one to match the other two if you want. It won't make that much difference.

PROCEDURE

(1) Before doing anything else, you need to order the parts you need. The quickest way is to order them from Digi-Key (1-800-344-4539). They can accept credit card orders, so have your credit card handy. Think ahead - most radios have 2 FM ceramic filters, so order 2 for each radio you want to modify. The filters are inexpensive. The part number you want is: TK2308-ND Single pieces are 72 cents, it is $6.27 for 10. The description is "FM ceramic filters" (with 150 kHz bandwidth). The Toko part number is: SK107M4-AO-10. Here is the catalog page: