The AM Forum

Finishing up an extensively modified radio- details to follow when it is done- and I want to improve the selectivity.

I remember reading somewhere that some old CB radios were a source of 6KHZ filters. Two questions:

1. What CBs had 455 khz filters? I'd like to watch eBay etc for one.

2. Can I use one from a transistorized set in a tube set or will the higher voltages take it out? I am assuming since it is in a grid circuit that it would not be a problem.

Thanks and 73
Carl
WA1KPD

None of the low end tube CB's that I remember had any 6 KHz filters. Some of the SSB/AM rigs might of had them. Why don't you just cascade two 455 KHz IF cans? Cheap and easy.

Hey Pete
OK- I hate looking dumb in public but  you give me more detail on that idea?
I like it as it fits in well with my whole concept. I don't think you mean just add another IF amp
Thanks,
Carl

Carl -

A few of the old Tram and Browning sets made in Laconia and Winnesquam NH used Collins mechanical filters, S-Line plug-in types. Tram Titans, Browning Golden Eagles, the high-end, hand-wire models. They sold new for $500 and up, and still fetch a decent price today. Unless you find a junker, you'd do better to sell the radio on ebay and buy the filter of your choice, then pocket the rest.

The Rolls Royce of CBs, when only the best will do, good buddy!  ;)

Check the schematic for the National NC-183D. It uses cascaded IF cans. You might have to diddle with the input/output impedance. I've seen where they use this concept and couple between the transformers a few pf's worth of capacitance.

Check digi key I think they sell cheap filters. Also ebay Racal and Harris used some nice crystal and mechanical filters for 6 and 6.8 KHz.

In a similar vein, I saw a Collins 75A2 at the Belvidere hamfest in Sept. that had a passive LC filter in the AM position.  IIRC it had a pair of mH range RF chokes and a couple of resistors.  I was going to take a closer look but forgot - anyone familiar with this approach?  The seller said it worked well, but I assume that means "broad as a barndoor"

73, Bob W9RAN

The old business band (2 meter high band) transceivers had very usable filters some small some large. Usually 455 KHz IF.  The older wide band 15 KHz deviation filters would be too wide but the narrow band filters are often very usable for voice, with some being much more steep skirted than others. John, K1DEU

Most solid state CB's from the '70's and '80's  (and '90's) have Murata 455 Khz AM filters in them.  I used to scrap various CB's in the '90's and got scads of filters out of them.  Most are about 6Khz b/w at 6 dB points.

Most MuRata and similar cermic filters use a letter to indicate bandwidth eg

CFU455B   +/-15Khz
CFU455C   +/- 12.5Khz
CFU455D   +/- 10Khz
CFU455E   +/- 7.5Khz 
CFU455F   +/- 6 Khz
CFU455G   +/- 4.5Khz
CFU455H   +/- 3Khz
CFU455I    +/- 2Khz

This is at the 6dB point . DIfferent families have steeper skirts past the 6dB and different ultimate rejections.
 
                                                                   Ian VK3KRI

Fair Radio has circuit boards outta various military stuff with appropriate mechanical filters for 30-40 bux.

Another option to go with that many seem to forget about is the Permakay filters out of old Motorola 2-way gear.  I think these are L/C filters and they come in a variety of bandwidths.  You're on your own as to which filter will do what, but I suspect a nice narrow split Permakay would be HIHI Fine Business for 6-8 KC or so.

I suspect if you use one out of a Motrac (solid state) you'll have to finagle impedance matching on the input/output as they are most likely low impedance (as you probably would for a mech filter anyway).  But one out of the tube rigs might be just the ticket.  I see them in hamfest junkboxes fairly regularly.  Something to look for.

The Kiwa filters work well in toob radios, either the Standard or Premium modules.  If you're trying to keep a radio looking vintage, they can be installed in an IF can and hidden away.  Better performance than a single ceramic filter from a CB radio board, but a bit pricier of course.

http://kiwa.com/kiwa455.html

There's also a discussion of using these in an SP-600/SX-42 here http://kiwa.com/goodwin.htm, and ans Osterwald review for ER here http://kiwa.com/errvw.html



Grant/NQ5T

Grant said:
Not to hijack the thread but maybe add to it, I ordered one of these Kiwa filters, (very small), to put in my 75A3. Can I place it in the same spot where the mech. filters would go with proper DC isolation? I have shunned the A3 because of it narrow 3.1Khz. filter and wonder if it would be ok if it were a little broader.

It is not a drop-in replacement.

One of the articles mentioned above talks about what is required.

The key is that the plate DC goes through the typical mechanical filter.
You don't/can't run DC through these filters.
The article states that (foolishly, imho) Kiwa uses only 50v rated isolation caps in front of their filters.

So you'll need to run the B+ to the stage before and maybe after, and use a higher voltage isolation cap, and a way to *bleed the charge* from the side of the cap that faces the new filter, so that it doesn't rise up to the B+ potential obviating the isolation cap.

Alternatively a 1:1 "transformer" wound on a tiny ferrite toroid would make that problem go away entirely...  ;D

The other factor is that these are "zero insertion loss" - but they do that with active amplifiers (likely some opamps). The Collins filter has some insertion loss, depending on the filter the amount varies - 10dB comes to mind with the newer filter series. So, you'll want to knock the gain down by the difference in *voltage* that you measure from the input side of the existing mechanical filter to the exit side of the same filter. Again, one way to do it is a transformer wound on a toroidal ferrite core with the proper ratio (the input to the next tube is very high-Z, so the impedance change is rather meaningless) or with a resistive divider (not as good, but usable).

I'd want to make it switchable myself... tiny relays come to mind for this application, or maybe PIN diodes? Or a wafer switch and coax?

The only reason I have any idea about this stuff is that I've been looking at putting a filter into my 500kHz. R-388 receiver.

A nearly optimal filter for AM receiving is going to be 9kHz wide at 6dB down. That gives you a passband of +/- 4.5khz. At least that's been my experience so far. At times a tighter filter can be of help, but that's "battle mode" to the max. Somewhat tighter is useable for AM, but not much more.

            _-_-bear