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Author Topic: INRAD filter in a 75A-3 ?  (Read 1963 times)
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WA2OLZ
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« on: March 11, 2013, 03:12:50 PM »

AM filters for a 75A-3 are apparently made of unobtainium. I'm looking at the INRAD 5.8KC filter they sell for the 75S-3 as a possible alternative.

1. Has this been done and, if so, was it satisfactory?
2. Is the input/output impedance of the 75S-3 filter the same as that of a 75A-3?
3. What kind of pitfalls am I about to fall into?

Finally, any alternative suggestions are very welcome.

73,
Jack
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WBear2GCR
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« Reply #1 on: March 11, 2013, 06:07:16 PM »

5.8kHz total width or +/- 5.8?

5.8kHz total is too narrow for AM...

I think I see filters all the time on ebay for the 75A-3 and other Collins receivers.

I bought one for my R-388 (of course that rcvr does not normally use mechanical filters) whose IF freq is 500kc not the usual 455kc. So, I'd expect that you could find one. dunno if
ur a stickler for the same form factor, but other form factor mechanical filters will function once connected electrically.

There are a few online resources with Collins filter numbers and codes, most not all use these codes. That way you can tell which filters ought to be doing what... Cheesy

Hope this helps some.

                   _-_-bear

PS. the other heretical option is a softrock board taking the IF out of the receiver ...

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VE3AJM
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« Reply #2 on: March 11, 2013, 06:28:49 PM »

I don't know about the INRAD filters, but a friend of mine years ago retrofitted a Collins F455 type 6kc AM filter meant to be used in a 75S3B into his 75A3. The S line filter is smaller as is the physical spacing on the input and output pins compared with what is needed in the 75A3.

He mounted the S line filter onto a small HB circuit adapter board, and added the appropriate pins under this board to match the filter pin spacing in the 75A3 and plugged it into the A3 filter slot. He said it worked well for him.

Al VE3AJM
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Steve - K4HX
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« Reply #3 on: March 11, 2013, 10:45:33 PM »

Kiwa Electronics

http://www.kiwa.com/kiwa455.html
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WA2OLZ
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« Reply #4 on: March 13, 2013, 08:10:21 PM »

Thank you for the replies, guys.

WB2GCR - my impression was was, perhaps wrongfully, an AM filter of 6KC was correct, about 2.7KC for SSB. If not, you just saved me a lot of grief! I have the Collins guide but, of course, the 75A-3 calls for a F455-C360 filter. The "C" format is not even mentioned in the guide!

VE3AJM - That is certainly a possible solution. I'm not hung up on form factor so long as it works. It can always be removed to revert to stock.

K4HX - What a wonderful resource. I didn't know Kiwa existed. I'll be doing more research on that solution for sure.

73
Jack
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K5UJ
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« Reply #5 on: March 14, 2013, 01:32:15 PM »

mech. filters have steep skirts making them useful for battle condx on the bands but when QRM is high, the m.f. needed is narrow--3 kc which is what comes stock with the 75A-3.  for "hi fi" listening, a m.f. is a waste--the steep skirts don't buy you much if the filtered IF passband is 8 or 10 kc or more.  Under those circumstances i.e. casual uncrowded hi fi listening you may as well use a 4 dollar Murata ceramic filter or even a simple LC filter.  There's an article in an old ER on making an IF filter for the 75A-3.     See ER # 64 "Putting the 75A-3 on AM"
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WBear2GCR
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« Reply #6 on: March 15, 2013, 08:04:55 PM »

Jack,

SSB is one sideband, so if a "normal" SSB bandwidth signal was sent in DSB it would then be 2x as wide or about 6kHz.

When we listen to AM typically we're wanting to hear about 5-6kHz. of frequency response. AM is DSB + carrier. So that bandwidth is 10kHz or 12kHz.

Since you can tune off to one side (you may do this on an AM broadcast radio) you can get away with a slightly tighter filter so that you still get nice highs and a slightly tighter filter than 12kHz or 10kHz. A bit how it ends up sounding in practice depends on exactly how steep the skirts of any given filter actually are. They tend to have a certain width on the top of the filter curve, and then a bit of widening (only say 3dB down) and then a sharper slope. The "better" filters are more of a squarewave looking affair, where the skirts (sides) drop off almost vertically, so what you get is nothing but the spec'd passband to listen to - these get rid of stuff outside the filter's passband pretty much completely. Lesser filters let more crud off to the sides...

So you have to look at how a given filter is spec'd. For SSB there is little question that the bandwidth mentioned is the bandwidth (at the top of the filter curve) that you listen to.  For AM, it pays to be a bit more careful and try to look at what the mfr says the spec means, and even look at the published filter curve to see how wide it really is.

I seem to think that on my TS-440s the IF board filter I thought sounded best was actually a 12kHz wide filter. It is nearly perfect when there are QSOs at 5kHz spacing... although it may be the 9.5kHz. version in there now - I seem to recall finding that one a bit too tight. These are little ceramic Murata 455kHz filters. Drop in and solder replacements for the stock ones. But one could take these and build an input and output buffer and put them in any 455kHz IF section pretty much and do it in less space than any usual Collins mechanical filter . If you cascaded them, you'd get really really sharp skirts, probably better than the mechanical filters. Not as pretty, but this sort of solution is worth considering.

For a mechanical filter I'd look for something in the 9+kHz range, the top of it being ~12kHz. for practical daily use...imo. They are out there, I have managed to find them after a few months of watching epay and reading what the labels say on the posted pix. A lot of sellers have no clue what they have!

                       _-_-
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VE3AJM
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« Reply #7 on: March 15, 2013, 08:49:44 PM »

Jack,

I've used the Kiwa filters in my GPR-90RXD and a Hammarlund SP-600. They work wonderfully and can really make a receiver more useable/versatile on AM, at a reasonable cost.

I chose their 4.5kc and 6.5kc standard type filters along with the switchboard for both those receivers.
I can choose between the installed filters that are on the board, or bypass it completely and use whatever inboard IF filtering there is in the receiver. Bonus with the Kiwa filters is that there is no insertion loss noted with their filters. Some of the older Collins mechanical filters often do not age very well for a number of reasons. You need to supply about 12VDC to power the Kiwa filters. Easily obtained from the receiver. The shape factors of the Kiwa filters approach those of a mechanical filter. The receiver can be returned to stock form easily if one wanted to, down the road.
 
With regard to the choice of bandwidth, that would be your decision of course.
For me, a 12kc wide Murata filter for general AM work would be useless, under even semi-crowded band conditions on 80m. The same for 40m AM operation at the top end of the band, unless you wanted to receive 2 or 3 QSOs at the same time. As you know, the 75A3 as is in the box will never be a hifi receiver. All one has to do is look at the size? of the audio output transformer and circuitry. Improvements can be made through negative feedback around the 6AQ5 stage etc. to help it out.

Al VE3AJM
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Steve - K4HX
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« Reply #8 on: March 15, 2013, 09:12:57 PM »

In my 51J-4 receiver I have 6 and 9.4 kHz mechanical filters and 12-13 RC filter. I most often use the 9.4 filter. It offers a nice blend of good audio and QRM rejection. I can only use the 13 kHz filter when the band is really quiet - daytime or late at night on 75 meters. The 6 kHz comes into play with the band is too crowded for the 9.4 kHz filter.
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KM1H
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« Reply #9 on: March 16, 2013, 11:57:43 AM »

My A3 came to me with the factory 6kc filter which is too sharp for a quiet band and all those good speech artifacts to 5kc and beyond are lost.

If you want steep skirts a pair of the wider Kiwa's do well but the last I looked they no longer carry the interface or even the online circuit to mate to tube receivers. If they are back Im glad. Ive used single and pairs of Kiwa's in several tube sets mainly to just get the steep skirts. Sets such as the NC-183, HRO-50 and the older HQ's really respond to them. Im thinking of doing it to the SP-400X, SX-73, and the 75A2.

Carl
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WA2OLZ
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« Reply #10 on: March 16, 2013, 02:25:45 PM »

I seem to keep digging the hole deeper and deeper  Angry

Last night I ordered a mini filter switch board with 6.5 and 8kc filters to play in my 75A-3. Now it seems maybe that won't work in a tube rig? Grrr!

A follow-up email just went to Kiwa to see what they have to say.

More to follow...............

73
Jack
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WA2OLZ
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« Reply #11 on: March 16, 2013, 03:43:49 PM »

I mentioned that I ordered the KIWA mini filter switch board and filters last night. One would think that is the end of the story other than the mandatory on-the-air report when installed. Normally, that would be the case.

The weather early this morning was terrible with cold, rain and snow and every joint in my body was screaming at me to move south. The worldwide web called to me as I had my first, second and maybe third cup of tea waiting for my XYL to wake up. The laptop led me to some obscure search patterns on EBay and….. wait a minute! What is that I just saw? Placing eyeglasses firmly over sleep-clouded eyes I read the for sale ad, read it again and, yes, a third reading was in order.

The EBay gods offered up a for sale ad for a non-working Browning Mk. II Citizen’s Band rig. So what, you may ask. Well, it just so happens the Browning Mk. II (and Tram Titan) CB rigs are equipped with a Collins F455C-60 AM filter, the very filter made of unobtainium and plug right into a 75A-3! The auctioned ended in 2 hours and, for once, I was in the right place at the right time. The CB wreck is mine!

My current plan is to use the Kiwa switched filters in filter position #1, remove the existing 3.1Kc CW mechanical filter and insert the 6.0Kc mechanical filter in its place.

The next episode of the ongoing saga will be presented on the same channel as soon as parts start to arrive.
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Todd, KA1KAQ
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« Reply #12 on: March 16, 2013, 05:57:16 PM »

Actually the filter used in the Browning uses a smaller case, the FA style IIRC. The 75A-3 used the F style which is half again as big, so you'll need to do some creative pin-building to use it in the A-3 if the receiver is stock. Mounting stud spacing is the same, the rest is not.

I actually have one of the 6.0 Kc F filters for the A-3. Have hung onto it for years and have a couple friends who would like it. Might offer it in trade for one of the Dave Curry 9 KHz A-4 filters. Just as soon as it leaves a pristine A-3 will fall out of the sky and I'll regret it.
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WA2OLZ
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« Reply #13 on: March 16, 2013, 06:18:05 PM »

The Browning actually has an F455-FC60. My 75A-3 has a F455-C31 CW filter in it stock.

I somehow thought the Browning FC case was a C case, which is what I need. My error. Dang, I'm going to have a whole room of stuff I can't use.
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WA2OLZ
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« Reply #14 on: March 18, 2013, 04:57:39 PM »

or not.

Craig from Kiwa just emailed me. My order for the switch board and filters is a non-starter because:
1. The switch board is no longer available
2. If it was, it is for SS devices, not tubes. Impedance is probably waaaay off.
3. He does not have any wide filters.

Other than that, all is well in filter paradise.

Now then, Todd - I'm from Jersey and know where you live. I'm about to send a couple of guys to visit and they will make you an offer you can't refuse for that A3 filter you are holding for me  Wink
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KM1H
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« Reply #15 on: March 23, 2013, 01:41:25 PM »

Sorry to hear the bad news from Kiwa.

What about adapting one from the R-390A? Someone here must have that info.

Carl
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WA2OLZ
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« Reply #16 on: March 23, 2013, 02:20:24 PM »

Hi Carl,

Yep - too bad, Kiwa looked like a good solution.

I bought an old Browning CB receiver that uses a F455-FC60. It just became a donor radio when I removed the filter and sockets in which it was mounted. I think I can mount it on some perf-board and, using pins (maybe from an HC6/U crystal) in the right spot, plug it into the filter 'A' position in the 75A-3. It sounds good in theory, but I will see what happens in real life.

As an aside, I never had a clue that some early CB sets had discrete transmitters and receivers.
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WA2OLZ
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« Reply #17 on: March 30, 2013, 09:16:53 AM »

 There's an article in an old ER on making an IF filter for the 75A-3.     See ER # 64 "Putting the 75A-3 on AM"

Apparently ER#64 is no longer available. Does anyone have a copy of the article they would be willing to share?

Copy received thanks to the generous members of the forum. Thanks!
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