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 Post subject: Permakay filter
PostPosted: Feb Thu 21, 2008 3:06 pm 
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Location: Florence, Al. U.S.A.
Does anyone have an old Motorola schematic with this filter? As used in 1959 cab radios?


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 Post subject:
PostPosted: Feb Thu 21, 2008 3:18 pm 
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Hi Carl,

What information do you need?

These were sealed units. They provided the high-selectivity filtering for the receiver IF.

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 Post subject:
PostPosted: Feb Thu 21, 2008 4:03 pm 
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Since I have several of these and they are 455 Kc, could they have a use in AM radios. FM then was I think about 5Kc wide.


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 Post subject:
PostPosted: Feb Thu 21, 2008 7:09 pm 
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Carl Neidert wrote:
Since I have several of these and they are 455 Kc, could they have a use in AM radios.

Hi Carl,

No, these were never used in consumer products. I suppose you could install one in an AM radio if you wanted. The Permakay was a very expensive item, probably several times the cost of a complete AM radio.

I expect you have some of the pulls that were available by the thousands when the FCC dropped the deviation from ±15 kc to ±5 kc for commercial two-way FM radios.

This was a real simple change... one plug-in RCA connector, one solder connection, and four screws. There might have been one capacitor change also. It's been 50 years since I did one of those conversions. :roll:

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 Post subject:
PostPosted: Feb Thu 21, 2008 9:57 pm 
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Location: Monterey California USA
I would forget about trying to use one of these as a filter on an AM radio. The filter bandwidth on the earliest Permakays is around 36 KHz and 25 on the newer ones, and the skirts are not steep. The bandwidth of the filter is not the FM deviation, it is the channel spacing and then some. In other words, they are far too wide to be of any use.

The majority of Permakays are well sealed and potted in resin so you can't do much with them. However, there were a couple of early models which were marginally potted and they can be safely taken apart. There was an article in Hints and Kinks in the early 1960's on how to modify one for an AM filter, but the effort was large and I still don't think it's worth the trouble when you can buy a nice ceramic Toko filter from Digi-Key for a few Dollars. The insertion loss of the Permakay is quite high as well. Some versions have a solid state amplifier inside, halfway through the signal chain, to compensate (e.g. Motrac TFN6017.)

There was actually a Permakay made for AM, and used in Motorola's aircraft radios in the early 1960's. But they are very rare and I would still pick the Toko.


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 Post subject:
PostPosted: Feb Fri 22, 2008 3:29 am 
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With R-390A IF assemblies being readily available Im using the 8 kC mechanical filter for AM in a couple of modified BA's.

If you buy one of those assemblies be sure to get a guarantee that the coupling cap hasnt leaked and blown out the filters.

Carl


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