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Author Topic: E. F. Johnson Company Low Pass filter 250-20  (Read 16979 times)
K2OWK
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« on: July 02, 2010, 09:22:12 PM »

I have an E. F. Johnson Company Low Pass filter 250-20. It is in excellent condition both inside and out. My question is, does anyone know its specifications? Its power handling capacity? Its 3 db bandwith? It has two adjustments on both sides, air caps. I am not sure how to adjust these. They seam to be locked in place with a dab of red paint on the locking nut. I have at my disposal a 60 MHZ Tektronix scope. A 30 MHZ RF frequency generator. A frequency counter. A VSWR Wattmeter and 50 ohm dummy load. I wounder if I can use these to check the frequency response and band pass and insertion loss? Also this filter has no marking for the input and output, and internally it looks symmetrical. I am wondering if this filter can be used as a receive filter as well as a transmit filter? I have some high frequency QRN where I am and a filter might help this. Any information about this filter would be appreciated. I believe it was produced in the late 1950 early 1960s or so. I think it qualifies as a Boat Anchor.Thanks, 73sK2OWK
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KA5N
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« Reply #1 on: July 03, 2010, 07:35:53 AM »

Don't tune it!!  These were pre-tuned at the factory and fiddling with the variables will just mess it up (assuming someone hasn't already messed it up).  A low pass filter of this type was designed to allow the HF bands to pass through unattenuated and greatly attenuate frequencies above about 60 MHz.  Since it is a low pass filter the notion of 3 dB bandwidth doesn't apply it has a sharp cutoff response characteristic.
The purpose was to block harmonics from a transmitter that might interfere with TV signals.  So it is hardly useful today with modern transceivers which have built-in filters.
It is symmetrical and 52 Ohm impedance.  I suppose it might help your interference problem since it wouldn't know it was attached to a receiver.  Just try it and see if it helps. 
Again, don't try to adjust it.
Allen
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K2OWK
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« Reply #2 on: July 03, 2010, 12:07:58 PM »

Thanks Allen, I will try it without adjustment. I live very close to an airport that uses high frequency transmitters. I get a buzz in my receiver that is very annoying. I do not get this buzz if I take the receiver to another location. I will try it for receive and let you know if it works. Thanks again.
73s
K2OWK
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K2OWK
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« Reply #3 on: July 03, 2010, 01:24:41 PM »

Hello again Allen, I tried the filter in line and it unfortunately had no effect. I was hoping for a reduction in noise on the receiver, and some filtering on the transmitter for my audio amplifier which picks up the audio from my transmission, but it had no effect. It does appear to have a 50 ohm input as it was unnecessary to re-tune my antenna tuner when I installed it in line on 40 meters. I guess it worked back in the day to suppress TVI on the old TV sets. My new set does not pick up my transmitter at all even though it is located just a few feet from my antenna. I like the looks of the filter, shiny round and very functional looking. I guess it would make a good display item. Thanks again for your input.
Regards and 73s
K2OWK
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W8JI
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« Reply #4 on: July 03, 2010, 04:57:21 PM »

Hello again Allen, I tried the filter in line and it unfortunately had no effect. I was hoping for a reduction in noise on the receiver, and some filtering on the transmitter for my audio amplifier which picks up the audio from my transmission, but it had no effect. It does appear to have a 50 ohm input as it was unnecessary to re-tune my antenna tuner when I installed it in line on 40 meters. I guess it worked back in the day to suppress TVI on the old TV sets. My new set does not pick up my transmitter at all even though it is located just a few feet from my antenna. I like the looks of the filter, shiny round and very functional looking. I guess it would make a good display item. Thanks again for your input.
Regards and 73s
K2OWK

Audio frequency device interference is always the audio device's fault, not the transmitter.  http://www.w8ji.com/consumer_equipment_rfi.htm

The noise you hear on a reciver is almost always at the actual frequency you are operting on, so a filter in front of the receiver will have no effect. If you have a buzzing noise that is broadbanded, it is probably power lines.
http://www.w8ji.com/power_line_noise.htm


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WB2WIK
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« Reply #5 on: July 03, 2010, 05:25:21 PM »

BTW in these days of almost everyone using cable or satellite TV, and most transmitters not generating any harmonic energy up in the TV band anyway, low pass transmitting filters don't have much of a market.  They will never help with interference to telephones, smoke detectors, intercoms, alarm systems, surround sound systems and all the other stuff that often picks up interference from amateur transmitters; their only real usefulness was to reduce "TVI" back in the day.

That day is mostly long gone.

However, having said that, that particular filter is really a very good one.  It was rated 1 kW "AM" power, which means it should easily handle "legal limiit" (1500W PEP) today.  A 1 kW AM transmitter back in the 1950s could easily generate a 600W carrier, and about 2.4 kW PEP output power, which is more than we're allowed, today.  It should have value to a "collector."
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K2OWK
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« Reply #6 on: July 04, 2010, 11:53:44 AM »

Thanks to everyone for their information. I will keep the old filter for nostalgia. I am a Ham operator who started in the mid 1950s. I retook the test a short time ago and recovered my original call through the vanity system. I am still using my SB401-1 transmitter and its original mike an Electrovoice 729SR cardioid. Still works as good as the day it was made. Again thanks for the filter information.
73s
K2OWK 
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G3RZP
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« Reply #7 on: July 05, 2010, 02:59:29 AM »

I'd use such a filter. Especially with older transmitters only using a pi network, you could well have harmonics above 30MHz when using the higher bands. The fact that it doesn't wipe the TV doesn't mean it can't cause trouble elsewhere.
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K2OWK
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Posts: 1279




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« Reply #8 on: July 05, 2010, 02:06:21 PM »

Thanks G3RZ8, I will take that into account. I use a Heathkit SB401-1 transmitter. I will take your advice and put this filter in line. It certainty can't hurt, and if it helps great.
Regards,
73s
K2OWK
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