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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 3:41 am 
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Location: ayer ,ma usa
I found a really nice looking Zenith that resembles the much bigger, common AM/FM jobs with the gold dial and external pointer(early50's?) ,often with the braided handle on top. Anyway, this is FM only, and seems to be about an 80% replica of the bigger one, even down to a smaller "mini" version of the brown bakelite cabinet. White external pointer, plastic with brass insert; back has a big green sticker"Armstrong System" "FM"..no AM. never saw one of these..I think it is pretty cool, even if it is a Zenith (this has happened to me before)Any thing I should know about it? (IF trans horror?- IF trans seems to only have access from above..maybe lower slug adjusts thru hollow upper one?? )7F03 chassis. model 7H918; selenium rectifier, 12AT7 converter, variable inductance tuner. Thank you, John<P>------------------<BR>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 3:50 am 
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Location: Maryland, USA
Hi John,<P>Unfortunately, the Armstrong system operated in the 42-46 Mc range, and is no longer operational. You could build a converter to drop the current FM broadcast band down to this frequency range so the radio would operate.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 4:13 am 
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Hi Leigh..No, it is on the 88-108 band(nothing else,no AM)....From reading about it, I think Zenith was one of the few who took a license in the very early days from the Major; That is what is so cool about this radio, I think! They were proud of that, to put such a big sticker on it!<P>------------------<BR>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 4:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: ayer ,ma usa
Further checking, it has the dreaded slug adjust transformers with no threads in the paper tube. However from reading about it in Rider, on this one, it says "alignment should never be necessary, unless settings are disturbed; if necessary to align, HEAT SLUGS FIRST WITH IRON!!" caps mine, no wonder they are all stripped; zenith sealed them to the paper tube with some kind of glue! New info!It also warns about pressing down on slugs with screwdriver"after which it will be impossible to adjust" Partied with that for a week last year, thanks to prior "tune -up" Bottom line -leave em alone...Rider has new respect from this fellow..<P>------------------<BR>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 5:30 am 
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Location: Cleona, PA
IIRC, Zenith made a big deal out of FM sort of to tick off RCA, since Sarnoff didn't want to mess with FM and was concentrating on TV at the time. Someone else may recall details better. Armstrong got the Sarnoff treatment, poor fellow.<P>Reece <P>------------------<BR>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 9:28 am 
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Location: Martinsville, Il. USA 62442
Hi Grady,<P>I have one of the same models and it works just fine. It's a model 7-H-918 manufactured in 1949. Zenith also made a matching AM only model too. I am in the process of restoring one now.<P>Best of luck,<P>Mike G.<P>------------------<BR>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 5:39 pm 
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Location: Gainesville, Florida
Hey grady. Funny thing I have that very same radio, Zenith 7M918, The Armstrong System. Has another sticker on it that says Super Sensitive Zeinth-Armstrong FM. Works great. Someone stuck a wrought iron cabinett handle on the top fastened through holes for it. Hopefully the holes are supposed to be there. Otherwise looks pretty good. Too bad about Armstrong. Got the shuffle from RCA then the worst. I like the way he used to climb the radio towers and hang from the very top. There was a complete history about this on PBS TV.


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 5:40 pm 
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Location: Jim Thorpe, Pa., USA
I have an early GE AM/FM set that has a similar sticker on the back. It uses a discriminator and limiter rather than a ratio detector as is found in some other sets. Is that the difference between sets that used the "Armstrong System" and others?<P>------------------<BR>Tom Lager


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Wed 10, 2004 8:51 pm 
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There's a lengthy article in the February 2004 issue of <I>Radio Age</I> by Ludwell Sibley which discusses the development of FM from the earliest experiments through current usage.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Fri 12, 2004 10:07 am 
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Location: Charlotte,NC
For those in the NYC metro area, on the top of the Palisades, north of the George Washington Bridge, in Alpine, NJ, is "Armstrong's Folly", a.k.a the "Alpine Tower", a large tower with 3 large crossarms. (I grew up within sight of it It is a really distinctive looking antenna, not your usual "stick"). This was one of the antenna towers Armstrong used in his developement of FM, or so I've been told. There was another near Paterson, I think.<P>------------------<BR>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Fri 12, 2004 4:49 pm 
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Here's a picture and background:<BR> <A HREF="http://www.fybush.com/site-021219.html" TARGET=_blank>http://www.fybush.com/site-021219.html</A> <P>Richard<P>------------------<BR>


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 Post subject: Zenith FM only.."Armstrong System"
PostPosted: Mar Sat 13, 2004 12:25 am 
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Location: Charlotte,NC
Yup, that's it. I could see it from my bedroom window at the bottom of the hill several miles away.<P>------------------<BR>


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 Post subject:
PostPosted: Oct Wed 01, 2008 12:47 pm 
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Location: Parkes, NSW , Australia
Cool story - many thanks for the wrap up of that stick!
I just took delivery of a 7H918. Thanks for the heads up on the cores!
I guess the selenium rectifier will have seen better days. What should I replace it with?
Peter


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 Post subject:
PostPosted: Oct Wed 01, 2008 1:42 pm 
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The selenium should be replaced by a diode and resistor. An easy way to do this is repplace all electrolytic caps, paper caps and out of tolerance resistors. Make sure the radio is operating properly then measure the DC voltage at the first filter capacitor. Then sub in a diode and add a resistor in series with the diode that will give you approximately the same DC voltage on the first filter cap as the selenium did. When I replaced the selenium in my LaFayette LT-50/KT-500 AM-FM tuner I added a capacitor after the diode before the voltage dropping resistor to provide better power supply filtering and IMHO it makes selecting the dropping resistor a little easier. Besides you can play with the value of the filter capacitor connected to the diode to fine tune the available DC voltage somewhat once you get close with the resistor. IIRC I used a 10 uF capacitor after the diode with a 100 ohm resistor before the first filter cap to get the 137 volts DC my LaFayette tuner requires to operate. That tuner has soldiered on faithfully for the last 3-4 years since I did that.


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 Post subject:
PostPosted: Oct Wed 01, 2008 4:48 pm 
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Location: Woodinville, WA USA
Incidentally, there is a website devoted entirely to FM-only radios.

http://www.somerset.net/arm/fm_only.html

It lists this model and many others.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


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 Post subject:
PostPosted: Oct Wed 01, 2008 8:27 pm 
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Location: New Jersey
The two 50s Zeniths I had with FM were permeability tuning on FM. Is that what defines the "Armstrong System" ? I was told thats what it was, but who knows. I had a Crosley with permeability FM tuning as well.

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 Post subject:
PostPosted: Oct Wed 01, 2008 9:04 pm 
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The "Armstrong System" decal could be referring to Zenith's use of a limiter and discriminator, just as designed by Armstrong. Some companies, such as Philco and RCA, devised their own circuits for FM detection to weasel out of using the Armstrong license.


Tim KA3JRT


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 Post subject:
PostPosted: Oct Wed 01, 2008 11:18 pm 
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Location: Parkes, NSW , Australia
Thanks TR for the tips on selecting the correct value resistor.
Peer


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 Post subject:
PostPosted: Oct Thu 02, 2008 1:15 am 
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Location: Ellington CT
TIM TRESS IS EXACTLY RIGHT ABOUT THE LIMITER/DISCRIMINATOR BEING THE BASIS FOR THE "ARMSTRONG SYSTEM" LABEL.
KEVIN

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 Post subject:
PostPosted: Oct Thu 02, 2008 2:52 pm 
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Check out the link below for a discussion of Armstrong and the Foster-Seeley discriminator:

http://antiqueradios.com/forums/viewtop ... 28905976d3

--Ed

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