Post #406,258
11/13/15 5:55:43 PM
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Now this I find a bit difficult to understand.
The reception for my FM tuner went completely to hell a few days ago. Today I did some investigation.
I have a high gain FM antenna up on the roof with a coax cable going down into the attic and through the ceiling to a coax connector on the back of the FM tuner, which is mounted at the top of a 72 inch high relay rack.
I went up on the roof to inspect. No problem found. A few feet of the cable in the attic I cannot inspect without great difficulty, so I haven't done so.
So I moved some heavy shelves to get access to the connection in the back of the tuner. It hadn't been disturbed for years - in fact access was so restricted it couldn't be disturbed. The connection looked a little messy, though, so I re-dressed the cable end and put on a new connector. Reception still really bad.
But, messing around there, I was certain I heard little bits of good reception, so I experimented. What I found was that if the cable end was brought very close to the connector, but no metal to metal connection from either the center wire or the shield, reception was excellent.
So, I cut the cable straight off, insulated the end with duct tape, and taped it up tight to the connector. Reception is, I'm quite sure, better than it has ever been.
Not being an RF guy, I haven't a clue why this is, but I'm glad it works.
If you can't fix it with duct tape, it can't be fixed
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Post #406,259
11/13/15 7:03:56 PM
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Re: fixing it with duct tape
Veterinarians would probably disagree with your conclusion.
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Post #406,260
11/13/15 7:16:48 PM
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What about neurosurgeons?
I think I know of one who would probably believe it, especially if he read it in a church bulletin.
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Post #406,261
11/13/15 8:55:15 PM
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Could you have some kind of a short (not necessarily a dead short) at the end of tuner connector?
Connecting the cable would simply short out the signal. The wiring inside the tuner beyond the connector would pick up the radiated signal from the cable end.
I would try some kind of contact cleaner before disassembling the tuner connector.
Also, dielectric grease on the connector might help the problem from reoccurring. Alex
"There is a cult of ignorance in the United States, and there has always been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that "my ignorance is just as good as your knowledge."
-- Isaac Asimov
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Post #406,262
11/13/15 9:37:57 PM
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Re: Could you have some kind of a short (not necessarily a dead short) at the end of tuner connector
The connector is nice and clean, and had a good plug in it. With the antenna cable disconnected there was no signal. with just the center wire making contact with the center of the connector - bad reception. With both center wire and shield making contact - bad reception. With just the shield making contact - bad reception. With the cable cut off and insulated with tape - excellent reception, provided the end of the cable is right up to the connector.
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Post #406,263
11/14/15 12:09:29 AM
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One explanation..
Assuming that yours is a 'component' FM tuner (du some old jour) and it has the usual layout of stages ... (Not a plastic cased Granco model with no RF-amp)
IF the AGC [automatic gain control] feedback loop on first RF-amp stage has suffered a well-drifted carbon resistor or leaky small cap/amidst its components:
could be that "it is no longer a real AGC", but is very unhappy with more than say, a few hundred µV input so it simply clamps to zero. Your air-gap evades the issue by not addressing it: happenstance maybe supplying just the right level for the IF to do its frequency magic and get a decent signal to the discriminator (or ratio-detector) where Mr. Armstrong's bitchin idea converts it to decent audio.
[Also if your antenna has a preamp--an added stage to vet by substitution: initially just a straight wire, right?] That you suggest: it may be better than (you recall, ever?) would imply that the tuner had a bad RF stage for donkeys' years.. Fisher 100, 200? H. H. Scott [??]
Anyway if you decide to sleuth it to a fare thee well, I have here a Sound Technology 1000A Broadcast-grade alignment generator (of vanishing-small distortion, IM or Harmonic)--an actual complete broadcast-grade source, as in--add a power amp and antenna: you're on the Air Indistinguishable from the Official (also legal) others. Lots of lesser-quality units are out there in small towns ... rarely challenged less'n they pick a frequency that drowns a distant station the locals like.
(I need to find a tech who still likes perfectionist repairing..) When I was dabbling in that, I couldn't/well.. wouldn't afford this state/art gadget ..and then Muricans drifted away from larnin stuff and into MBAs, and well, you know. Might even be worth the RT shipping, should you get a perfectionist heat-on.. knowing exactly what you are transmitting makes such a weirdness as you describe: almost a pleasure to screw with!
Else, carrion. If you can bear to never get to.. the bottom of this magical phenom. ;^>
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Post #406,264
11/14/15 12:46:34 AM
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I was hoping you'd reply.
:-)
I never developed an intuition about radio, but your explanation makes a great deal of sense to me. Even without an explicit amplifier stage, antennas do sorta "amplify" signals if they're the right size and design. Having a tiny air-gap with a wonky preamp would seem to explain the observations.
Cheers,
Scott.
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Post #406,268
11/14/15 12:40:46 PM
11/14/15 12:41:35 PM
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Re: One explanation..
No, the antenna doesn't have a preamp.
And, being the Philistine I am, I'm not going to mess with it until it stops working, I have far too much else to do than to go back to messing with the sort of stuff I messed with 40 years ago - unless I absolutely have to.
But, thanks for the well thought out reply.
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Post #406,285
11/15/15 4:31:33 PM
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Non-guilt seconded here, re ignoring gadgets + a PS on du Pré
(Besides, the fun in all that is {..a holy phrase for one R.P. Feynman} the pleasure of finding things out.
Butcha needn't find-out some one thing, 100X.
I now consign n fixable objects to recycle; many just to remove one more from my horizon (so I can glance momentarily at today's newest homo-sap fucked-tribe obscenities in livid-4G Cheney-colour. )
Daily across the U.S. some bitchin Tektronix, hP and other standard-Standards are relegated similarly (while physics hasn't changed much and all were/are quite Useful: but only to tribes not yet entirely plugged-in 17/24 to computer generated sonic mass-distractions.
I suspected we'd go out with mondo whimpers ..but not with non-stop pewlings and appeals to the great-god Mammon (that predator who eats your guts from-the-inside-out.)
PS: KDFC, the once S.F. classical FM station which went dark for years of Not One!/in the so-called Paris of the West: Now is web-wide and organized, part-funded by USC! in your bailiwick: now via crowds, worldwide one supposes.
Played Jacqueline's Elgar cello cto recently (unsure if I have a copy still) ... indeed the modern Yo Yo cannot raise the visceral goose-bumps of that tragic beauty's rendition which I heard as a new release.
From the first robot-pisno-machines onward, we are so fortunate to have preserved certain Masterful performances forever. (Amelita Galli-Curci, anyone? a colorature soprano to Die-for: once seen live by My Gramma.)
Rescue something Still-alive today, especially something starving or recently abused by a biped. Often intentionally, as things fall apart with alacrity.
Screw the machines, except when you need one to get out of quicksand.
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Post #406,283
11/15/15 4:04:27 PM
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Well, it worked great for a day.
Next day the sound started to break up, and wiggling the antenna wire couldn't fix it.
Fortunately, I was able to find another FM tuner (never used) in the garage, and it's happy with the antenna (though I had to do some Micky Mousing because it has some European style antenna connector). It's not as fancy - I seem to recall it was designed for disk jockeys (of the party entertainment sort), but I need decent sound, not features. My FM tuner stays permanently tuned to KUSC.
So, scratch one Teac FM tuner.
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Post #406,284
11/15/15 4:09:48 PM
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So, when was the Teac FM tuner made?
I suspect you got your money's worth. Alex
"There is a cult of ignorance in the United States, and there has always been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that "my ignorance is just as good as your knowledge."
-- Isaac Asimov
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Post #406,286
11/15/15 5:26:34 PM
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Re: So, when was the Teac FM tuner made?
Well, finding that out would take a lot more time than I want to spend on it. I neglected to list it on my personal information page, so the purchase is buried somewhere under a few thousand business purchases. I'm pretty sure, though, that it didn't last as long as I usually expect from such equipment, maybe 4 or 5 years.
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Post #406,287
11/15/15 6:18:41 PM
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So, no date of manufacture on device itself?
That's what I thought you'd have. That goes back to the days when new models weren't created every 6 months, but had lives measured in years. Alex
"There is a cult of ignorance in the United States, and there has always been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that "my ignorance is just as good as your knowledge."
-- Isaac Asimov
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Post #406,291
11/15/15 8:02:19 PM
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Re: So, no date of manufacture on device itself?
Nope. A serial number, but no dates.
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Post #406,292
11/15/15 8:09:44 PM
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A Google Image search can sometimes narrow things down.
E.g. is it this?  That's from 2003. Many more are here. Have fun. Cheers, Scott. (Who mostly listens to NPR in the car and his CDs on Google at home.)
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Post #406,295
11/15/15 8:44:41 PM
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Not the one.
It was a T-R670, which, looking at reviews, was sold from 2005 to at least 2012.
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