AM Radio reception fades in in and out

[62 Val]

My '62 Valiant stock AM radio works, but reception fades out and then comes back (very loud) and then fades again recurrently. I have adjusted the antenna gain but the problem persists.
Would a weak tube cause this problem? If so, which one would be the likely culprit?

 

[pauly v.100]

not a radio expert but i'm old enough to remember when am radios were the only game in town. they have weaker reception than fm's and will almost always fade in and out depending on where you are. i think the reception is related to line of sight with the radio tower. try going into a parking garage or tunnel, you won't pick up anything at all. atmospheric conditions can have an effect too. i used to be able to pick up a station in alberta, canada from a tiny am transistor radio in seattle.

 

[62 Val]

Thanks for your response, Pauly. I would tend to agree that signal strength from the transmitter may have something to do with the receiver processing the signal, but I have a feeling that one of the amplifier tubes may be malfunctioning. I reckon I'll have to go to work and try to find a shop with a vacuum tube tester in the Seattle-Tacoma area. If my recollection serves me,
I believe that the FM signal is dependent on line of sight, as FM stands for Frequency Modulation, and AM is more likely to travel a greater distance, as AM stands for Amplitude Modulation, and its signal travels in more of a wave-pattern.

 

[Longgone]

Do you have a telescopic antenna? They used to make an antenna about 60" long. I would definitely suggest one if you don`t have one yet. The long antennas look weird but they do a good job of picking up and holding AM broadcasts.

If you adjusted the trim on the radio I guess there`s not much more you can do there. Check that the antenna base is grounded well. Is your radio not solid state?

 

[doktor_x]

Did those radios have a switch for local/distant, loc/dx or something of that nature? I'm wondering if it could be a capacitor going south or maybe just some oxidation in such a circuit. My electronic skills are very rusty, so I may be way off.

I have a tube tester, but it doesn't do much good at this distance.

 

[Claudius]

You are correct in that FM is line-of-sight. This is why travelling up and down through hills results in sporadic loss of FM (and cell phone) coverage due to the shadowing by the topography. As for AM, it tends to travel long distances by "Tropospheric ducting", via the atmospheric layer for which it is named.

Going into a concrete structure means that you are going into a Faraday Cage created by the metal re-bar that strengthens the structure. If the gaps in the re-bar are smaller than the wavelength of the signal, nothing passes through. If you happen to be travelling near high voltage power lines, the electric field radiating from them may be saturating your receiver and blocking AM reception. Although the power frequency is only at 60Hz, the high amplitude of the level (ie: >300kV) can be spawning multiple harmonics (120Hz, 180Hz, etc) until one of them is within the AM band and lands on your station. Your radio is only looking for a signal whose strength is measured in microvolts (or smaller).

Thanks for your response, Pauly. I would tend to agree that signal strength from the transmitter may have something to do with the receiver processing the signal, but I have a feeling that one of the amplifier tubes may be malfunctioning. I reckon I'll have to go to work and try to find a shop with a vacuum tube tester in the Seattle-Tacoma area. If my recollection serves me,
I believe that the FM signal is dependent on line of sight, as FM stands for Frequency Modulation, and AM is more likely to travel a greater distance, as AM stands for Amplitude Modulation, and its signal travels in more of a wave-pattern.

 

[62 Val]

Thanks guys,
My microvoltaic head is now clearer. I do have the original, very long, telescopic antenna, although I don't keep it fully extended as to not hit the top of my garage door. I like the theory of a failing capacitor, but couldn't a tube react the same way? My radio is not solid state, and has 5 vacuum tubes. I think my next recourse is to test the tubes and proceed from there. The tubes, according to my shop manual, are:
12GA6
12BL6
12AE6A
2N573
MN-29 or 2N176
I'll try to keep ya posted...

 

[Fishthatkills]

Electrolitic Capacitors dry out over a period of time. They do not last forever.
Since this is 47 year old radio I would bet it's time for a rebuild.

John D. Beckerley
Austin, Texas

 

[Claudius]

This is probably a very good idea as electrolytics do dry out. FYI, when replacing the electrolytics, please observe these guidelines:

1) Stick with the original values of capacitance (ie: microfarad rating).
2) You can safely choose a higher voltage rating than original if you need to without affecting performance or longevity. NEVER go with a lower voltage rating as it will have a shorter life span.
3) Strictly observe the polarity of the electrolyic device (+ and - leads) or you will blow the device apart with the first application of power and have other issues to deal with.
4) If there is a question about the Equivalent Series Resistance (ESR) of a given device when you are comparing multiple options, choose the lower value one because it filters noise out more effectively.

Geek lesson over.

Electrolitic Capacitors dry out over a period of time. They do not last forever.
Since this is 47 year old radio I would bet it's time for a rebuild.

John D. Beckerley
Austin, Texas

 

[doktor_x]

Thanks guys,
My microvoltaic head is now clearer. I do have the original, very long, telescopic antenna, although I don't keep it fully extended as to not hit the top of my garage door. I like the theory of a failing capacitor, but couldn't a tube react the same way? My radio is not solid state, and has 5 vacuum tubes. I think my next recourse is to test the tubes and proceed from there. The tubes, according to my shop manual, are:
12GA6
12BL6
12AE6A
2N573
MN-29 or 2N176
I'll try to keep ya posted...

Those numbers beginning with "2N" look like transistor numbers. Testing tubes is a sensible step toward narrowing down whether it's an amplification issue or something on the tuner side.