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  #1  
Old 02-12-2018, 09:09 PM
Jeffhs's Avatar
Jeffhs Jeffhs is offline
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Zenith H480W clock radio - AM works, FM very weak

Greetings.

I have a Zenith H480W AM/FM/stereo FM clock radio which works very well on AM, but FM is extremely weak. I must turn the volume up to maximum to hear anything, and even then the sound is very weak, with a hum in the background. This radio was working well on both bands until I decided to clean the slide controls for volume and tone; as soon as I sprayed the slide pots and tried the set again, FM reception all but vanished. The built-in FM antenna (a coupler attached to the FM antenna terminal, through which the line cord passes) is connected.

What could I have done wrong? I can't believe a shot of contact cleaner (Radio Shack brand, long before RS left my area) could do this much damage. In addition to very weak FM reception, I also lost the dial lights and the stereo FM indicator (the latter now does not light up when the radio is tuned to a stereo FM station, and the dial lights quit at the same time). Did the contact cleaner short out something? I'm thinking the cleaner must have knocked a heck of a short across some common component--common, that is, to FM and stereo FM, destroying that component and all but silencing the radio on that band and mode. I would like to restore this radio to its original operating condition, as it was my first "good" AM-FM-stereo FM radio. Could the contact cleaner have caused an internal fuse (if there is one) to blow when I turned the radio on again, or just what on earth could have happened? Is there a fix for the problem, or did that contact cleaner destroy a transistor or an IC?

Note: I do not have a schematic or a service manual for this radio, and don't have test equipment (save for a digital multimeter) either. If I can repair the set without too much trouble, OK, but if the problem is caused by a failure of multiple components (as I am afraid it is), I will just leave the radio as it is and use it on AM only. As an aside, the power-reserve backup battery for the clock is long gone; that is, it is still installed in the chassis, but since the radio is well over 30 years old, the cell died some years ago, resulting in the clock resetting itself to 11:00 after a power outage.


Thanks much for any and all replies.
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Last edited by Jeffhs; 02-12-2018 at 09:22 PM.
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  #2  
Old 02-12-2018, 10:39 PM
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jr_tech jr_tech is offline
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Bit expensive, but there is a service manual right now on ebay...

https://www.ebay.com/itm/Rare-Origin...kAAOSwRLZUGuM9

not affiliated
jr
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  #3  
Old 02-12-2018, 11:08 PM
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ChrisW6ATV ChrisW6ATV is offline
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It could just be a coincidence, or it could also be something such as a wire came loose from the band-selector switch or another place (if you had the cover off and the chassis open to do the cleaning). If you sprayed the slide controls from the front with the cover still on, maybe enough cleaner did leak into/onto other parts-did you look inside for wet areas or similar evidence?
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Old 02-12-2018, 11:09 PM
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MadMan MadMan is offline
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Quote:
Originally Posted by jr_tech View Post
Bit expensive, but there is a service manual right now on ebay...
...but there's a Make Offer button!

If the contact cleaner destroyed anything, it would have to be the slide pots. More likely, the resulting drainage dried up and is only disabling something. Or, could just be because you disturbed it. Might be a cracked solder joint just gave out completely simply because it was disturbed.

Get out the magnifying glass and start looking everything over. Failing that, get yourself a toothbrush, and lukewarm soapy water and just go to town on the PCBs. Rinse with clean water, dry as best as possible with paper towels, finish with compressed air, let dry.
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  #5  
Old 02-13-2018, 10:01 AM
dieseljeep dieseljeep is offline
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Quote:
Originally Posted by Jeffhs View Post
Greetings.

I have a Zenith H480W AM/FM/stereo FM clock radio which works very well on AM, but FM is extremely weak. I must turn the volume up to maximum to hear anything, and even then the sound is very weak, with a hum in the background. This radio was working well on both bands until I decided to clean the slide controls for volume and tone; as soon as I sprayed the slide pots and tried the set again, FM reception all but vanished. The built-in FM antenna (a coupler attached to the FM antenna terminal, through which the line cord passes) is connected.

What could I have done wrong? I can't believe a shot of contact cleaner (Radio Shack brand, long before RS left my area) could do this much damage. In addition to very weak FM reception, I also lost the dial lights and the stereo FM indicator (the latter now does not light up when the radio is tuned to a stereo FM station, and the dial lights quit at the same time). Did the contact cleaner short out something? I'm thinking the cleaner must have knocked a heck of a short across some common component--common, that is, to FM and stereo FM, destroying that component and all but silencing the radio on that band and mode. I would like to restore this radio to its original operating condition, as it was my first "good" AM-FM-stereo FM radio. Could the contact cleaner have caused an internal fuse (if there is one) to blow when I turned the radio on again, or just what on earth could have happened? Is there a fix for the problem, or did that contact cleaner destroy a transistor or an IC?

Note: I do not have a schematic or a service manual for this radio, and don't have test equipment (save for a digital multimeter) either. If I can repair the set without too much trouble, OK, but if the problem is caused by a failure of multiple components (as I am afraid it is), I will just leave the radio as it is and use it on AM only. As an aside, the power-reserve backup battery for the clock is long gone; that is, it is still installed in the chassis, but since the radio is well over 30 years old, the cell died some years ago, resulting in the clock resetting itself to 11:00 after a power outage.


Thanks much for any and all replies.
First thing, carefully remove that ni-cd battery. I've seen them leak and cause damage to the PC board and surrounding components. A connection might have been damaged and when the cover was removed it came loose.
IIRC, they were made in Korea by Goldstar.
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  #6  
Old 02-13-2018, 03:58 PM
zeno zeno is offline
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Location: New Hampshire
Posts: 3,001
When on FM without a station do you get a loud static OR blank sound ?
If its loud static with a weak station its probably the RF amp.
If its pretty quiet but you can get a weak station its prob IF.
Stereo light will NOT light on a weak signal.
The push buttons are trouble on these, clean them. I have the same
vintage mono Zenith & the switches are VERY touchy.
The hum is probably the main caps. Now that its quiet you hear it.
As far as the spray goes it can cause detuning BUT I dont think that is
it. Try it again when well dried.
Safe way to clean the PCB is 91% isopropyl alcohol. If its residue that
should work. When done dry it under a hot lamp.
Remove the back up battery. They do leak & can ruin a radio.
And yes most the Zenith radios of this era were made by Gold Star
to Zeniths specs. The better ones had TRF AM. IIRC yours did.

73 Zeno
LFOD !
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