Radio problems with FM band

[Mike Self]

All right, all you electronics engineers.  The FM band on my Alpine AM/FM/cassette stereo (vintage around 1987) has decided it no longer wants to work.  It started out intermittently cutting out, and now it's no longer there on any FM frequency, and not even the strongest stations come it at all.  However, the AM band works just fine.   

 

Not even knowing enough electronics to be dangerous, here's what I've done diagnosis-wise:

• The unit is obviously getting power as the AM band works, the dial display lights up etc.  
• cleaned the contact at the bottom of the Bosch (Blaupunkt) antenna where the mast joins the contact at the base of the windshield
• substituted another antenna connected directly to the antenna lead on the radio.  
• With the antenna disconnected, no AM either, but reconnecting brings the AM band back
• switching from stereo to mono has no effect, nor toggling the local/distance switch

 

Am I overlooking something obvious, or is this a problem in the radio's circuitry and thus will need a visit back to Alpine (or a replacement unit).  

 

TIA

 

mike

 

[stephers]

Sound like an internal problem      need antenna hooked up to get AM but not needed for FM   ( not the best sound but it will work that way )

 

Thanks, Rick

[Steve Tochi]

Mike,

I don't know the answer but I've been dabbling in the resale of vintage car radios.  Alpines from that era in good working order are commanding some big money these days, so you may be in for sticker shock. I have a guy who should be able to repair it so PM me if interested.

All you old guys go dig out your Alpine and Pioneers, and might be worth something, although Audiovox is not quite there yet.

 

Steve

[Chris_B]

Mike, a free air antenna (hand held, not mounted on a ground plane- i.e., the car body) is not going to work as well as but should provide enough signal to have reception on stronger FM channels. Sounds like the FM section  of your head unit is FUBAR. The definitive test is to swap in a different head unit, even if temporarily integrated just enough to work (ground, power, one channel of output, and antenna feed in).

[calw]

General info:

You should be able to pick up FM stations with a piece of wire a few feet long out in the open.  AM suffers due to the electrically "short" antenna that's practical on a car, and at least older radios had a trimmer capacitor adjustment on the back side which makes a difference when someone bothers to adjust it.  Newer car antennas (think shark fins and the like) are amplified and eliminate this deficiency by brute force. 

 

Specific info:

It sounds like an internal problem, yes.  Having been intermittent, it could be as simple as a connection that's vibrated loose in the radio at the input to the FM front end- a cold solder joint, wire jumper, etc.  Particularly if the intermittency (if that's a word) was an receives fine/ receives not at all situation- not a strong/weak signal situation.  With my moderate electronics abilities, I'd look inside and see if the circuitry is made of mostly discrete components with solder traces visible to the naked eye that I could get at with my soldering skills and have a go at making sure there aren't any loose connections or cold solder joints.  Probably not too likely because by that era circuit boards were all wave soldered under pretty tightly controlled conditions, but worth a look.  On the other hand if its just a few integrated circuits, I'd be putting the covers back on and letting someone else try to repair it if it's worth the cost.

[Chris_B]

4 hours ago, Mike Self said:

All right, all you electronics engineers.  The FM band on my Alpine AM/FM/cassette stereo (vintage around 1987) has decided it no longer wants to work.  It started out intermittently cutting out, and now it's no longer there on any FM frequency, and not even the strongest stations come it at all.  However, the AM band works just fine.  

Mike, if you are willing to give up your vintage cassette unit for a more modern head unit, I have several spares. I used to be somewhat of a car audio freak, but at this point all of my older cars have been upgraded and my newer cars came with audio systems not really needing to be upgraded. In any event, I have a number of head units that I experimented with at various times, including Sony and Pioneer CD players and I believe one pioneer bluetooth unit that will do mobile phone, pandora/USB input and the like, but doesn't have a "hardware" media player. Your are welcome to have one, if you pay for the freight...