Zenith H480W - very weak FM after slide pot cleaning

[Jeffhs]

Hi all:

I have a Zenith H480W AM/FM/stereo FM clock radio from around 1980. It worked very well for years, until I tried to clean the slide potentiometers (volume, balance, tone). When I powered the set on after cleaning, I found that the AM reception was still as good as it's always been (all local stations being received with plenty of volume), but FM is extremely weak -- I can hear stations (exonerating the RF signal circuits), but I must turn the volume up to maximum, and even at that I can barely hear anything. The stereo indicator doesn't illuminate on stereo stations, either. What on earth could I have done to nearly kill FM reception just by shooting contact cleaner into those slide pots? Is there any chance at all of bringing this set back to what it once was on FM?


Thanks in advance.

[jr_tech]

Antenna connection? Perhaps there is a "line cord" antenna connection that was broken if the back was removed?
jr

[Reece]

If you squirted control cleaner in an FM RF circuit, it may have thrown the tuning off. It may dry up by itself eventually and fall back into tune.

Try wiggling the FM and IF tubes. Maybe one got jarred into an oxidized spot. Clean the tube pins and plug in and out of sockets.

[jr_tech]

Here is one on the 'bay... looks solid state to me. perhaps clean the push button switches?

http://cgi.ebay.com/Vintage-Zenith-D...-/300524174437

not affiliated,
jr

[Jeffhs]

I cleaned the slide pots over a year ago, so any traces of contact cleaner have long since evaporated. I am concerned that some of the cleaner might have gotten onto a circuit board and shorted out a component, say a transistor. Since the stereo indicator doesn't light on stereo stations, I'm thinking I might have damaged the stereo decoder IC as well, by virtue of shorting out several pins. What puzzles me is why the AM section still works as well as it does, and why I also lost the pilot light on the slide-rule radio dial (but not the clock display) when the FM all but went silent. Could there have been a short across a transistor or the MPX decoder and the power supply to the pilot light, or a short that damaged a regulator transistor?

I do not have a service manual or a schematic for this radio, so will have to look on eBay. Until I do have at least a schematic, I don't want to poke around inside the set and make things any worse than they are now.

BTW, the H480W radio is indeed solid-state, with most of the set on a PC board. To the best of my knowledge and belief, Zenith never made a tube-type stereo-FM clock radio.

[ctc17]

I have the info on the F480, what MPX IC is in that radio 905-46B?

[ctc17]

I found the zenith service manual for the H480, what part do you need? I can scan some of it later.