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Save my Stereo Bacon

xanadu1797

xanadu1797

Active Member
Hi All,
Quick question:
My Fisher 500c stopped locking into stereo mode recently. I'm assuming the stereo beacon amp tube might be bad. I have little experience with working on this unit though so I'm open to other suggestions.
Assuming replacing that tube is a good idea which should I replace it with the EC97 or the 6GK5 tube? My understanding is they are different tubes but they are listed in the 500c manual for the stereo beacon.
 
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Use the 6GK5. Apparently there is a mistake on the tube layout and manual for this tube. The EC97 is a totally different tube. The 800-C manual also shows EC97/6GK5 for the Stereo Beacon tube, But for the V1 position shows EC92/6gk5. Neither of the "EC" tubes are correct. STICK with the US Designation on the tubes as much as possible and you'll be fine.
 
Yes it is. Another basic check you can do is to watch the Stereo Beacon lamp when you first turn the set on. The Stereo Beacon lamp should come on immediately when the set is first turned on. After a couple of seconds however, the lamp should to out, indicating that the control tube has warmed and is now pulling current through the relay, causing it to pull in, and extinguish the lamp. When a stereo station is received, the trigger signal from the MPX unit actually turns the Stereo Beacon control tube off, allowing the relay to return to its resting state, and turning the lamp back on again. As you might be able to guess then, if the Stereo Beacon control tube is bad, the symptom will be that the lamp will not turn off when it should -- not that it won't turn on when it should.

Finally, if the relay is not activating as it should, then besides the lamp not functioning correctly, neither will the set go into stereo mode when the selector switch is set to automatic, as it is the relay that triggers the audio switching circuits as well.

Dave
 
I did some more checking on the EC-97. Apparently it's a EURO version / compatible xref for the 6ER5 which is a close substitute for the 6GK5. So conceivably the EC-97 would work with a possibly some degradation of operational parameters. My original statement still stands. Use the 6GK5.
 
Ok, so what other reasons might the stereo not be working? My stereo beacon light comes on and shuts off during power-up...
 
Lack of an antenna, or poor antenna performance due to placement or orientation. At minimum a 6ft dipole antenna placed as high up as you can get it, and oriented 90* to the direction of the signal. An outdoor Antenna is better.
How far from the transmitter tower are you?

Do you have obstacles between you and the transmitter (hills, industrial bldgs higher than your house, Power lines, etc.)

FM is line of sight. It does not bounce off the ionosphere like AM does. The further you are from the transmitter the weaker the signal. FM STEREO requires as much as 4-5 times as much signal strength as MONO FM.
(Google FM Multipath).

Turn the selector to FM STEREO FILTER and see if you get a lamp. Can you hear the difference between mono and stereo? Or does the sound stay the same, if the lamp comes on in FM STEREO FILTER. Bump the tuning slightly left and right as it could be slightly out of alignment. On one of my 800-C's I have to turn the knob slightly off center to get the Stereo to kick in. That indicates that the alignment is off. And in that case, you need to find a shop to align it.
 
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