12-23-2017, 08:19 PM
I replaced the electrolytic in the ratio detector circuit.
While I was at it, I also replaced the ratio detector transformer with one from a Fisher FM-100-C tuner, along with a number of resistors in that circuit.
Why?
Ratio detector transformers from 1962 and later tube-type Fisher tuners and receivers are "wide band" transformers which are much better suited for FM Stereo. The coupling between windings is different than in the older ratio detector transformers. This tuner (built around 1960) was not designed for our FM Stereo standard and is in fact mono, but has space on the chassis for an FM Multiplex decoder. With this ratio detector transformer it will be able to not only handle stereo (after the decoder is added), but it will perform better than would the original, narrower band ratio detector transformer.
So I tried it out on FM again and...nothing.
Then, noticing the FM band tuning eye was not moving at all, I disconnected it.
And after that...I now have FM.
The reception is very low, but it is there. It will need a complete alignment, especially with the replacement of one IF transformer and the ratio detector transformer. But FM is at least working now.
I will have to replace that tuning eye, also I suspect it is shorted internally...just enough to knock out FM reception while allowing the voltages in the FM section to appear normal.
While I was at it, I also replaced the ratio detector transformer with one from a Fisher FM-100-C tuner, along with a number of resistors in that circuit.
Why?
Ratio detector transformers from 1962 and later tube-type Fisher tuners and receivers are "wide band" transformers which are much better suited for FM Stereo. The coupling between windings is different than in the older ratio detector transformers. This tuner (built around 1960) was not designed for our FM Stereo standard and is in fact mono, but has space on the chassis for an FM Multiplex decoder. With this ratio detector transformer it will be able to not only handle stereo (after the decoder is added), but it will perform better than would the original, narrower band ratio detector transformer.
So I tried it out on FM again and...nothing.
Then, noticing the FM band tuning eye was not moving at all, I disconnected it.
And after that...I now have FM.
The reception is very low, but it is there. It will need a complete alignment, especially with the replacement of one IF transformer and the ratio detector transformer. But FM is at least working now.
I will have to replace that tuning eye, also I suspect it is shorted internally...just enough to knock out FM reception while allowing the voltages in the FM section to appear normal.
--
Ron Ramirez
Ferdinand IN
