[Ron Ramirez]

Arran and Tim,

I meant to post these schematics - one of the 100-T, the other of the FM-100-C - a few days ago. But then my "taste of retirement" (ten day holiday) ended on Tuesday morning and it was back to the salt mines, so I sort of forgot about it.

I've been studying this circuitry, and it seems that the older (pre-FM multiplex) Fishers took the output from pin 6 of the ratio detector transformer through a 270 ohm resistor. From late 1961 on, though, after the advent of true FM multiplex stereo, some Fishers took the output from the junction of the two 6.8K resistors while others continued to take the output from pin 6 of the ratio detector transformer through a 270 ohm resistor.

The Wide-Band ratio detector transformers (made from late 1961 on for FM multiplex) have the internal diodes reversed with respect to the internal diodes in the older FM mono ratio detector transformers. The circuitry is the same - only the 8 uF capacitor gets installed in reverse polarity with the newer transformer, because of the diodes being connected differently in the Wide-Band transformers.

It is known that the Wide-Band transformer can be substituted for an older-non-Wide Band transformer in an older Fisher where the user plans to install a multiplex decoder for FM Stereo. The only thing that needs to be done to the circuit is to switch polarity of the 8 uF electrolytic; otherwise it is a direct replacement. However, that statement does not apply to the 100-T, as I later found out.

I installed a Wide-Band transformer in my Fisher FM-100 tuner, which (like this Coronet) has space for an add-on multiplex decoder. I also reversed the leads of the 8 uF electrolytic. It works just fine, no problems. And its Magic Eye still works properly after the switch, just as it should.

This is why I suspect something is wrong with the FM-100-C transformer. The circuitry is similar enough that it should have been a direct replacement, plus switching the leads of the 8 uF electrolytic which I did.

Now, when I reinstalled the 100-T's original ratio detector transformer, I also switched the leads of the 8 uF electrolytic back to factory specs.

See the attachments for full schematics of both tuners. I apologize that the 100-T Coronet schematic is low resolution, but it should be good enough for you to see it. The FM-100-C schematic is higher resolution, easier to read.