The 1963 Fisher R00 is acting (I believe) strangely--maybe it's normal. I can't be sure. The AM audio signal is twice as strong as the FM/multiplex audio signal when the audio output level controls are adjusted for normal listening level. Additionally, the meter shows very low signal strength for AM stations compared to nearly full scale readings for FM. Unfortunately, I find no way to change a part value to cut back on the AM audio-- or no separate adjustments for either AM or FM output level, just the overall audio level controls. I wonder if either diode CR1 or CR2 have anything to do with the AM output level; maybe the unit is having an AVC issue (?).. The AM bandwidth switch operates properly. It's just that I have to turn the amp volume down considerably when switching from FM to AM. That plus the weak AM meter readings make me wonder if this unit has some malfunction. Many thanks for any thoughts you may have.
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Hello,
I would check the input audio signal level (approx 200 mV) (pin 4C ) and the L (pin 4h) and R (pin 4k) outputs of the MPX decoder.
Check and replace C4 25uF and C421,422,425,426 4uF coupling capacitors .
An extra 10- 47 Kohm potentiometer can be installed to set the signal level of the AM output,beetwen the C25 0.1uF capacitor and mode selector switch.
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