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In Reply to: Fisher MPX-100 problem posted by morrowaudio on October 24, 2006 at 11:52:46:
I happen to be working on a -100 right now which has a different problem.The stereo switching circuitry works SEPARATELY from the left/right decoding matrices. It looks for the presence of the 19khz stereo pilot sent out by the FM station and switches on when detected. This function is performed by the 6AB4 tube.
There were two significantly different circuits used in the -100. Tube differences are that earlier one used a 6AU6 and a ECH84 (in addition to the 6AB4, 12AX7 and 12AT7). Later circuit used a 6BL8, 12AX7, 2 X 12AT7 and the 6AB4.
My guess is that one or both of the coils (19 and 38Khz)coils may be mistuned.
Follow Ups:
I've sweat bullets over this sucker. Can get the separation better than 25dB (up to 10 kHz) but it has high (and audible) harmonic distortion. Not going to fart around with it anymore. It's off my list as a decent decoder.
HiI'm also working on one as well (the later version).
Do you know where replacement coils can be obtained from by any chance?
Probably your only chance will be out of another unit.
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