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Any techs out there? I have a Fisher MPX-100 where the stereo indicator lights up but the unit stays in mono. All tubes check good and the relay contacts have been cleaned. I heard that this is a problem with these units. Any idea?
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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.
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.
Check the resistors, especially in the output. There's often, I don't know what you call it, a clever voltage adder/divider where the L+R and L-R signals get added for output. Everything back then used carbon comp resistors, which wander all over the place with age, heat, voltage, and their own desire to travel. If the wrong capacitor has gone south, that could do it too. Good luck! I wish I had a station here worth listening to.
Hi, morrowaudio:Do you have the proper schematic for the unit in question? I believe that Fisher made two distinct versions of this MPX decoder.
I had one years ago and really did not like it much. The Scott 355's I have owned seem to be better units and they also work much more universally with a wide range of vintage monaural tuners.
Start with the correct schematic, check the voltages and double-check any diodes in the circuit.
I wish you well!
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