PaulWC
Active Member
I recently purchased a wood cabinet for my Fisher MPX 100 and before installing it in its new home I needed to work on the unit since it was not consistently recognizing the stereo signal. You also needed to tune the tuner slightly off it's highest best tuned position to get the MPX 100 to get some stereo indication. Wouldn't hold on to it consistently.
I had previously worked on the unit several years back to recap, clean contacts/controls, tube check and bin cleaning, etc. I also changed out the selenium rectifier for a modern full diode bridge. If memory serves, it seemed to have worked better then than now, although it was never very stable on stereo. Haven't used it much since we don't listen to FM much and the Executive's mono is glorious.
Opened up the unit to check all caps, resistors, etc, and all are fine, except for the voltages to the first plate on the MPX signal V1 tube is way off. The schematic states it should be 150 volts, it's 104. The Schematic also shows the second plate to be 60V but I believe that is incorrect and should have shown 160 V (based on review of lower serial number schematics). I checked the resistors in that power line and all are OK. Something is going on that I can't figure out. Is the replacement rectifier bridge at fault? I needed to change out the resisters in the power supply to get the schematic's voltages.
I could just change out the resistors to V1's first plate and add a resister to V2 to get the voltages in-line. But I wanted to check with you all to see if I missed something. See the pics and any help would be welcome.
I had previously worked on the unit several years back to recap, clean contacts/controls, tube check and bin cleaning, etc. I also changed out the selenium rectifier for a modern full diode bridge. If memory serves, it seemed to have worked better then than now, although it was never very stable on stereo. Haven't used it much since we don't listen to FM much and the Executive's mono is glorious.
Opened up the unit to check all caps, resistors, etc, and all are fine, except for the voltages to the first plate on the MPX signal V1 tube is way off. The schematic states it should be 150 volts, it's 104. The Schematic also shows the second plate to be 60V but I believe that is incorrect and should have shown 160 V (based on review of lower serial number schematics). I checked the resistors in that power line and all are OK. Something is going on that I can't figure out. Is the replacement rectifier bridge at fault? I needed to change out the resisters in the power supply to get the schematic's voltages.
I could just change out the resistors to V1's first plate and add a resister to V2 to get the voltages in-line. But I wanted to check with you all to see if I missed something. See the pics and any help would be welcome.