Early MX110 MPX alignment madness
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| Posted on May 31, 2022 at 21:21:45 | ||
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Posts: 1
Location: TX Joined: May 22, 2022 |
Good evening, I'm attempting to complete a MPX alignment on this MX110 that is turning into quite a chore. I hope someone can chime in here. Per the service manual. Adjusting the 67KC trap - maximum voltage: I'm reading about 15V DC off of pin 6 from the 38KC transformer and chasis ground. This voltage does not change when tweaking the trap slug. Continuing the other alignment steps results in a similar behavior: its just not responding to alignment. Some background: All electrolytic caps have been replaced and the initial FM alignment has been performed. All other inputs produce clean/clear audio as expected. MPX has never functioned correctly on this unit. MA113 was discovered shorted open and replaced with an equivalent GE transistor. The stereo LED now glows on startup, but goes out after unit has warmed up (despite strong FM signals). Another issue is a popping sound from the output when selecting FM or MPX mode. Assuming some DC is entering the circuit somewhere. Removing MPX tube 6U8 makes the popping go away when operating the FM selector modes. Clean FM does play at this point. Swapping in known good 6U8s does not resolve popping. Voltages to most of the MPX tubes are within spec, the exception is the 12AU7, pin 2, at only -300mV vs the -1.8V per the schematic. I tested all the GE diodes on the MPX PCB and they appear fully functional. Potential solutions? Happy for a little direction here.
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