R3131 total gain failure - more theory please

[stantelson]

Two R3131 self-test failures - First - CPU - easily fixed with new 3.6V RTC battery and reset button, with great info already in this forum. Second failure - I then have a self-test "total gain" failure. Same failure when cold and after 30 minute+ warm-up. From postings I have already found, suggested to replace R120 in output stage of RF board from 390 to 470 ohms. Other suggestion, to replace 4072 opamps (posting said two, but there are three on the IF board near output connector). Haven't started either of these - would like more confidence that either change makes sense. Thoughts? Thanks.

[stantelson]

as an update, I replaced the 390 ohm R120 with a very small 1K pot and determined Total Gain passed in the range of about 430 to 800 ohms. Planning to replace R120 with an 0805 680 ohm R. Stil curious why this failure is occurring - anyone know root cause of the gain drift?