Follow up. Also posted to? - QMX Failure on power up - .
Checked resistances on 5 V and 3.3 V buses on problem (#2) QMX and compared to my good QMX (#1). They were the same.
The #2 QMX connects to the computer just fine when all parts, including the bad P/S board, are in place.
So, I felt that the #2 QMX main board, 3.3 V p/s, display board, and control board were not damaged.
The following were done without transmit testing.
I put the #2 display board and the #2 control board into the undamaged #1 QMX. All worked fine with normal current draw.?
So, I took both P/S's from the #1, good QMX, and put them into the #2 QMX with #2 control board and #2 display board.
#2 QMX worked perfectly well with normal current draw.
Conclusion:
5.5 V P/S board on #2 QMX is only fault, so far. Will need to check for proper reception and power out on Tx yet, but that will await repair of the 5.5 V P/S board.
The only apparent damage in #2 QMX is to the 5.5V board with the destruction of C107 (470mcF, 6.3 V tantalum cap) and Q108. The drain pad for Q108 was also destroyed, but there doesn't seem to be any shorts here, . R111, R113, D104, D106, and D108 all test fine. I have the C107 replacement. So, I am awaiting replacement AO3415A (Q108) transistors to finish repair to the board.
Analysis:
Why did this fail?? My conclusion is that C107 had infantile failure (internal short) for who knows what reason. That drew tremendous current through Q108 and destroyed that as well. As the rest of the 5.5 V p/s components would not draw extra current with this failure, I do not believe they are damaged - most have tested good. The main, control and display PCBs are not damaged, so high voltage from the 5.5 V P/S could not have been a failure mode. (There was no short from Q104 to Q103. Of course, that would not have caused my issue anyway.)
Any thoughts are appreciated, especially if my reasoning is faulty.? ?Thanks!
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73, Dan? NM3A
