I managed to build my QMX successfully, and in two days of playing with it fried the receive function.? W1NC Jeff Moore diagnosed and returned the radio to full function:
IC402 was destroyed. ?I used to see this frequently in Rev 2 QMXs. ?But this is a first for a rev 4 unit. ?It’s hard to say why this happened. ?Usually it is from back EMF when switching from TX to RX mode. ?But there is a protection diode in place for that after Rev 2. ?I tested that and it seems ok. ?Your power out is high compared to many other QMX rigs. ?Maybe if you backed down the voltage to 11 volts or less, it might help prevent that in the future.
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Here is what I found and did:
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1) Disassembled all boards, cleaned with 99% IPA
2) Inspected under 10X stereo microscope
3) Tested on bench AF, image scan and rf scan functions - all found to be very poor
4) diagnosing on the bench found IC402 bad - replaced that chip
5) retested on the bench AF, image scan and rf scan functions - all found to be 100% now
6) Tested TX functions on all bands at 12V.? At 12V power out is 3.6 - 5 watts depending on band - no attempt was made to optimize beyond that
7) reassembled into case and retested everything one last time - now 100% RX, TX all modes, all bands
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Total time spent - about 2 hours
I had fun this morning snooping on tons of FT8 on 17m using a z-matched random wire indoors.? Weather is a bit wet to string up my outdoor antenna just yet.
Thank you Jeff.
72s?
WB0ZJJ-Alan