Re: QMX RX dead after testing TX with dummy load - can FST3253 replace CBT3253?
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I've been following this thread and added the diode.
I thought I would include the image of what I did, and sounds like the same placement.
73 and Happy New Year!
Chris?
Bev, post a picture of the diode mod. It will be useful to all.
JZ
SOLVED
I replaced the smoked CBT3253 by an FST3253 from my home stock, everything works.
Then I was looking for the "safety diode" installation mentioned here:?/g/QRPLabs/message/115735
The "right end" of C508 was easy to find, it is the one not connected to ground, can be checked by a multimeter for continuity. Then it appears from the PCB sketch in assembly manual that the place for diode anode is the "opposite" end of L503. So if you look at C508 and cathode of the diode should be connected to the "south end" of C508, the anode the has to be soldered to "north end" of L503. The distance is just about far enough to acommodate through-hole 1N4148 with leads bent? tightly. Leave the bent leads of 1N4148 just long enough to keep the diode tightly above Q508. Before placing the diode to PCB, make sure to have ends of leads tinned by fresh solder, then solder diode leads to C508 and L503, adding a micro droplet of solder if necessary.
It took me several hours to figure out how to solder the diode, especially with regard to L503. In the process I mechanically damaged Q508 (BSS123) but luckily I had a few spare 2N7002's (exact equivalent) so I was able to finish everything without having to wait till morning when they open electronic parts shop :)
I hope the diode will finally prevent IC402 from further failure for good because my QMX produces 6 watts on some bands. Since I finished the kit, I had to do two repairs and one mod and I think now I deserve enjoying some fun with the QMX. It's a beautiful and effective radio, I have never had such satisfaction working other QRP rigs (NorCal 2030, KX3).
73 Jindra "Bev" OK4RM
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