12V short in RF board 1.0 (classic bands)

[vk2tty]

G'day! My (tr)uSDX with the RF board attached wakes up on 5V USB but not on 12V via the DC jack. If I remove the RF board, the main board wakes up OK. There's only 6Ω between the 12V and GND pins on the edge connector of the RF board.

I tried tracing from pin to pin to find the short but figured out pretty fast I have no idea how to interpret any of the measurements. How do I troubleshoot from here? Where should there be connectivity? Where shouldn't?

I've got a multimeter, an RD6006P bench supply with a current limiter, and an oscilloscope I've never used but'll give a shot if you tell me where to measure and what to look for.

In case it's relevant, I've got nothing in Q1–4 because they've been replaced by an FDT86256, says the sheet from UpTech that came with the kit.

[dl6sez]

Hello OM,

look for shorts due to soldering.
If you don't find any remove FDT86256.....
The only lonely possible thing for me
You killed it maybe already due to statics discharge ESD.

73 de Chris

[vk2tty]

Oh, bugger. I was hoping to be able to use my radio during my holiday. Dead components will make that very, very difficult.

It's only four pins, I've found pages on testing MOSFETs, and removing components makes it easier to isolate shorts. I'll take it off and have a look.

Is it safe to give the RF board 12V with some current limit in the hope I can find the short radiating with an infrared camera? Any other tips?

[dl6sez]

Hello OM,

yes you can do so 200mA should be okay. Why not only remove one end of L15 the dark grey toroid in the near of the Q1 to Q3 holes, if it then doesn't draw more than 100mA it is your FDT86256.....

73 de Chris

[vk2tty]

That, I will try right away. Also, trying to figure out with the schematic where the remaining 100mA is supposed to be going.

[vk2tty]

After un-hooking one end of L15 on the RF board the unit wakes up and draws 67mA idle at 12V.

While I'm buying an FDT86256, what other cheap spare parts should I buy just in case? I'm collecting a list of (tr)uSDX killers to avoid so we're not leaving the next poor bastard up a tree, too.

[vk2tty]

I've ordered an FDT86256 and an ABM-713-RC electret microphone in case I over-heated the one that came with the kit. If there's anything else I'll likely need, please let me know, else I'll hope I haven't blown anything else.

[vk2tty]

yes you can do so 200mA should be okay. Why not only remove one end of L15 the dark grey toroid in the near of the Q1 to Q3 holes, if it then doesn't draw more than 100mA it is your FDT86256.....

I've ordered an FDT86256…. If there's anything else I'll likely need, please let me know, else I'll hope I haven't blown anything else.

Chris got it exactly right. I had blown the FDT86256, and nothing else. When I replaced it, my (tr)uSDX sprang to life. Thanks, Chris!