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Tonight I had made15 contacts via ft8 with no issues on 40 meters through a vertical with 1.2-1 swr.? I switched to 20 meters, and adjusted my vertical till I had a 1.1-1 swr.? Mid-transmission, my sbitx shut down.? No smoke, just shut down.? I unplugged it and tried cycling the power again, but nothing.? There is no power to the pi, because the red led is not on.? Where should I begin my diagnosis?

Yes it is getting power. I swapped power supplies, and it still did not turn on. I also used its power supply on another radio, and it worked correctly

Scttmtclf,

I would verify that the LM338T provides the 5volts to the Raspberry Pi (RPi).? You could verify that the RPi is still working by removing it from the radio, connecting an HDMI display, a USB Mouse, a USB keyboard,? and providing a USB-C voltage supply.? If the regulator has failed, I would wait for the upgrade kit from HFSignals that has the replacement regulator and finals.? If the RPi is the problem, you need to order a replacement.? The RPi 4 is hard to find at a reasonable price.

To reinstall the RPi, follow the assembly manual:


73
Evan
AC9TU

Scttmtclf,

What you describe is exactly my experience. ?The root cause is the LM338 failing and you probably lost one or both of the PA transistors as well. ?The overall problem is the heat sink clamp for the differing sizes of the devices. ?The cure, unfortunately, is to replace the PA transistors and the voltage regulator. ?The good news is Ashhar has just shipped replacement devices.

The replacement voltage regulator is a buck design that is extremely efficient and does not require a heat sink. ?The allows the PA transistors to be solidly clamped to the heat sink. ?I’ve not had one problem since I replaced these devices.

I’m not sure if there is a step-by-step instruction guide but it’s a pretty easy replacement. ?The only tricky part is mounting the regulator.

Good luck, Scott

?

Begin by looking at the 5v regulator. It is the 388 or its replacement switching regulator just next to the 12 volt input to the unit.

Pete WK8S

Scott,?
Do you have a part number for your buck converter?? I think it's amazing that Ashar is sending replacements out.? However I am hoping to do a POTA activation next Thursday and I'd like to move as quickly as I can with the repair.? Tell me if I'm wrong, but the buck converter will replace the LM338, and I just swap 2 new IRFZ24 FET's onto the heat sink?
Thanks you for your explanation
Scott

This is the one I have been using for a while.?

I also need to swap my IRFZ24 FETs but that was due to a high SWR scenario, the buck converter is still working fine.?

Anyone know if it is safe to run the sbitx as a receiver with the IRFZ24s removed (assuming you don't transmit)?

Hi Scttmtclf and Mark,

I used the same regulator and yes, you can test the radio with just the regulator and you are correct, don’t transmit. ?Testing the PA transistors is easy but I would not trust them. ?Digi-Key has the IRFZ24NPBF version for $0.82 each. ?I ordered 10.

The regulator board is very touchy to adjust. ?Be very careful and adjust it for about 5.3VDC. ?I added a low pass filter on the input side and a filter cap on the output of the regulator. ?I replaced one of the main PCB mounting screws with a standoff so I could mount the new board. ?You likely do not need the filter(s) but it was stuff I had in the drawer so what the heck.



Good luck, Scott


. ?

Thank you for the explanation.? Have you had any issues with this regulator using the full 40 watt output?

I follow this group with interest because I might later join the move and buy a sBITX. But I'm quite surprised by the multiple instances of final PAs being cooked. I currently use a Hermes Lite 2 SDR XCVR with an Alexloop antenna. A magnetic loop must be finely tuned on a very narrow band and when tuning the SWR varies from more than 40:1 to below 2:1 when tuned. The PA uses AFT05MS003N mosfets, which are given to accept SWR>65:1 and I have never cooked them;?
73 - Pierre - FK8IH