GOOD NEWS!!? ?The sBitx Lives Again!
Allison -- It was 250mA combined; I had previously set it at 360mA combined trying to improve the horrible spurs, which turned out to be due to shorted diodes in part of the complex circuitry that determines which LPF is selected.
Although I could not find my trove of SMD 2N7000/2, I found the leaded ones and on my 2nd try I got one soldered in.? ?That was not easy for me!? ?I put a 200 ohm resistor in series with the DRAIN lead taking the suggestion by Dr. Schmidt, K9HZ -- excellent suggestion!? ?
When the old (destroyed apparently) Q101 was pulled out, the probably basically disappeared.? ?When the new Q101 went in I can't tell for sure there is any difference, but it works, and that is what counts.? ?At 12.5 or 12.8 volts, NO OSCILLATIONS when the unit is powered up, or when sBitx is started.? ?And I also tested with NO LOAD.? ?Whew!? ?Passed everything.
I have now managed to reconnect the "new" switching power supply and its associated inline current meter and wow! the startup current is indeed WAY LESS, so I was correct in my original concern -- that oscillation was chewing up a lot of power!!? ?I would guess now that it was 3Amps or so (but I don't have this well calibtrated).
Here is a photo of the destroyed resistors that the oscillation caused:? (Look just to the left of the orange transformer secondary wiring)
Here is a photo of my amateur attempt at soldering in a new MOSFET to replace the destroyed Q101:
Yes, the iron is probably way too hot.? ?You can see the 200 ohm SMD resistor tombstoned on the drain solder point, and I have bent around tthe drain from the leaded MOSFET to match.? ?For me, this is ticklish work.
I would STRONGLY suggest that the design get modified to put a current limiting resistor (or other solution) to protect Q101 **
Otherwise, I would suspect there are going to be a fair number of sBitx's that reach the secondary market because of "smoke released" and the owner is clueless why.? ? This has apparently happened to me TWICE now and most of my work on this particular unit has just been hours and hours of testing with a load attached, which significantly reduces the amplitude of the oscillation in my case.? ?Units that are turned on with NO LOAD may see far higher oscillatory power once Q101 fails, IF it fails in such a manner that it turns on the IRF4905.? ?(Some unknown percentage, right?)? ? ?This appears to be a preventable failure.
Pretty sure my "Developers Unit" has a leaded Q101 (Qmumble) installed, so it will be considerably easier for me to insert a resistor in the drain lead there.? ??
Thanks for all the insights from Evan and Allison and Dr. Schmidt -- this took only about 24 hours to solve and get the unit working again thanks to your input!
73
Gordon KX4Z
