Do We Have A Fix?
Time to roll the drums, take a deep breath, plug it in, start the camera, duck (just in case) and throw the switch...
Now that the unit is back from the dead, it is time to assess its remaining issues. As mentioned in the interlude, three of the five output filter caps I could easily pull out for measurement were in poor shape and definitively required replacement, two of them from the 5V rail. Predictably enough, with the 5V rail having the two worst capacitors of the main output lot, the effect on output quality is obvious here with periodic pulsing to 6V.
I do not have 10V 2200µF or better fresh capacitors that fit under the heat sink due to circuit board components, including other capacitors, being packed too tightly around the capacitor footprints to accommodate replacements any wider than the original 10mm diameter. However, I do have a HiPro power supply from an IBM ThinkCenter PC that had a catastrophic primary-side failure and was packed with 10mm 10V 2200µF Teapo SC capacitors for the 5V, 5VSB and 3.3V rails which seemed like perfect replacement candidates – definitively nice upgrades for the original Fuhjyyu TNR and Teapo SEK assuming the Teapo SC are still good. Since the donor power supply presumably had a quick demise rather than a slow capacitor decay one, I am fairly confident its secondary caps will be in reasonably good shape. And sure enough, they all tested well within tolerances for capacitance and were indistinguishable from the Panasonic FM on my ESR check. Reasonably convinced that my salvaged caps would perform substantially better than the dead or dying caps they are replacing, I decided to put the SCs in.
I could not find a direct replacement for the 10mm 4700µF Fuhjyyu TMR, so I just slapped one more 2200µF SC in its place. At a glance, it seems 2200µF is the largest capacitance most manufacturers offer in 10mm diameter low ESR format with 10 to 16 volts rating.
My donor power supply did not have 8mm caps for the 5VSB output either, so I had to leave my 10mm 1200µF FMs flapping in the breeze until I order 8mm parts for that. Conveniently enough, the Rubycon 1000µF 10V ZLH I planned to order for the SL300 will fit nicely here too.
After letting my repaired unit simmer overnight to see if the 10Ω resistor would survive with the main outputs enabled and powering a 20W 12V halogen desk lamp, I went back to have a look at power-up transients. Everything looked fine. The 5VSB output overshot by 200mV while the main outputs only overshot by about 100mV, well within ATX's 10% power-up transient tolerances. The main outputs rise from 0% to 100% in 15ms, which also meets ATX's 20ms maximum requirement.
Once these measurements were done, I disconnected the power supply, opened it again and felt around for anything unexpectedly warm. The first thing I checked was ZD4 and components immediately around it. I also took a look at my new R29. Its markings were still white, a sign that if it did heat up, it did not get hot enough to discolor the ink. It seems ZD4 is clearly not meant to dissipate any significant amount of power when good capacitors are present and provide the bulk of flyback clamping for the standby/auxiliary transformer's outputs.
What are those zeners for? They are primarily intended for clamping leakage inductance. While the bulk of flyback energy in the 5VSB/auxiliary transformer ends up on the 5VSB output, a small amount of it will still get dumped on other windings due to leakage inductance. Since the auxiliary windings may have negligible load attached to them, the stray inductance energy can cause auxiliary capacitors to slowly creep up to arbitrarily high voltages. The zeners simply put a ceiling on how high this is allowed to go.