Just a quick addition - the dead Xicon electrolytic had no visible signs of failure - no bulging or leakage at all. The DMMs, however good they are, were mostly useless in this case as they were simply confused by what was happening - no DMM or analog VM would have helped much - an analog VM would probably have been worse as it may have reacted as a true-RMS meter which would have hidden this issue completely. At least the DMMs went a bit bonkers, which made me think a bit!
The 'scope is what nailed it; the gate on the FET was clear, as was the mad HT output line. Nick On Thursday, 14 September 2023 at 14:22:00 UTC+1 Nick wrote: > So, > > I have had an original NixiSat from new, so a long time now - I detailed > it's restoration to a more modern time source and working condition in > these two threads: > > https://groups.google.com/g/neonixie-l/c/vcNOdSB3nYM/m/3AmeUTtCAQAJ > ...and... > https://groups.google.com/g/neonixie-l/c/evzX0noEGXg/m/JrVaZG3YFogJ > > However, a couple of days ago it had completely died; the PSU, a > switched-mode 12V 2A wall-wart was fine, but the clock was completely dead. > > A quick check showed that the tiny 1.5A fuse had blown... and the IRF740AS > FET was TOAST, almost literally. It had got so hot that it had burnt the > PCB, (1cm charred circle) lifted the track and died to the extent that it > no longer identifies as any sort of semiconductor... it's shuffled of this > mortal coil and gone to join the heavenly choir invisibule... . > Interesting smell, well known to EEs (no, not pizza, burning FR4). > > I replaced the fuse, did some clean-up of the crispiest bits, removed the > CPU, dead FET , the tubes (position recorded for each tube as it was > wrapped and stored) and HV drivers and did some voltage checks. The MAX771 > was running and the 5V rail was fine... the HV drivers and CPU were looking > good. > > So I replaced the FET with a more modern FDB14N30 which is also in a > TO-263 (D2PAK) and is just generally a better FET (and I had some) - > specifically lower Qg & RDSon; similar VGS(th). Tracks repaired just for > the FET drain with 28G TCW. > > Using a good lab PSU (Tektronix PS2521G), 'scope (Tektronix 2465A) and > DMMs (Agilient U1272A & U1253B) plus an Aglilent FLIR, I started looking > for trouble... it arrived quickly. > > With 12V in, the current went right up to 260mA and the FET temperature > started climbing rapidly - at 70C I cut the power. The HT rail was not > easily readable and was the first indicator of what might be happening. The > FET gate was looking fine with variable width pulses at about 45kHz but > much more "mark" than "space" - not what was expected for an unloaded HT > SMPS boost controller - it should have settled really quickly into more > "space" than "mark"... > > Looking at the HT rail using the 'scope was revealing - it was all over > the shop, from about 120V up to 290V - sharp rise time and then decay until > the next hit from the inductor... > > Just for sanity, I checked the feedback resistors - 1M5 and 12K7 - they > were fine, but obviously what was happening was that the wild swings were > driving the MAX771 insane, that was keeping the FET mostly on and that in > turn was connecting the FET between 12V and GND with about 0R25 or series > resistance (the current sense resistors, 0R125 and the inductor 100uH and > 0R11) plus the RDSon of the FET (which is about 0R290). > > What on earth would cause this? It's obvious if you think about it :) > > The boost converter output tank capacitor should be 4u7F @ 400V. I removed > it and checked - it came in at 5p5F ! Another dead component. For > reference, it was an Xicon. > > Without a tank, the output will do exactly as I observed and the FET will > cook. Lucky that other stuff like the HV drivers and the MAX771 didn't get > damaged too... > > Replacement cap - I had a nice, good quality Nichcon 10uF @ 300V with the > same pin spacing, so used that. > > All done. Without the tubes, the NixiSat idles now at about 25mA with the > FET stone cold and the gate staying at 0V as the tank was a nice, steady, > 182V. Replaced each tube in original position. All good. > > Clock fixed for almost nothing, but an interesting lesson for those who > haven't fiddled with these sorts of supplies... > > Nick -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/27d627c8-162d-47c0-b04f-f94aa0918d81n%40googlegroups.com.
