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 neonixie-l+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/4923e894-fd37-4ccd-8356-ea6f20cf74f3n%40googlegroups.com.
