Nice that you got it running and thanks for reminding us all of the breakdowns of capacitors, happens to often and a lot of people never suspect them.
I nice forum that has helped me to repair stuff on a few occasions is: https://www.badcaps.net/forum/, lots of helpful people there and sometimes good schematics for the item you are trying to repair. /Martin On Thursday, 14 September 2023 at 15:37:36 UTC+2 Nick wrote: > 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. 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