I'm not all that much familiar with D5XXX models; but: in Samsung nomenclature (for 2009 TV models and up), "MICOM" most certainly means Weltrend (WT61P*) chip - in recent Samsung TV models, it should be rather called sub-micom, or standby-micom, but never mind
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This Weltrend chip has internal, built-in flash ROM (48kB, or perhaps 64kB - depending on the exact version). In case of accidentally executed "MICOM upgrade/update" from factory menu, this internal flash ROM contents will be no longer valid, and that's most probably the root of your problem. Replacing
only the Weltrend chip (as long as it's exactly the same version - and preferably from the very same model/mainboard revision), should actually do the job.
Sadly, there are no publicly available detailed datasheet[s] for those Weltrend chip[s]. That's quite unfortunate, because they apparently have built-in JTAG/ICE interfaces (plus, usually, some kind of their own serial console). I bet the WT61P* MICOM bad-flash situations are in fact very much recoverable (i.e., without replacing/resoldering the chips), we just don't know how to do it
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BTW, I (vaguely) remember: some time ago, some clever guy (from Russia, AFAIRC) managed to dump the Weltrend chip flash contents via JTAG interface (???). But I don't know any details - who, and how exactly.
Still no turning on, but at least, the SEMS21 (Decoder IC) now it?s getting hot (normal heat)... but nothing more...
So... now my question is... if this SEMS21 IC have some inside too, as all the traces from all the NANDs, EPROMS and I2C and HDMI inputs, goes all to this IC.
SEMS* == Mstar-designed & Samsung-manufactured main TV SoC (in D5XXX models - guessing from T-MST5* firmware name - it's actually Mstar X5 chip). I your case, I don't think it is all that productive to touch/replace it (or any of it's peripherals). Also, usually there is separate I2C EEPROM (24C02 or so) connected directly to Weltrend MICOM.
Was there
any reaction at all from your 2nd (supposed to have an "electrical problem" - whatever that means) mainboard - ?
I mean, even in case if the mainboard is otherwise badly shot, in standby mode (as long as Weltrend chip is up-and-running, the A5V standby voltage is provided to the mainboard, and the 5.5V -> 3.3V DC/DC converter is OK), usually the front LED will at least flash while receiving signals from remote control. If not, perhaps you replaced the badly-flashed Weltrend chip with otherwise-electrically-damaged Weltrend chip
