HI everyone, and thank you all for your input, much appreciated ! Looks like we have some people well versed in theses old Tek scopes, I should be covered then !
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Yes last night was a bit of a rush... was trying to get as much done as possible befonre going to bed. Not gonna repeat that tonight... need some sleep eventually ! LOL...
So, sitting back a little tonight, digesting the information...
FET :
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I removed what I thought was a funny looking/custom Tek thing... and in the end that funny package was just a big plastic piece wrapping the FET to press it against the heat sink. The FET is indeed, phew, just a regular TO220 package. So that's a relief, there are no custom/impossible to find component involved in this repair, I should be able to fix it for cheap, and enjoy my scope for a long time to come.
Part number ? Indeed as one of you said, Tek doesn't stick to the BOM religiously at all : BOM said MTP 6N60E... my scope comes with 6N55.
I Googled a bit and found a replacement for the 6N60E. An IRF BC40A. Exact same specs (600V, 6A, 1.2 ohms), still available today from Farnell and only 1,5 Euros or so ! Happy I am.
The plastic connector of the FET is cooked, but the FET itself looks absolutely sparkling new ! No sign of over heating anywhere, be it on the terminal or the plastic body. However... I tested it with an ohm meter and I get either 2 ohms, or a dead short, no matter what pins I check and no matter the polarity ! OK I expect a short when the drain-Source diode is forward biased, but when in reverse I would expect an open circuit since it's an enhancement type FET...
As for the Gate-Drain or Gate-Source, of course I would not expect a short there either...
So, the FET is definitely cooked.
Thanks for the debugging tips, will proceed. The schematics are great and supply vottage and waveforms n various places, how nice from Tek...
Looks like the preregulator runs at 60kHz or so, I guess any FET should be able to cope with that...
the inverter is even more lazy, 20+ kHz.... well I guess the transformer downstream couldn't cope with more than that...
Schematics indicate 145V once the mains is rectified and fed to the FET... so that must be for the 110V mains in the USA. On my side of the pond it's 220/240, so I expect 300V+ or so.I assume the preregulator is designed for European market so as to still give the 40+Volts anyway, not 80...
OK so now that hardest part is to come : get access to th board, so I check all the components, and e able to remove and replace all the cooked components.
The heatsink is right in the way : most of the components involved in the accident, are right at its feet so to speak. It being very tall, makes it a nightmare. So I have not much choice but remove it :-/ The small three terminal regulator hooked onto it will have to hang in the air... should alright just for the time of the repair... Fan mountng hardware, EMI board and mains socket all get in the way too, need to move them out of the way.
Not sure if the analog board, can come off the chassis without taking the whole scope to bits... crossing fingers. Will check the service manual for info on that...
So, first get a good access to the part of the board that's hosting the PSU, then I can start working on it...
Oh, as for isolation, I do happen to have an isolation transformer, ancient thing I have never used.. that will be the opportunity, great. Rated at 100VA max, hope it will be alright...
Ah, need to check that bulb in series thing, not familiar with this trick...
Thanks again for the input, will update the thread as I go...