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That's perfect, thanks a heap Askild :-o)

I aided keysight first head office in USA for that but, instead the 'applications' engineer reminded me it was out of service and as it was upgraded to 6813B some years ago from the 6813A including all the extra DSP for harmonic flicker tests etc it was still registered as the A version. Whereas keysight in Australia calibrated it as the B version and added the correct front panel id sticking etc.

Your 3rd edition appears spot on for the A9 output filter, which appears intact though the reverse diode variance on the FETs seems 1% variance when I'd expect lower than 0.5% - not surprising as they are date coded 0425. All gate impedances better than 600 Meg Ohm.

Looking for two sets of IRF 450 TO-204AA devices ie 8 each total 16 or batch of 20, though at least one set plus two spares all same date code would be ok. Infinion (used to be international rectifier) list them as 'Active' and afaik some milspec equipment in USA still use them but, qualified retailers list them as obsolete :-(
I guess I could replace with later irfp devices but, fiddly as heatsink not compatible, though can be worked around. Would need to ensure gate impedances matches so no introduced ringing which could compromise gate breakdown...

It troubles me? though upon close inspection of heatsink minimal if any silicone thermal paste & one side only, cracked TO-3 mica insulators and very odd misalignment of heatsink to PCB such that (only) the air gap between each to-3 devices leads as little as just under a mm. Fortunately though MOSFETs socketed :-)

So will be sleeving the leads & replacing all insulators and new silicone thermal paste as old silicone paste can be compromised via bacteria or fungi - humidity related. Fortunately heatsink floating (no warning sticker on it though), on insulated thermoset blocks so devices not ground referenced to AC coupling of Drain or Source etc. What an interesting set of circumstances, still awaiting feedback from Infineon on equivalent MOSFETs if there's no existing stock. I have been told Infineon occasionally run a batch to restock for the milspec supporting USA military service. The IRF 450 part of their HiRel range and offer a Rad Hard option too and listed as still Active.

Next is to look at all the components which drive the A7 inverter MOSFETs, seems same in 3rd edition as 4th edition. I'm now curious how compatible the later 4th edition A9 output filter (plastic pack MOSFETs) is with the other modules especially A7 inverter from early 2000s A or B versions which used metal can MOSFETs.

If anyone has IRF 450 eg 20 off and can confirm static safe storage and static safe shipping and all same date code or at least 10 each same code then please email or message through here, thanks?

Thanks again for your link spot on, all the best for now, cheers

Regards

Mike Massen
Perth, Western Australia