So there you go, under specced (or dodgy) transistors, surprise, surprise!
Genuine high power transistors are not cheap, and have been scammed on the grey market for
-gasp- surely a top quality brand like Korad would not have bought a lot of transistors from some shady chinese parts broker that sanded off the real manufacturer and put some other, more expensive part number on them?
As for ISO : there is nothing wrong with that. It is just a standard for traceability and process. But, and this is the key point : it does not define the actual process.. It only ensures you can track things. Yes it is a quality standard, one that measures the quality of the papertrail. I also prefer to but stuff from iso companies. You wont find any shady fake parts dealers there. And if you do, you will be able to track where the stuff came from and get your money back.
Now, as far as the hammering on calibration companies goes : i am not going to blast triocal here, i got no quarrel with them and do not know how they operate, but : i would assume that they would have tested these supplies under max volt, max load for 24 hours. Meaning: set the supply at 30 volts and pull 5 amps from it. Leave foer 24 hours. If no magic smoke escapes. Pass.
Since korad doesnt specify anything else like impulse behavior or load regulation.. Can't really verify..
In general ( again: i do not know triocal and have no quarrel with them ) , it is my experience that calibration companies only verify that a product falls within manufacturers specs. If korad only gives two numbers there is little to verify. If the manual lists a thousand numbers under different test conditions they will have a lot more work. So if this korad thing, with the meagre manual fries under some weird condition you can't blame triocal. There were no specs. Like dave said : safe operating area ? Unknown. It may very well be that you can only draw 5 ampere at 30 volt. Dial it down to just above the relay switchover and the thing may go outside of the soa as that is the point where the transistors burns off the most heat. Its got full voltage from transformer and lowest at output , not causing switching down of transformer. And then smoke comes out.... If this is not documented... Point finger at manu for not documenting this. The manu may be ISO, but if its not in there its not in there. There is a paper with specs, check , ISO stamp...
Second point. Most callabs ( i'm not fingering triocal, i dont know them. This is again just my experience) do not have the skill or know how to adjust a machine and bring it back into spec. It's pass or fail. Or they send it to the manufacturer for alignment. There is a lot of small callabs that do cal a multimeter for 50$ they also do a scope for 100$..
.. Really ?
Third point : some callabs jave no clue what they are doing. We got two power supplies rejected because they outputted -0.3 volt at 60 mA when the output was 'off'. I contested that and told them to read service note xyz of the manufacturer. For a particular range of serial numbers this was normal. This note was an addednum to the specs. Their answer was ' we can't know everything... I kicked them out. As a callab it is your job to know those things. How can you perform a verification if you don't know what you are verifying....
Fourth point: it is your job as user to read the manual of the machine you are using and know its limitations and specifications. Of course, if none are given your guess is as good as mine. Serious manufacturers will have calibration intervals given in the manual and the verification / adjustment procedures will be available, free / paying / or there will be a procedure to return it to the manu for calibration. If there is no procedure ,( either as a document , paying or free, or a statement it must be returned to manu ), ... Run for the hills....