TE : I finally succumbed... decided not to buy or build a dedicated ESR meter to test the caps in my Tek 575. Not economical.
Instead I bought the dreaded super cheap AVR based open source component tester millions before me have already bought... thinking for 10 Euros it can measure ESR.. and test common components on top of it, which I desperately need to sort my shit load of salvaged semiconductors over the years. Lab V2.0 is in the making and that involves turning boxes of shit into tested, identified, organized stuff in labelled boxes... so I can actually use this stuff, not just stare at a pile of component thinking "hey maybe something in that pile could do the job ! "....
Did a few sanity checks, looks OK, readings are both plausible and repeatable. Tested a few resistors, as well as my ton of 40+ year old salvaged electrolytic caps, only to find out that.... 99.5% still are within spec capacitance wise, AND still have low / acceptable ESR ?! The worse that I could find, was of course in the tiny packages, like 0.1uF 16V or something... the size of a (small/tender) pea. There, I found 2 or 3 that read high at 25/30 ohms. but that's about it !
Gee I am glad I didn't scrap all these caps "assuming" they HAD to be all long gone ESR wise ! They are not !!
All my big ones measure between 0,00 (resolution of the tester) and 0.100 or something, so just fine.
Then I tested the can caps in the 575, the x4 2000uF 20Vols caps that are in parallel to smooth the low voltage supply for the Base Step Generator.
I had previously measured them with a DMM at 3000uF instead of the nominal 2000uF, and one at 2750.
Cheap tester agreed 100% with that ! Then tested ESR as that was the goal wasn't it... 3 of the 4 caps read just fine at 0,15ohm, unbelievable, but one is toast, read TWO uF not 2,000uF, and ESR is 150ohms !
So this one definitely is not happy.
Overall very happy with this cheap tester, the best 10 Euros I have spent in a long time !
Being able to do new measurements is I find even more fun and exciting (and useful...) than buying yet another 35th old scope... maybe there is a trend here... maybe I will wake up one morning and get rid of half my scopes and concentrate on getting more test gear instead !