WRT ESR testers/testing:
I routinely use ESR testing, in circuit, as a way of determining overall capacitor health and condition. This is partly because it is a convenient first step (as ESR testing in circuit is more likely to work than testing for capacitance value in circuit), and partly because that it's been my experience that high ESR readings are a very good indication of capacitors that are in need of replacement. I am not talking about 1 or 2 ohms here, but readings that are considerably higher, say 5 to >40 ohms. Regardless of whether or not you feel that ESR is an important factor in sound quality, if you have a given make / model of capacitor and you know that they typically read, say, >2 ohms ESR when new, then if you encounter that same cap in a device and it's reading higher ESR, I think you can safely infer that that cap is, at the very least, off spec. Typically, on removal, these caps tend to read low in uf value too.
As a real world example, I recently had a client bring me a Focusrite ISA-220 which he had purchased used, "in mint condition". Externally, the unit was spotless, but it was exhibiting a hum issue when 48v was engaged, and amplitude sweeps looked like the HPF and LPF were both engaged when they weren't (and, the signal just plain dropped 20 dB when the Limiter circuit was engaged...and not from GR). A quick poke around with an ESR meter showed many caps reading high, and, when those caps were removed and tested for value, many of them (like around 30%), were pretty much empty cans reading 0-1uf for 10, 47 and 100uf caps. That was the case with the nine caps in the limiter circuit, and the filter cap in the 48v circuit and a good many others. All of these failures were heat driven. I suspect this unit came from a professional studio where it had been left on 24/7-365 since new, thus allowing the umpteen 5532's in there to cook the life out of the caps. I have certainly seen devices that would simply not pass audio because the capacitors had completely dried out (like the GB8 mentioned earlier in this thread, and sections of a Neve V3 I have worked on), and in most, if not all cases, those caps read high ESR in circuit. After replacing 100+ capacitors, the ISA 220 was as good (or perhaps better, since I used longer lasting caps) as new.
And, to bring this back on to the OP's topic: At one point Soundcraft had what they simply called the "Small", "Medium" and "Large" console power supplies. Your's looks like the "Medium".
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