I know about the load and was absolutely sure that I tested it under the load. As you can see it’s the original KGBH board and not KGSSHV but the + 15/-15v sections are almost identical so it should not matter really in my case.
To make long story short I was having problems with the amp board which eventually lead to problems with PSU. My old 7915 regulator, do not even remember which manufacturer, was working fine there before I screw it up. So after trying STMs I thought that my input voltage a bit high for this particular regulator. I am really glad to look dumb stupid now , if not this thread I would put the project on hold again. Now everything looks like working right, all I need is to find time to powder coat my panels and put this beast together.