Well, that is the normal behavior at certain times, the power won't feed into the chips unless commanded by the control board, which is why you can't do the voltage testing using just the stock control/firmware, the fixture tools do that for you, they sent continues voltage so you can test the voltage without a rush, otherwise, the stock control board would only feed the voltage in the initial test and then stops, the official Bitmain fixture tool sold by Zeusbtc isn't that great in that aspect too, you only get a few seconds to test (better than stock of course), there is another tool that ships from Russia (or at least used to) I think the official website is https://tester.asic.repair/en, this one is a lot easier to use and gives you all the time you need to test the voltage.

With that said, I think this goes beyond just understanding how to use the tool, fixing hashboards needs some general understanding about electronics, if you don't have that -- it would be hard to fix them.