Using a transistor as a diode results in something with much less leakage than a typical signal diode, for much less money than a low-leakage diode.
It's an old trick - the oldest reference to it that I have seen was from Fluke in the '70s, but no-doubt others knew about it before then...
PS: Sorry to say, but that meter looks like toast. If you do get it going, it would be hard to trust long-term - those carbon deposits could cause leakage and breakdown at high voltages... Tenma don't make good meters. In fact, Tenma don't make them at all - you probably have a badge-engineered Uni-T. We had a batch at work, and most started playing up after just a couple of years of light use. I - chief skip diver than I am - managed to through them away with no pangs of guilt, after doing a "tear-down" for the course that was in, to demonstrate why you should buy Fluke instead
As I say, sorry to give bad news, but remember the risks here