I too have a 50 year old Discwasher Zerostat that still works. I can check the operation with a small probe that consists of a 10 nf capacitor in parallel with a neon bulb. A wire from one leg of the capacitor/bulb circuit is held near the Zerostat output while operating it. A second wire from the opposite leg of the capacitor/bulb circuit is formed into a small loop that you can grip between thumb and finger. The bulb will briefly glow on the slow pull of the trigger and the slow release of the trigger.
Found a Milty Zerostat alternative
I was thinking about buying the Milty Zerostat but came across a discussion regarding this plasma arc lighter.
https://www.amazon.ca/Lighters-Electric-Rechargeable-Lighter-Windproof/dp/B09CYQ92PF/ref=dp_prsubs_sccl_3/130-2888405-1836766?pd_rd_w=YAj0t&content-id=amzn1.sym.7f829f53-1df3-4cd3-91bf-9933c19106a6&pf_rd_p=7f829f53-1df3-4cd3-91bf-9933c19106a6&pf_rd_r=XCDACDZA93ZQEQD27N82&pd_rd_wg=LfOt5&pd_rd_r=6ea7c5de-5354-4a91-9826-e178d9a2b653&pd_rd_i=B09CYQ92PF&psc=1
It really works well! I took a record out of it's sleeve and held a tissue against it. The tissue stuck to it, even when held upside down. I turned on the plasma lighter and held it about 1.5" away and made two or three circular passes around. Tried the tissue test again and it would not stick to the vinyl. It's also well constructed and doesn't feel cheap.