Advice needed: MIT Z Isolator HC becomes hot when using isolated power bank.

Hello everyone. I bought an MIT Z Isolator HC (early 2000's) unit secondhand a couple of years back. I've noticed that when plugging anything drawing more than a negligible wattage load into the isolated power bank outlet, the unit becomes hot. The wattage limit is 500, and it will heat up with 150 to even something as low as 30 watts. I've tested it with very low wattage inputs and it's perfectly fine there. The high current non-isolated bank works perfectly fine, so I've resorted to only being to input power cables into those two slots.

MIT cannot help due to the unit being so out of date, so I have to resort to other means of advice/help. I realize few will really know the inner workings of the particular unit, but a long shot is still a shot. I can hopefully get it open without issue if pictures are needed of the inside makeup, but sometimes screws can be an issue (like button headed allen wrench types). While I know people that work in the a/v and lighting industry, they don't really have any experience working with something like this (but may be able to help if the issue is pinpointed ahead of time).
Thank you for any and all help.