A complete waste of time?

Well,

I decided it would be a good idea to buy this from eBay:


Its an old HP DVM and Frequency counter; with a glorious Nixie display.

Initial testing revealed some issues with the frequency counter part of the functionality which I traved to an IC that was basically a set of NOT gates, so I removed it and replaced it with a good old 7404:


So, now the frequency counter is working, but the digit in the image below shouldn't be a 4 but a 0:


It works when the digit is supposed to be displaying a 4, 5, 6, 7 or blank:


So the rules of BCD tell me that the line that represents "4" is always stuck LOW no matter what the actual signals tell it to do.

There's a bunch of logic in the system, but there's basically a 4-bit latch that seems to have appropriate signals coming into it for this digit, but the "4" line is just stuck low.

The part is labeled a HP  1820-0116, and I think it's one of these:


So I've ordered some - the "2" on the end of the display also seems to be a similar problem so more to investigate.

Is it normal to buy test gear, to fix the test gear you just bought on eBay?

I think I've got a bad case of TEA. I'm told there is no hope or indeed cure.