Vinegar and classic repair



First of all I'd like to thank this forum. Normally I'm quite confident repairing old electronics. But I've never thought of using vinegar to remove battery leakage from metal parts.

* So hereby HUGE THANKS to this forum! *

Normally I've just used water, soap, brush and a little time to soften it and yes it's not as good as it can be. I usually give it a good spray of soldering lacquer afterwards to prevent further corrosion. I an HP-35 laying in my junk box for either parts or - and luckily it turned out this way - for repair. It's PCB underneath the battery compartment was quite green and looking like a big mess On the component side fortunately there was only very minor corrosion and none inside the keyboard switches. An old toothbrush and a bath of vinegar later and it looks almost like new with shiny copper and gold!! (yeah did remember to rinse thoroughly with lots of water) Afterward I can see "lots" of gold in the vinegar. The battery leak has corroded underneath the gold and of course it then comes off when brushing. To replace the protective gold it got a layer of soldering lacquer as usual.

Afterward at first it didn't just work as expected. It showed corrupted numbers. Thought I killed it and decided to swap the cathode driver (the right of the chips) with another 35 in my junk box which has a digit exhibiting bad ghosting (or what to call it) of a digit. Unlike expected it did do nothing to the ghosting neither to the one that just got the bath. I was about to pull my hair out but took a magnifier and a _really_ close look to the underside of the bathed PCB and found out that a small scratch somehow got filled with copper (yes copper - not a gold leaf). Perhaps is has been there from start but bath helped it do the final short circuit. Anyway removing it with a knife and the bathed one now works *perfectly* without ghosting or anything else bad. Yeah it still has the swapped cathode driver and to not put any further stress on the PCB that'll just be that way although I probably just could swap them again without damage as I'm quite confident in those kind of things but each time there's an unavoidable risk to the PCB. But now I know the one with a ghosting digit the cathode driver is not to blame. So now I'm keeping my eye out a cheap defective classic one to use as a donor for display parts other than the cathode driver.

What's your experiences?
What's normally the cause of a ghosting digit? Bu manually pulling it with a resistor of 180 Ohm the ghosting can disappear at the expense of increased power use and the ghosting digit still stands out when powering on.
The anode driver, creeping currents on a PCB with corroding (although I can't see any) or perhaps the LED package itself? I really looked over and over on the PCB with the ghosting digit but can't find a fault here (in contrast to the bathed one).

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