Can't calibrate component tester

I got one of these a few years ago, and it's given very good service: Banggood Hiland transistor tester but it finally needed it's battery changing, and now I can't re-calibrate it. It gets stuck on the "1-3 capacitor >100nF" (or something like that) message. I connect the capacitor between terminals 1 and 3 but the message doesn't go away and the calibration never finishes. Am I doing something wrong? Suggestions welcome!

Do you have this manual?

This is just a guess, but can you try the selftest call as per below?

A capacitor with any capacity between 100nF（0.1uF）and 20uF
connected to pin 1 and pin 3 is required for the last task of
calibration. To indicate that, a capacitor symbol is shown
between the pin number 1 and 3, followed by the text " >100nF".
You should connect the capacitor not before this text is
shown. With this capacitor the offset voltage of the analog
comparator will be compensated for better measurement of
capacity values. Additionally the gain for ADC measurements
using the internal reference voltage will be adjusted too with the
same capacitor for better resistor measurement results
with the AUTOSCALE ADC option. If the menu option is
selected for the tester and the selftest is not started as menu
function, the calibration with the external capacitor is only done
for the first time calibration. The calibration with the external
capacitor can be repeated with a selftest call as menu selection.

I tried it that way after it had failed the first couple of times. The calibration routine is the same. I'm wondering if I'm connecting the cap too early.

throbscottle said:

I tried it that way after it had failed the first couple of times. The calibration routine is the same. I'm wondering if I'm connecting the cap too early.

Click to expand...

There is some language interpretation needed....

You should connect the capacitor not before this text is
shown.

You may want to search the many comments on their site to see if someone says something relevant. I would probably connect it soon as well as late and I would try values near the upper and lower range and all of that stuff. Of course, I don't have one so it is easy to say

Ok, spent half the day tinkering with it. I noticed that every minute or so it flashes the value of the cap on screen very briefly. It's showing 0 for anything under about 1UF. But even with a value it actually recognises it still doesn't complete the calibration. So I'm now wondering if something bad happened to the arduino.

That sounds like a possibility and they have a caution:

Attention: Allways be shure to discharge capacitors before
connecting them to the Tester! The Tester may be damaged
before you have switched it on. There is only a little protection
at the ATmega ports.

Maybe you have a blown port bit (A2D) on the ATMega...

Though if that's the case you'd expect it to fail the first part of the calibration where you short the 3 terminals together. The only other thing I can think of is that it's got a bit damp.
Maybe it's just time for a newer model

Did you notice this?

Is that your model?

Now that is the difficult question. I try looking for info on the one I have and get every other version. I think mine is a (relatively!) very old model.
It looks as though that is the chip for the "graphics version" of the kit which has a bigger display and a bit more functionality, and the listing for the kit gives the catalogue number of the chip, which is the one you linked to.
So that got me looking for catalogue numbers, and hey presto, in the customer q/a section for my kit, there is the cat number of the chip that goes with it, 966305, which seems to be this one: https://www.banggood.com/Original-H...t-p-966305.html?rmmds=search&cur_warehouse=CN
So I'll just have to order one and see what happens!
edit: I just noticed you said "did you notice this"... yes I had done and had been umming and ahhing about it.