Brown Glue

So, I learned something new today!

I have an old, old scientific calculator, bought for school use almost 17 years ago. It has survived multiple examinations, being carried across international borders, and usage of varying frequency.

Yesterday, it suddenly developed an error. It physically would not let me enter 2-digit numbers. I could type, for example, '5', or even '5 × 5', but not '55'. Opening up the case, I noticed that the circuit components had been carelessly attached, with glue smeared all over the circuit, which was not entirely unexpected of a relatively cheap school calculator. More importantly, the single button cell was rusted. That's the problem, I thought. I cleaned up the contacts, and inserted the new battery.

No dice. Exactly the same problem as before.

I reopened the calculator and had another look inside – the keypad design was fascinating – but nothing stood out, other than the smears of brown glue. Wondering if there was something to it after all, I looked up 'brown glue on circuit board'. When Google auto-completed the phrase for me, I knew I was on to something.

It appears that the glue was chloroprene rubber adhesive. It starts off yellow, but slowly turns brown over time. It also becomes conductive. And this dried, conductive material had been carelessly dribbled all over my calculator's circuit board.

I very carefully chipped the dried glue off the conductive tracks using a small screwdriver, dabbed away the dust with a damp cotton swab, allowed the circuit board to air-dry, and replaced the casing. The calculator effortlessly informed me that 55 × 11 = 605.

It's nice that my old friend will continue to function at least a little longer, but I'm also pleased to have found something new to look out for when repairing electronic devices. There's something very fulfilling about getting a broken device to work again.