Here is a writeup from another radio enthusiast about his newly purchased Mega 328 uC based component tester. They come in handy for testing electrolytic capacitors especially for ESR – equivalent series resistance. The tester will also test silicon and germanium transistors, handy for making a judgment on how leaky it is. I have a couple of these testers, one in a clear case with the 9V battery for portable use. The other doesn’t have a case and I use a 6VDC AC adapter to power it. It complains that the battery is low, but it still regulates and says the supply voltage is 4.93 volts. This one was from 2015 and instead of the pushbutton it has a rotary switch with momentary contact when the shaft is pressed. A long press does something, I forgot what. More info about them in my blogs below. Here is his Antique Radio Forum post.
https://antiqueradios.com/forums/viewtopic.php?f=8&t=378010
My blogs on the component testers: