Not too long ago I restored a C-3 Condenser (Capacitor) Tester. It's older than the example at
http://old-timers.tuars.com/c-3/c-3.htm .
Photo shows the two 8 uF electrolytics on the left. I replaced these with 10 uF units, one of which I decided to just use a 250V instead of 500V. There is a 0.5uF capacitor which is connected between one of the power input lines and the chassis, that one was replaced with a X1 safety cap. I went further with safety and since the power cord needed replacement, I used a three prong cord and connected the ground wire on it to the chassis. Most of the 22K 1/2W resistors that determine the leakage voltage settings were replaced with 1Watt units. I added a 500K pot across the 470K resistor and set it so that with a 10 Mohm resistor across the capacitor testing posts and the leakage voltage set to 150V,
the eye just closes.
One important thing to remember with testers similar to this is that during the capacitance measurement there can be around 60 Vac on the measurement terminals. Some modern ceramic capacitors are only rated for 50V or 25V.