I have bought a Nikon SB-24 for a bargain price. I checked it, worked well.
…excepting the fact, that after 30 seconds the flash always went sleep. From this mode was easy to wake up: just a short impulse on the middle pin in the hot shoe.
On the net you can find the repair manual https://elektrotanya.com/nikon_sb-24_repair_manual.pdf/download.html
This flash has some settings in the inside: you can set the security voltage (the maximum after which it will shut off to protect the capacitor and the circuits)), the monitor voltage, which is that threshold, on which the ready light goes on, and others, for example the light quantity for a Manual or an Auto flash.
On my flash the ready light on the back was NEVER on – and for this flash is a normal behaviour, that case he can not observe the ready light (better to say, the capacitor can not reach a predefined voltage (ca. 330 Volts) after 30 seconds goes to sleep.
So, be careful: a flash is not a joke. Works on 300-350 volts, can KILL YOU.
Before repairing it, always CHECK the capacitor, case it is charged, discharge it through a small power 230 volt lamp, or a 1 kohm /5 W resistor.
This is the normal behaviour after 5-10 seconds
And this is mine, with NO ready light. The flash worked – but disconnected after 30 sec.
Case you open the body, begin with the bottom, then the red front cover. More is not necessary for this operation..
Under that cover you will find the measuring point of the capacitor voltage (deep under the plastic, so you will need a long multimeter probe), GND is on the front, in a 3 pin socket, under a cover, and the 5 different potentiometers. VR2 is the right one, if you watch it from the front (see the picture with the screwdriver).
Turn it clockwise (CW), and the monitor voltage limit is lower. Conterclockwise (CCW) – will be higher.
And worked. Imade a setting fo 320 volts. Lower, than official (so will have a little bit less power) – but I will use it only as an auxliary flash with a radio remote control in Manual mode.
For the whole description see the link https://elektrotanya.com/nikon_sb-24_repair_manual.pdf/download.html
By the way: the hot shoe middle pin voltage is +4,50 volt, with the GND on the side pin of the hot shoe.