There are two kinds of old SLR. On one the shutter is timed by electronics, and a flat battery means a camera that does nothing. On the other the shutter is timed by springs and gears, and the battery runs only the light meter. Take it out and you still have a camera — you just have to know what exposure you want.
That second kind is why bodies made fifty years ago are still in daily use, and it is worth understanding before you buy anything.
What mechanical actually means
Press the button and a spring releases. A train of gears decides how long the curtain stays open. Nothing is measured by a circuit, nothing is held by a capacitor, and nothing stops working because a part is no longer manufactured. Everything in that chain can be cleaned, adjusted and re-lubricated by anybody who works on cameras.
Cameras built this way include the Nikon FM and FM2, the Pentax K1000, the Olympus OM-1 and the Canon F-1. There are many more.
What it buys you
It is repairable. A worn mechanical shutter is a service. A failed electronic shutter is usually a board, and a board for a camera discontinued in 1983 is whatever somebody has left.
It works cold. Batteries lose capacity in the cold. Springs do not.
It tells you what it is doing. You set the speed, so you know the speed. That sounds obvious until you have spent a roll wondering why everything came back dark.
What it costs you
Nothing is automatic. You read the meter, or you read the light, and you decide. The first roll is usually a mess. The second one is not, and by the fourth you stop thinking about it.
And the meter is still electronic on most of them. On a Pentax K1000 or an OM-1 a dead battery means no meter, and many of these cameras were designed around mercury cells that are no longer sold. The workarounds are well known — an adapter, a different cell, or a handheld meter — but it is worth knowing before rather than after.
What to check on a mechanical body
Fire it at every speed, slowest to fastest, and listen. The slow speeds are where a tired camera gives itself away: 1 second should sound like a second, not like half of one. If the slow speeds drag, the shutter needs a service.
Look through the back with the shutter open on B and check the curtain for pinholes against a light.
Turn the wind lever through a full stroke. It should be smooth and it should stop firmly.
Light seals: assume they need replacing.
Which to start with
Any of the four named above will still be working in twenty years if it is serviced once. The differences between them are about how they feel in your hand and which lenses you want, not about whether they are good — they are all good, which is why they are all still here.
Everything mechanical that passes through Unseend is fired at every speed before it is listed, and the slow speeds are timed rather than guessed. If a shutter drags, that is on the page.