The Pentax Spotmatic was a break-through camera in the 1960's with it's behind-the-lens metering system. For a moment in time, parent company Asahi surged ahead of the market leaders Canon and Nikon. But the 42mm universal screw-thread mount for their lenses would end up causing some engineering trouble when it came to offering an automatic exposure model. With a couple of small changes to their house lenses, Pentax was able to create the ES model (Electronic Shutter or Electronic Spotmatic, if you prefer) around 1971.
The Pentax ES was a stunning beauty. Some special machining allowed certain new lenses to seat on the body with exact alignment to permit aperture-preferred automation (you choose the lens opening, the camera picks the shutter speed). It worked great.
[The Pentax "K" series a few years later solved all the engineering problems of automating with single-pin M42 screw-mount lenses by dumping the special ES mount in favor of a completely new bayonet arrangement.]
Production of the wonderful ES only happened for a few years and the price for this technology was high. Sales were restricted to well-heeled enthusiasts or pros. Thus, a Pentax ES is not easy to come by for vintage camera collectors; so I was shocked to see an example at a local garage sale going for $2. It came complete with it's case, a Honeywell electronic flash and instruction book.The owner said the camera was jammed; the mirror would not come down but the shutter worked fine. Well, as a shelf queen it looked great and the price was certainly right.
A little internet investigation revealed that an ES mirror stuck in the up position could be cured. Quite often, the bearings on the shutter drum/spindles just needed lubricating. The gear at the end of the assembly sticks slightly so that the mirror operation never gets a chance to cycle completely. Sometimes this occurs at one shutter speed, often at all. A touch of oil is all that's needed on the bearings to help clear the problem.
Before assuming you need to do surgery, first try taking the camera off the automatic setting and wind and shoot using the manual speeds. If that doesn't work proceed to the information below.
Now, I'm no camera repair expert and you're on your own with regard to how successful this is and how good your mechanical skills are. But all you'll need is a small screwdriver and some light oil. It's a pretty easy amateur operation.
First remove the four screws on the bottom plate of the camera with a very small Philips-type screwdriver (jeweler's type). Lift the bottom plate off straight up. You'll now see a green circuit board covering the guts of the camera.
Next remove the three screws holding the circuit board. Once done, the assembly is free to be lifted up. Lift the left edge slightly (in order to clear the tripod socket assembly in the middle) and pull the circuit board gently to the left to disconnect it from the electrical connector.
Go slowly, do not force anything. It should separate with a calm, even tug or with a wiggle.
The target for the oil is a gear located to the lower-left of the tripod socket. You're trying to reach ball bearings between the gear and the large screw center. Yep, those are some mighty small bearings.
Using some form of hypodermic, pin, penpoint, flat-head jeweler's screwdriver, etc., apply a very, very small amount/beadof thin lubricant to the area just outside of the screw that holds down the shutter drum/shutter spindle arrangement.
I realize that you might want to know what kind of Lubricant to use. It's easier to say what I would NOT use: lithium type, graphite, 3-in1, and especially WD-40. Bad choices. You're looking for very thin oil, nealy as runny as water. Sewing machine oil might fill the bill if you can find it. I just happen to have some specialty machine tool lubricant given to me by a friend.
OK, a very small drop is what you're after. You don't want to douse the bearings. Wipe up any extra lubricant off the face of the gear with a cotton swab.
Test the shutter a couple of times. If the mirror returns to the typical down position after the release - success! That's it! Simply reassemble the circuit board and bottom plate in the reverse order from the first step.
If the mirror still stays up, well you haven't damaged the camera, but the problem is likely located at the other end of the spindle drum, deep inside the camera. That's a professional fix. Sorry.
Hopefully, you'll find that your ES will spring back to life. The ability to make a repair like this to a forty-plus year-old camera is testimony to the wonderful precision engineering by Pentax. Good luck on your example.