Pentax Spotmatic F: Prism Foam and Meter Wiring Repair
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
Servicing a Pentax Spotmatic F. Photo three shows the prism removed and the deteriorated foam on the prism and the prism housing. This is what can come loose and fall into the viewfinder causing the dark spots on top of the fresnel screen. Photo two shows the circuit board lifted aside and the meter removed to repair a wiring issue.
The Spotmatic F is an interesting model of the Spotmatic series. It was one of the last screw mount cameras in the early 1970’s from Pentax. Unique in that it used a PX625 battery where most of the other Spotmatics used the 400 size battery. Locating a battery for these is fairly simple as you can use the 1.5V alkaline battery since it uses a similar meter circuit as the K1000. The meter movement itself has two coils inside and one coil gets input from the CdS photocells and the other gets input from the variable resistors (shutter speed, iso, diaphragm). When these are balanced out, the meter needle will be in the center. The camera also used wide open metering.
Another interesting feature of the Pentax Spotmatic F was that it did not use a meter on/off switch. There was a separate photocell to measure the light level and when it dropped below 2 EV, the meter would shut off. Of course, there was always some very little amount a current flowing, but it was in microamps and would not drain the battery. So, probably best to use a lens cap when the camera is not in use. The K1000 used the same scheme.
