Hello Pentaforum community:
I am Sergio González Pagliara, from Argentina, electromechanical engineer, postgraduate degree in welding and amateur of photography.
Always admire Asahi Pentax's technology and achieve my first Asahi Pentax ME equipment in 1978. Use it intensively until 2005 without any problems in your electronic exposure system. This equipment featured Asahi Pentax 1:7 lenses and an Vivitar 75-205 telezoom, which still work perfectly today.
I entered digital photography in 2005, with various cameras until finally in October 2015 I decided to have a reflex system again. Having optics in good condition, I decided on a Pentax KS 2 equipped with a DA 18-135 Pentax telezoom, which at the time was a good economic/technical option to re-enter good photography and in passing be able to use the old optics.
Everything went well with the KS 2 until August 2019 and with 7000 shots when the diaphragm opening problems or dark photographs began. At first occasional and then it was aggravated to almost impossible its operation.
So I started researching at Pentaforum and found a Photogem post where it describes the history of the development of solenoids applied in Pentax machines. Here's the link for anyone who wants to read it.
A LITTLE BIT OF HISTORY: Development of the solenoid in Pentax cameras - PentaxForums.com
What struck me most is that the main constructive difference between the green solenoid (Chinese) and the old white solenoid (Japanese) is the construction of the reel. PET green, cheaper material and PTEF white, more expensive Teflon material. This led me to think of some considerations regarding the operation of the solenoid:
a) The PET reel increases the coefficient of rubbing of the Plunguer inside the reel when moving.
b) Batteries after 4 years of use have lost efficiency and their voltage decreases rapidly after being recharged (there are many reviews that using good quality AA batteries on the K50, problem is controlled)
c) The material with which the magnet body is built and the Plunguer himself may have gained some magnetism after many cycles
These three considerations led me to try to control the first. For this I obtained a Teflon-based lubricant (PTEF) which I applied only to the PET reel. The consequence was that the opening Block directly stopped working. When the lubricant is passed to the Contact Mirrors of the Plunguer and magnet body, they are glued by surface tension and viscosity of the lubricant, and the diaphragm was always closed.
In the face of this result, disassemble the solenoid of the camera again, disassemble its parts and whit isopropilic alcohol clean the two reels internally, the magnet body and the Plunguer.
When I reassembled the camera at first its walked well, but after two or three days without using it, its took dark photos again in the first few shots.
This led me to believe that the pulse of electric current sent by the camera does not last more than 1/5 of a second, as the opening block is designed to reach a speed of up to 5 shots per second. If the batteries are already weakened, the rubbing coefient is high, and there is suspicion of remaining magnetisms in the metals, there is no other left for the solenoid to stick and as a result the closed diaphragm. From this reasoning is that some forum participants suggest beveling the edges of the Plunguer and decreasing the area of contact with the magnet body, so that the effects of the three listed causes can be lessened.
Already about to proceed to vary the geometry of the Plunguer, I thought I'd use a solid lubricant. It was a coincidence that on my workbench it had graphite powder for the lubrication of locks. Place a very small amount of fine graphic powder inside each reel, and with a small brush I scrub the Plunguer with this graphite. Mount the solenoid and the camera hasn't failed again. (it is very important to keep the cleaning of the reels and solenoid metals. The fat of the fingers can cause problems)
I have almost 1500 shots after this solid lubrication and the solenoid is doing wonders with a very smooth and safe operation.
The following reasoning may apply:
Graphite decreases the coefficient of pet friction, forms a thin separating layer in the mirrors of the Plunguer and magnet body, which improves or decreases the attraction force that can be generated by the eventual magnetism of the metals. This improvement makes even with weakened batteries the solenoid operates and allows the opening block system to select the diaphragm opening obtained by the processor.
Graphite powder from the soft mine of an HB pencil can be easily obtained.
Logically you should be very thorough in the welding procedures of the cables, observe that the graphite is used in the amount necessary so that it does not contaminate other camera devices, observe the correct alignment of the Plunguer with the rest of the solenoid and the lever of the opening block and ensure that the polarity of the magnet and/or cables is correct.
I'll allow myself to use some Photogem photos to define the names of each part of the solenoid.
My native language is Spanish. I apologize to the community for my little Englishman. I wrote the text in Spanish and translated it into English with an automatic translator. Then correct this translation so that it can be understood.
I hope it will help many to repair their camera at no higher cost and confirm that Pentax is perhaps the best camera on the market.
Best Regards and Happy 2020.
Sergio Gonzalez Pagliara