A little bit of history about the development of the solenoid in Pentax cameras up to the K-70:
The
legendary patent named "Automatic Camera Shutter" was applied for July 30, 1968 and granted Jan. 4th, 1972.
You can down load the patent
HERE 1. The first very simple solenoid I have found in the
Pentax ME and ME-Super.
No permanent magnet yet but only a plunger and an electro-magnet to induce the magnetic force to pull the plunger: 
It was in 1983 that Pentax introduced its first SLR which offered fully automatic exposure ("program") mode when coupled with a matching Pentax-A series lens:
The
Pentax Super-A (Super Program), followed by the
Program Plus (Program-A) in 1984.
This was the first time solenoids where used the way we know it until today up to the Pentax K-70. 2. A very nicely built and sophisticated solenoid: 
Not yet a rare-earth-magnet as later used but an alnico-magnet, i.e. an alloy mainly of al-uminium, ni-ckel, co-balt, invented by T.Mishima 1931 in Japan: 
Backside: 
The solenoid in action: 
The force of this alnico magnet pulled a kind of cap connected to the leaver which moved the aperture-mechanism and kept it in place.
Taking a picture, the two coils acting as an electro-magnet receive 3 Volts DC from the cameras battery ( 2 x 1,5V SR44 or 1 x 3-V CR1108).
Those 2 coils cancelled the magnetic force of the permanent magnet and the top-cap opened, the leaver moved. Impedance was 14,3 ohms.
The electro-magnetic coils: 
The partnumber given was G-100: 
This partnumber
G-100 remained the same at least up to the Pentax K20D and K200D, possibly later on as well but I have not yet seen a service manual of those.
Samsung named it G-100 as well in the GX1, GX2, GX10 and GX20.
The next solenoid as we know it was introduced in some the MZ-Series (MZ50, MZ6, MZ7 and a few others up to the 35mm Pentax *ist).
This Solenoid was manufactured by
Shinmei Electric Co., Tomioko-Cho, Futaba-gun, Fukushima, Japan (see 3.a) and by
TDK, Japan (see 3.b)
Those solenoids were also sold to ALPS and Matsushita.
This first
SLR Shinmei and TDK solenoids look identical to the later
DSLR made in Japan versions and had the same impedance.
This solenoid was driven with 6V/DC (rated with 4,5V/DC) by the solenoid-driver, impuls-voltage was 6.45V.
Thus the holding-force and body thickness were slightly different and sometimes with opposite polarity. It is not recommended to use it.
3. a) Here a photo showing the 3.rd version of a solenoid, this one I took from a MZ50 tested in a K30: 
One can see, the plunger tilts slightly in relation to the round part of the leaver which it moves.
The next drawing shows this early SLR solenoid (i.e. not yet DSLR!): 
It has
3.4mm bobbin-size (
versus 4mm bobbin-size for the later
DSLR-solenoids).
Data Japan-made "SLR" solenoid:
- Operating Voltage DC: 3 - 6V
- Coilresistance: 30 ohms
- Attraction force: 2,2 N min
- Backtension: 0,8 N
- Operating stroke: 2mm
3.b) To make things more complicated, Pentax also used a white Japan made SLR solenoid made by TDK: 
The early introduced MA-307-9 was developed for 3-6V use (2 x CR2 /900mA) = so for the 35mm SLR Pentax of the MZ/XZ-Series!
And most important:
The bobbin thickness was 4m and
not 3,2mm as the Shinmei-version. So even more difficult to differentiate from the DLSR version!
Later they introduced the MA-307-26 für 3,7-6,5V (2 x CRV3/3300mA!) = DSLR *istDS/DL, K10D
What can be seen very cleary is the difference in holding-force (attracting force) and return-force (back-tension)
4. A very different solenoid was used in many other MZ/ZX such as the MZ5 bodies: 
4. With the introduction of the Pentax *ist D came the "made in Japan" white DSLR solenoid which never failed in any of those bodies up to the Pentax K-r: 
The manufacturer of those solenoids
remained Shinmei, Japan and manufacture was in Japan.
The next drawing shows that this new introduced Shinmei Japan solenoid for DSLR bodies had now a 4mm bobbin-size instead of 3.4mm: 
As mentioned, this difference of 0.6mm is hardly noticable but this is why aside from another small difference in holding-force and often opposite polarisation the early
SLR solenoids with 3.4mm bobbin-size
sit slightly bent when installed into a DSLR body!
This DSLR Solenoid had a live-timespan of remarkable 100.000 actuations but lasts actually much longer.
Data Japan-made "DSLR" Solenoid:
- Operating Voltage DC: 3,6 - 7,2 V- Coilresistance: 30 ohms
- Attraction force: 2,5 N min
- Backtension: 1,0 N
- Operating stroke: 2mm
According to an engineer who worked for Pentax and with teams designing cameras (I have the email from him as prove)as well according to a Ricoh representative from France the material used was a mix of PTFE's (Teflon) and thus it could be injection molded. I cannot guarantee this to be the case but there is no reason not to believe specialists.
5. Green-China-Solenoid
When Shinmei moved production to China, difficulties started!
Shinmei was now the sole provider to Ricoh for Pentax DSLR cameras!
Manufacturing place was now in Shanghai. Problems due to saving costs arised, such as
bad pay and
bad treatment of workers
Manufacture in Tomioka, Japan, stopped earlier on but the plant itself was finally shut-down in 2010.
This means not more manufacturing possible in Japan.
The material of
the body changed to the green colour and instead of using a mix of different
PTFE's (teflon) it was now made out of
PET.
The alloy of the plunger changed as well.
PTFE (teflon) is a very good bearing material, it is used as well for
bearings in turntables. This bearing was patented by W. Firebough. He describes the bearing very well in
This interview. I could verify the amazing bearing qualities myself. When one inspects a heavy used Japan-solenoid closer against one made in China, one can tell the difference, PET is worn off much quicker.
This China made solenoid was first used in the flash circuit of the Pentax K100D (year 2006), K100D, K200D, K-m, K-x and K-r (until year 2011) but as far as I know mainly in those delivered to Europe. So a testing period of 5 years without any failure.
Rated voltage was now 3,7 - 7.5V (impuls 8.32V) but live-time was drastically lower: 20.000 actuations.
Data China-made solenoid
- Operating Voltage DC: 3,7 - 7,5V
- Coilresistance: 30 ohms
- Attraction force: 2,8 - 3,0 N min
- Backtension: 1,2 N
- Operating stroke: 2mm
Here you can see the measurements on a Pentax K-30
The next drawing shows the datasheet of the green China-solenoid: 
The dashed (----) line shows the 30ohms version used for Pentax. The lower live-time of 20.000 actuations is due to
PET instead of
PTFE used in the early Japansolenoid.
Some claim (without prove, because it cannot be proven) that the voltage used for the Chinasolenoid would be more in the region of 2.5V because it releases smoothly at 2.5V. That's a lot of nonsense because none, the early Japan made SLR, the next Japan made DSLR solenoid nor the China made version work well with just 2,5V/DC! They kind of work with 3V but are driven with 6V in the SLR Pentax cameras and 7.2V in the DSLR Pentax cameras. If one studies the curve for holding-force one can see this very clearly! I have done tests with 3V, 6V and 7.2 Volts. Measuring release-time (and thus holding-force) can be done from 5V - 9V. If one compares different solenoids it just is important that the applied voltage is the same for all solenoids one compares! The applied Vpp since the Pentax *ist-D is exactly 8.32VDC. Earlier pre DSLR bodies such as the MZ50 had 6VppDC. The solenoid is driven by a transistor (BJT/SOT23) and protected by a simple diode.
While the China-made solenoid was tested in the flash-circuit, the solenoid used for aperture-control remained still the white made in Japan unit!
This test-period went on for about 6 years (the K100D was introduced 2006, the K-30 introduced 6 years later in 2012).
So 6 years without trouble, enough time to use it for the aperture control.
This photo shows the green China solenoid for the pop-up-flash in a K200D: 
And here built out: 
This change to manufacture in China turned out to be the beginning of difficulties due to several reasons
described here as well and led to a problem now known as:
ABF = Aperture Block Failure (also named: "dark-image-syndrome" or "dark exposure problems")
The plunger of this solenoid suddenly didn't release anymore but remained stuck! The diaphragm/blades/aperture of the lens could not open and
thus particular wide open and low f-stops (f1.4 / 1.7 / up to f 8) resulted in totally underexposed pictures, almost dark/black. If one changed to fully closed (such as f22) one gets a normal exposed picture.
For quite a long time it was not yet clear that the cultprit was just the solenoid!
The complete "diaphragm-control-block" was exchanged or possibly just the whole camera because exchange of the block was very work + time intensive. But then due to all the research
mainly here in this forum (!) Ricoh/Pentax realized that it was just the solenoid itself.
It was about
Dec. 2015 that Ricoh started to modify the solenoid which we now call the
6. "2.nd generation China-Solenoid":
But quite a lot KS-1's, K-S2's and many K-50's and all K-500's still used the earlier
1.st generation China-made green solenoid because they were manufactured prior Dec.2015! Thus quite a lot of them failed and do still fail.
This is mainly due to fact that the metal body + plunger of the China-Solenoid is made out of relatively higher coercivity material:
It thus retains its magnetism after the field strength is removed!
This also explains why in many cases ABF happens more to those Pentax bodies which hadn't been used for a longer period and/or those with a low shuttercount (a low shuttercount implies little use anyway up to the case of two NOS Pentax bodies which I repaired, they hadn't been used at all but cought ABF, so the best proof for this explanation which I got from a forum-member who is a Chartered Electrical Engineer with Nuclear Electric.
It is very simple:
When the Pentax is not used the plunger sits all the time near the permanent magnet and the magnetized body.
No use will enhance the "glueing effect", i.e. the solenoid sticking inside the body.
Regular use is no guarantee but it is clear and researched that ABF happens less often.
!!! The holding force of this 2.nd generation China-Solenoid was still stronger compared to the Japan-made Solenoid !!!
and it does not fire as quick.
Close-up photos of the solenoid used in the K-70 show now a very nicely machined plunger with a smoother surface: 
Very different to the surface of a solenoid modified by filing or sanding: 
The surface of the alloy which is hurt by sanding or filing starts to corrode and can actually rust pretty bad.
So due to the still stronger holding-force and the alloy
it turned out that this 2.nd generation China-made solenoid wasn't perfect yet!
ABF happened less but there were still reports of K-70's failing.
Some repairshops tried to deflect from the true cause (the solenoid) and invented a socalled "resistor-chip" which now would fail and cause the solenoid to stop working correctly. We had some
hot heated discussions about it, it turned out to be a fairytale and the repairshops quickly stopped "exchanging resistorchips".
6. Fake Solenoids (but recommended by their sellers to be suitable for Pentax = Fake facts) a) Meanwhile more and more green solenoids turned up on ebay, ali-express etc. If purchased in quantities via alibaba China one single solenoid was just a few cents, BUT: Those are even inferior and were NOT MADE FOR DSLR but for Lenovo DVD-drives: 
You can see, the 2 pins facing sideways are missing!
The permanent magnet is longer and stronger and thus they have a way too strong holding force!
They were constructed for a very simple "press-button-open-tray" circuit
Beware of those by all means! b) Beware of Blue-coloured China-Solenoids from ROM-Drives: 
Wrong impedance (15ohms)! Danger of exodus for the "solenoid-driver".
c) And of course:Beware of green solenoids which are filed/sanded!
Further information why you should avoid this you can study
HERE d) Some sellers on ebay offer white Japan solenoids which actually are SLR solenoids, no matter if 4mm bobbin-size made by TDK!
See 3.b) Further Warnings: (based on long-time and verified studies) - Lubing: Beware of any attempt to lube the plunger of the solenoid!
Even very thougthful attemps or rubbing graphite-powder into the plunger didn't solve it.
WORST Scenario I ever came across was to spray contact-cleaner or similar stuff from
HERE, i.e. the mirror-box towards the region of the solenoid. You don't want to spray anything inside your camera, most sprays contain oily ingredients such as naphta or petroleum together with fast evaporating ingredients to make it dry quickly. The worst that can happen is if such stuff comes into contact with the mirror or hinged submirror, damping material for the mirror, the shuttercurtain of your sensor.
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With the Pentax K7 Pentax introduced a stepper-motor instead of a solenoid. This demands a different powersupply, more current from the battery, extra space within the body, i.e. the body has to be larger plus some weight. This stepper-motor was used in the K7, K5, K3, KP and K1.
Size comparison of the stepper motor on its board against the solenoid:
7. GOOD NEWS:
3.rd generation green China-solenoid used since Jan. 2021 in the K-70
and of course now the KF:
Shinmei changed the size of the magnet, it is again shorter and thus has less magnetic power. Here you can see all 3 stages of the China-solenoid (plus the very bad Lenovo solenoid on the right side): 
So we have 3 steps of the China-made green solenoid used since the K-30 in the aperture block:
- Verson 1: The non-modified version as we know it was used until Dec. 2012, less clear defined dimple in the plunger
- Version 2: The 1.st modification as used since Dec. 2012 with a smaller magnet, changed alloy and clear defined dimple in the plunger
- Version 3: The 2.nd modification: Same alloy as in the 1.st modification but even shorter magnet (since Jan. 2021)
Originally posted by photogem 
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