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A LITTLE BIT OF HISTORY: Development of the solenoid in Pentax cameras
Posted By: photogem, 06-27-2019, 01:50 AM

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:


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 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.

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:

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 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 modification but even shorter magnet (since Jan. 2021)

Last edited by photogem; 09-28-2023 at 06:17 AM. Reason: Latest info added by Photogem
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06-27-2019, 03:21 AM   #2
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Very interesting! Thank you for posting this history of the development of the solenoid.
06-27-2019, 07:26 AM - 1 Like   #3
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QuoteOriginally posted by photogem Quote
A LITTLE BIT OF HISTORY: Development of the solenoid in Pentax cameras
A nice piece of reverse engineering and analysis. Thanks for posting!

I am frequently amazed at the content of Pentax Forums and the many talented members.

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06-27-2019, 11:38 AM - 1 Like   #4
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Excellent. Thanks for sharing this information. I confess, I wouldn't have expected to find the solenoid development interesting... but I enjoyed reading this

06-27-2019, 11:47 AM - 1 Like   #5
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The best part is the pictures, which makes sense since this is a photography forum, but photos do a much better job of conveying information than words.
06-27-2019, 01:39 PM   #6
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Link broken?
06-27-2019, 01:41 PM   #7
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QuoteOriginally posted by monochrome Quote
Link broken?
Which one, Paul? The two patent-related links at the top of the post are working for me...

06-27-2019, 05:24 PM   #8
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QuoteOriginally posted by BigMackCam Quote
Which one, Paul? The two patent-related links at the top of the post are working for me...
It’s good now.
09-24-2019, 01:04 PM   #9
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Is there any place that we can purchase the "NEW" green solenoid for repair of our K30, and how would we know we got the NEW solenoid
09-24-2019, 10:25 PM   #10
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QuoteOriginally posted by hnikesch Quote
Is there any place that we can purchase the "NEW" green solenoid for repair of our K30, and how would we know we got the NEW solenoid
I had wondered too and tried but hitherto no success.

And yet, the "MODIDIED" (I would not call it new) green Chinese solenoid has sometimes failed as well in just the same way those of the K30/K50 failed, but to a much smaller percentage. (Same for K-S1 and K-S2)

As we know, the quality of manufacture of the white made in Japan solenoid is just superior and thus I recommend to find this one!
09-25-2019, 12:06 PM   #11
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Does the istD use a white solenoid for the pop up flash and if it does can I use it repair my K50?
09-25-2019, 06:46 PM   #12
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QuoteOriginally posted by martym Quote
Does the istD use a white solenoid for the pop up flash and if it does can I use it repair my K50?
As far as I can remember yes.
You can use any solenoid from the istD up to the K-r but at least in Europe most Pentax from the K100D up to the K-r use already the green solenoid in the pop up flash but the white one for aperture control.
09-28-2019, 09:02 PM - 1 Like   #13
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Incidentally, that mechanism still works correctly in my "Super Program" which I purchased in 1983, "retired" in 1995, but still use periodically.
10-06-2019, 01:41 AM   #14
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Well, this mechanims with a Japan-made solenoid still works without any problem in any DSLR body prior the K30 (and except the K7 which has the new design with the stepper-motor). It works as well without problem in those SLR bodies but as said, it slightly different.
05-25-2020, 03:32 AM - 5 Likes   #15
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QuoteOriginally posted by martym Quote
Does the istD use a white solenoid for the pop up flash and if it does can I use it repair my K50?
*istD has 2 solenoids, one in the flash-circuit, one for aperture/diaphragm-control
*istDS and DS2 (as well as Samsung GX1s) have only 1 solenoid (Aperture)
*istDL/2 (Samsung GX1L) have again 2 solenoids

1 solenoid to be found in K10D, K20D (Samsung GX10/20) as well (Aperture)

2 solenoids to be found in:
K100D, K110D, K200D, Km(K2000), Kx, Kr (but in EU and Switzerland most of those have already the green solenoid in the flash-circuit!)

1 green solenoid to be found in:

modified better green solenoid found in:
K50/K-S1/K-S2 from December 2015 onwards

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