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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 analoge *ist). It looks identical to those used in the later DSLR bodies and had almost the same impedance. But its holding-force and body thickness was slightly different. Some which I and others had found had opposite polarity as well but others applied it with success in DSLR bodies, so it looks like that there were different versions used. Another type of solenoid was used as well for example in the Pentax MZ5/n. But Pentax didn't keep it up later on. This solenoid was driven with 6VDC (rated with 4,5VDC) by the solenoid-driver, impuls was 6.45V.

3. 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). It looks almost similar to the DSLR solenoids but aside of a tiny differenz in bobbin-size it has a slightly different holding force and often me and others have found polarisation to be opposite to the later introduced DSLR solenoid.

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

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 was Shinmei, Japan... also manufacturing similar solenoids for ALPS and Matsushita.

The next drawing shows that this new introduced Japan solenoid for DSLR bodies had now 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 new DSLR-solenoid made in Japan (also for ALPS) It had a live-timespan of remarkable 100.000 actuations!

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

When Shinmei moved production to China, difficulties started!

The material of the body changed to green colourand instead of using PTFE (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 tried in the flash circuit of the Pentax K100D, K100D, K200D, K-m, K-x and K-r but as far as I know mainly in those delivered to Europe.
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 do 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, which some mistake for a resistor or chip-resistor. This diode looks almost identical to an SMD-resistor, and yes, it resists into one direction (infinite ohms) but not into the other (zero ohms)
Nevertheless, it is a good idea to resist such chips were you don't want them or were they just don't belong to.

Some repairshops quote that they exchanged a chip-resistor but following the circuit very closely I just could not find any. It wouldn't make sense either, because if one studies the curve and quick response the solenoid has to actuate, any resistor would basically block any quick response. The solenoid does depend mainly on "surge-power", i.e. milli-amperes delivered quickly and not on voltage. If one studies the curve, one can quickly find out, that there is a range which is best for minimum holding-force. I challenge anybody who insists on some chip-resistor to prove it to me. I am curious for the outcome. And should I be wrong, I will happily "throw my hat" on the ground and apologize!

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:

But this change turned out to be the beginning of difficulties, due to several reasons described here as well:

The plunger of this solenoid often remained closed/stuck! The diaphragm/blades/aperture of the lens could not open and this led what is now often called ABF = "Aperture-Block-Failure", "dark-image-syndrom", "dark-exposure-problems" etc.

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. The made in Japan solenoid was no longer available, machines were already since a long time in China, the plants in Japan closed, so no chance to go back to this Japan-made solenoid!

Some early KS-1's, K-S2's and many K-50's as well of course K-500's still used the earlier China-made green solenoid and thus some of them failed and still do fail.

It is mainly due to fact that the metal body + plunger of the China-Solenoid is made out of relatively higher coercivity material, i.e. it retains its magnetism after the field strength is removed!

This 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.

Combined use of the AA-Adapter with rechargeable AA-batteries (best are Eneloops) + regular use has proven as the best way to avoid ABF.

I know of (very few) cases were this is the case. Usually high-shuttercount + ABF indicates non-use for some time.
But not always, I have encountered cases, were ABF just happened the next day.

But good enough: As far as I can tell the green solenoid now used in the Pentax K-70 is superior to the early version!
And yet: The holding force is still stronger, it does not fire as quick as the white made in Japan-Solenoid!

Close-up photos of the solenoid used in the K-70 show a very nicely machined plunger with a very smooth 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.

Beware of those green China-Solenoids now being sold on ebay, aliexpress and other sources,
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! They have again a way too strong holding force, also because they have a longer and thus stronger permanent magnet!
They were constructed for a very simple "press-button-open-tray" circuit

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.

- Beware also of blue-coloured China-Solenoids from ROM-Drives:
Wrong impedance (15ohms)! Danger of exodus for the "solenoid-driver".

- Again: Beware of green solenoids which are filed/sanded!
Further information why you should avoid this you can study HERE

- Beware using white Japan-made SLR solenoids from some MZ/ZX SLR-bodies!
Recently I did a vice-versa test by inserting the China-made green solenoids into a MZ50, MZ7 and MZ30.
They didn't work at all!

Some companies offer modifications "in weight, structure and/or surface", sounds impressive, but if you study such a modified solenoid:
They are just filed, which of course alters weight, structure and surface!

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:

GOOD NEWS (for the latest green China-solenoid used now in the K-70 and for repair:

I don't know when Ricoh demanded of their manufacturer another very good modification.

They changed the size of the magnet, it is shorter and thus has less magnetic power:

So we have 3 steps of the China-made green solenoid used since the K-30 in the aperture block:

1. The non-modified version as we know it was used until Dec. 2012, less clear defined dimple in the plunger

2. The Modification as used since Dec. 2012 with a smaller magnet, changed alloy and clear defined dimple in the plunger

3. The 2.nd Modification: Same alloy as in the modification but even shorter magnet.

And as can clearly be seen: The solenoid on the Lenovo (fake Pentax) solenoid on the right has a larger magnet
Worse than Version-1, i.e. the original non-modified China-solenoid.

Last edited by photogem; 04-14-2022 at 12:30 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...
Itís 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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