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Tutorial: Detection of aperture block failure on Pentax DLSRs with solenoid
Posted By: photogem, 01-25-2020, 09:30 AM

How to detect aperture block failure on any Pentax DSLR which uses a solenoid
for aperture-control:



2 different possibilies which give you 99,9% certainty!


- Method 1: For all Pentax-bodies without Live-View (LV), but of course it works as well for those with LV as well!
- Method 2: For all Pentax-bodies with Live-View (slightly different with the K20D*)

If the aperture control (diaphragm control) doesn't work any more, then in almost all cases the solenoid driving the aperture control is stuck.

This problem started with the introduction of the Pentax K-30.


Older Pentax DSLR bodies such as:
- all *ist DSLR bodies

- K10D, K20D
- K100D, K110D, K200D
- K2000/m, Kx, Kr

and Samsung equivalents never actually had this problem because all those had the superior Made in Japan Solenoid!
So all these are good donors in case you can't find a white made in Japan-Solenoid!


If you want to know more details and the history about all this, feel free to read this thread:
History of Solenoid



Method 1 with kit-lens or any other AF-lens without A-ring:
(such as DA 18-55, DA 18-50,DA50, DA35 many Sigma/Tamron lenses etc.)

a) Set the top dial of your Pentax to AV-mode:



b) Set aperture "wide open":

for example: - DA 18-55: f3,5 (at 18mm) -- DA 18-50: f4,0 (at 18mm) - DA 50: f1,8 - DA 35: f2,4



c) Take a photo:


This photo should be dark/underexposed!



Next step:


d) Set aperture "full stopped down" for your given lens:

for example: -DA 18-55: f38 - DA 18-50: f36 - DA 50/DA35: f22


e) Take a photo(if necessary use the flash if daylight isn't bright enough):


This photo should be fine!




The test shows you that the solenoid doesn't need to actuate when the lens is fully stopped down, that's why this photo is alright!
But with the lens wide open the solenoid has to actuate but can't because it is stuck.

If you done the test this way, you can say with 99,9% certainty, that the solenoid is stuck and needs to be replaced.


Method 2: Using Live-View mode with any A or AF lens with or without A-Ring:
(such as DA, DA-L, FA, F or A and those of other manufacturers with A-ring)

If you use a lens with an A-Ring set it on A!

This is best done with a prime lens, such as DA50/2,4, DA35/2,4, F/FA50/1,7/1,4
or any other which allows a good view into the front of the lens to see the aperture blades.

This photo shows the F50/1,7 set on A:




a) Set the mode-dial to any mode such as AV, TV, TAV or P:

b) Turn your camera (switched ON) so you can see into the front of the lens!

c) Now press the Live-View button:




If the aperture in your lens moves to f4, the solenoid works alright:






But if the aperture is fully closed,
the solenoid is stuck and damaged:






Often (but not always!) you can still take photos in LV (Live-View), while photos taken through the OVF (Optical-View-Finder) are underexposed!

This is due to the solenoid actuating twice plus the important fact that the mirror is up as soon you activate LV!

Mirror-up drains more power (mA) from the battery.
As soon as you press the shutter-release-button, the mirror goes down and this sudden stop of holding the mirror up allows some extra power and thus the solenoid often (but now always) now actuates this second time.


But: Because the solenoid was stuck the first time metering was not perfect.
Metering was done full stopped down instead of f4,0!
So you get some kind of alright photo but it isn't really correct!

You cannot you check the function of the solenoid in M-Mode and preselect aperture!
In M-Mode the solenoid is not used by the camera!
But all the other parts of the complex aperture control mechanism are used, this is the reason why manual lenses and green button metering does work. Anybody who has a Pentax K30/50/500 open can check this easily:
As soon as you have disassembled your Pentax you close the flash (otherwise the flash capacitor will charge, danger of receiveing a shock as explained here, plug a lens on and insert the battery: Do the tests in AV/TAV and then M-Mode and you can see the difference. Of course if the solenoid is stuck you can see it just doesn't actuate at all except possibly in Live-View-Mode, but as soon as you have exchanged the solenoid for the white Japan-made solenoid, you can do the testing.

* The Pentax K20D has different Live-View similar to to all semi-professional Pentax bodies like the K7, K5, K3, KP, K1 and possibly K-01 as well.
Live-view is not a fixed f4.0 but is advanced and changes according to the light situation!

So if the diaphragm remains closed i.e. fully stopped down the solenoid is stuck (allthough this never happened to the K20D anyway or if it happenes,
it is due to somebody having swapped solenoids!)
After the K20D all Pentax semi-professional bodies had no more solenoid but a stepper-motor which hardly ever breaks!
So the K10D and K20D (Samsung GX10 + GX20) were the last and yet, particular those reach incredible high shutter-count!
The K10D was used in very rugged situations such as the famous PENTAX K10D in Space III.


Of course in most cases repair will also work if you apply the dreaded filing/sanding-method,
but why you better stay away from this method you can read here:
Why you shouldn't file/sand the plunger of the green China-solenoid but replace it


Any discussion, commentaries etc. please post here:
Dark exposure problems on K-30 or K-50? Discussion Thread - PentaxForums.com

Last edited by BigMackCam; 01-27-2020 at 06:24 AM.
Views: 1,547
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