[photogem]

A lot of hot air about something, that already has been solved.

[Cee Cee]

A lot of hot air about something, that already has been solved.

well with CV19 being what it is I have some time on my hands to consider these issues and expel any unwanted gases !

come on, don't you want to fully understand the science here, after all this could possibly lead to novel solutions which don't involve having to track down an old camera from which to remove the solenoid and then replacing the solenoid in their own camera, which involves de-soldering & soldering the coil by people with no soldering skills, which in my experience doesn't always end well.
Do you find it interesting that the "filing the edges of the plunger technique" (to reduce the contact area) has had varied and inconsistent results, perhaps the inconsistency is more to do with which polarity they refitted the plunger! rather than the filing technique.
I must admit the finer points of magnetic theory are perhaps above my pay grade, But reversing the poles on the horse shoe plunger should counteract any residual magnetism, and perhaps provide a simple solution, if not then residual mag (as the primary issue) could be eliminated from the discussion and other theories explored.

[malc77]

Thank you for your reply Cee Cee.

I believe the problem with the Chinese solenoid is that the plunger material is of relatively higher coercivity material than the Japanese one (ie it retains its magnetism after the field strength is removed). Reversing the plunger wouldn't work long term as the plunger would just get re-magnetised.

Low coercivity material such as soft iron (low carbon) is very easily found, cheap, and machining a few plungers simple- and far easier than dismantling otherwise good cameras. What a great pity Pentax didn't catch on earlier, for almost no increase in production cost the issue would have been obviated.

Malcolm (Chartered Electrical Engineer with Nuclear Electric)

PS But others may know better of course!

[Cee Cee]

I believe the problem with the Chinese solenoid is that the plunger material is of relatively higher coercivity material than the Japanese one (ie it retains its magnetism after the field strength is removed). Reversing the plunger wouldn't work long term as the plunger would just get re-magnetised.

it will be interesting to see how you get on, good luck. If it took 5 years for the build up of magnetism to cause the camera to malfunction (in my case) then by reversing the plunger it could take a further 5 years to build up to that point again - by my simple logic. But this is, of course if residual magnetism of the plunger is indeed the only problem.

[photogem]

.....come on, don't you want to fully understand the science here, after all this could possibly lead to novel solutions which don't involve having to track down an old camera from which to remove the solenoid and then replacing the solenoid in their own camera, which involves de-soldering & soldering the coil by people with no soldering skills, which in my experience doesn't always end well.

Do you find it interesting that the "filing the edges of the plunger technique" (to reduce the contact area) has had varied and inconsistent results, perhaps the inconsistency is more to do with which polarity they refitted the plunger!

As I have written HERE there is no chance that turning the plunger would have any other effect than losing its magnetism but not because of it being turned around but because it was away from the magnet and lost remanence.

I must admit the finer points of magnetic theory are perhaps above my pay grade, but reversing the poles on the horse shoe plunger should counteract any residual magnetism, and perhaps provide a simple solution, if not

a) then residual mag (as the primary issue) could be eliminated from the discussion and

b) other theories explored.

a) is a very good idea because there is no reversing of poles on the plunger possible

b) is not really necessary, we don't need endless theories but only facts and facts are "on the table"

And we have a reference:
The white PTFE Japan-Solenoid.
It never failed.

As I have shown in my report about the history of solenoids built and used by Pentax and particular that one used in the Pentax K-70:
Ricoh tried to do the best and it for sure became better but not perfect.
(My own K-70 had failed and I had a second for repair)
And this report will answer your other questions about sanding etc.

[photogem]

Hello again,
Thank you again for your replies. Presumably (although I haven't taken my K30 apart yet) the older 'made in Japan aperture block solenoid' has an armature made from low coercivity material such as low carbon (soft) iron, and does not retain its magnetism once the field is removed. Whereas I am presuming the Chinese version does not use a sufficiently low coercivity material armature?
As a clock and watch maker with a workshop, I wonder if it is reasonably possible to make a replacement armature using soft iron etc (eg from an old solenoid or electric bell etc) to replace the Chinese armature? I guess it would be a simple machining job.
Anyway,
Regards,
Malcolm.

Because this was an interesting suggestion I made an experiment with a K-30.
I left the green solenoid and just ex-changed the plunger using the plunger from a white made in Japan solenoid.

Because I have many chinese green solenoids here (from repaired Pentax bodies) I first made the test comparing the holding force against each other
but nothing changed, the holding force of the green solenoid with the plunger of the white one was still too strong.
So anyway I decided to install that modified green solenoid into a K-30 that but leave it open (for quick exchange)
It first worked alright which often is the case if you just manually release the plunger but this method works only very short term.

So I left it just on a shelf and then, partly due to Corona, forgot about it completly and now checked it:

The problem is right there, dark images, the solenoid won't release.

And yet the alloy is different, one can see it and thus the main body which holds the permanent magnet could magnetise differently because it is part of the whole assembly.


But this does not explain problem #1: The holding force which just is too strong.

Anyway, the solenoid of a K70 with late manufacture has a less strong holding force now but it failed.
The reason could well be the magnetism of the alloy but also it being vulnerable against the sudden impact of the plunger onto the main aperture.
I have seen change of surface this main aperture. And yet I see also the different guiding-ability (bearing) of PET against PTFE (Teflon) as one reason, because
PET just does not act that well as a bearing compared to PTFE.
I have seen bearings made out of PTFE which had very strong and long use but last very long.

It could even be that just soldering the pins on the PET body already deformes it slightly while PTFE can withstand much higher temperatures.
One can verify this very well with RCA sockets or RCA connectors. Most of them have PTFE or Teflon (the name DuPont gave it).
The melting-point of PTFE is 326.85 °C (620.33 °F)
The melting-point of PET is 250 °C (482 °F)
That is quite a difference and could play a role

[photogem]

I believe the problem with the Chinese solenoid is that the plunger material is of relatively higher coercivity material than the Japanese one (i.e. it retains its magnetism after the field strength is removed)....

Malcolm (Chartered Electrical Engineer with Nuclear Electric)

I take up this thread again because I just heard from Malcolm today that he repaired his K30 by using the white Japan-Solenoid.

But the main reason is that his explanation is pointing right there where one of the two main problems is located,
i.e. the difference in materials getting magnetized (the other is PET for the China-Solenoid vs. PTFE for the Japan-Solenoid)

But it isn't just the plunger but the whole body.
This photo shows the magnet and those parts which it can magnetize:

It is obvious, the main metal body which holds that tiny but very strong "rare earth" magnet is larger than the plunger.

That means, during non-use of the Pentax the magnet does magnetize not only the plunger but the metal body as well (of course).
Also: When the plunger is released, it moves just for a fraction of a second away from the body with the two coils and then returns.
Not enough time to lose magnetism.

So Malcom's explanation shows exactly why particular:

a) Pentax bodies with low shuttercount

b) Pentax bodies not having been used for a longer time

are more prone to ABF (aperture block failure) than those used on a regular basis*.


Today I repaired this NOS (New Old Stock) blue Pentax K30:

NOS of course does imply a shuttercount-reading below 6 but this snapshot reading of 65 is because the person who brought it to me tried first to find out what was wrong and after the repair with a white Japan-Solenoid I myself did some testshots as well!

So Malcom's explanation brings this to the very point, just that it's not the plunger alone but the whole metal body!

*of course high-shuttercount Pentax bodies can catch ABF as well, but particular if not used for a longer period!
Its the none-use which is more important, just that low-shuttercount implies low use anyway!

[sergiogonzalez]

Thank you for your reply Cee Cee.

I believe the problem with the Chinese solenoid is that the plunger material is of relatively higher coercivity material than the Japanese one (ie it retains its magnetism after the field strength is removed). Reversing the plunger wouldn't work long term as the plunger would just get re-magnetised.

Low coercivity material such as soft iron (low carbon) is very easily found, cheap, and machining a few plungers simple- and far easier than dismantling otherwise good cameras. What a great pity Pentax didn't catch on earlier, for almost no increase in production cost the issue would have been obviated.

Malcolm (Chartered Electrical Engineer with Nuclear Electric)

PS But others may know better of course!

Dear malc 77.

Please see this thread.

Dark frames or dark expositure problems in KS2, K30, K50, etc. - Page 10 - PentaxForums.com

Best regards