Pentax M 28 / 2.8

JimKnopf · 07-30-2013, 05:12 AM

Hello,

today I got a cheap Pentax M 28 / 2.8. I got it cheap, because the aperture blades are closing a bit slowly. I took the risk to buy it because I think it can be repaired. I once repaired an old Tokina RMC 28 / 2.8, which was not that difficult. Does somebody know, if the Pentax M is similar to the Tokina with respect to dismantling the front and rear lens elements in order to clean the aperture blades?

Thanks,

Jim

wombat2go · 07-30-2013, 05:28 AM

I have one (7162082) with same problem. It works better when warmed up so it is probably a dry lubricant problem. Aperture blades are visible and they are dry and clean.
Can you take off the mount first, to see if there is a pivot in there you may be able to put a drop of solvent on?
I will be interested to follow your progress, however i use a Vivitar 28/2 mostly.

MysteryOnion · 07-30-2013, 06:57 AM

JimKnopf, quick test also if you have a chance... Gently and while the lens is set to infinity, tap the front against a not too hard surface... a book, or sheet of cardboard. Then check to see if any change in blade action. My M28/2.8 seemed the same slow blade problem and dry lube didn't solve. When I took the lens apart is when I found that the set screw broke loose and allowed the front portion of the aperture assembly to press down too tight on the blades in the lower cup portion. Since servicing the lens and re-set and thread-locking the screws, no issues since.

JimKnopf · 07-30-2013, 08:15 AM

@wombat2go: I'll keep you posted ...

@MysteryOnion: What helped with my Tokina was to really get out the lens elements and then putting some acetone on the aperture blades. Basically this would be the first thing that I would try. However, before opening up the whole thing, I would still need to try the lens out more thoroughly, as I need to analyze under which conditions I get overexposed images.

Last edited by JimKnopf; 07-30-2013 at 08:54 AM.

tom1803 · 07-30-2013, 08:19 AM

DO NOT APPLY ANY LUBRICANT!!

The aperture blades are meant to be run dry. Acetone will certainly clean the blades of oil but it should not be applied in situ. The aperture assembly should be taken apart and then cleaned. Acetone has a nasty habit of attacking any plastic parts, paint, etc.

I have no experience with the Tokina lens, so I can't help you there. However, the M28/2.8 is fairly easy to take apart and it's more or less the same procedure as disassembling the M50/1.4 or similar. There are plenty of articles on this forum and the web on how to do it.

07-30-2013, 05:12 AM	#1
JimKnopf Junior Member Join Date: Jul 2013 Posts: 37	Pentax M 28 / 2.8 Hello, today I got a cheap Pentax M 28 / 2.8. I got it cheap, because the aperture blades are closing a bit slowly. I took the risk to buy it because I think it can be repaired. I once repaired an old Tokina RMC 28 / 2.8, which was not that difficult. Does somebody know, if the Pentax M is similar to the Tokina with respect to dismantling the front and rear lens elements in order to clean the aperture blades? Thanks, Jim

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Pentax M 28/2.8	Schmidlapper	Lens Sample Photo Archive	16	05-01-2022 10:54 PM
For Sale - Sold: Pentax-M 50/1.4, Pentax-A 28/2.8, Tamron 70-300	jlandecker	Sold Items	10	04-16-2013 05:13 AM

07-30-2013, 05:28 AM	#2
wombat2go Veteran Member Join Date: Jan 2010 Location: Michigan Photos: Gallery Posts: 2,207	I have one (7162082) with same problem. It works better when warmed up so it is probably a dry lubricant problem. Aperture blades are visible and they are dry and clean. Can you take off the mount first, to see if there is a pivot in there you may be able to put a drop of solvent on? I will be interested to follow your progress, however i use a Vivitar 28/2 mostly.

07-30-2013, 06:57 AM	#3
MysteryOnion Giveaway winner! Join Date: Dec 2007 Location: beantown Photos: Albums Posts: 944	JimKnopf, quick test also if you have a chance... Gently and while the lens is set to infinity, tap the front against a not too hard surface... a book, or sheet of cardboard. Then check to see if any change in blade action. My M28/2.8 seemed the same slow blade problem and dry lube didn't solve. When I took the lens apart is when I found that the set screw broke loose and allowed the front portion of the aperture assembly to press down too tight on the blades in the lower cup portion. Since servicing the lens and re-set and thread-locking the screws, no issues since.

07-30-2013, 08:15 AM	#4
JimKnopf Junior Member Join Date: Jul 2013 Posts: 37 Original Poster	@wombat2go: I'll keep you posted ... @MysteryOnion: What helped with my Tokina was to really get out the lens elements and then putting some acetone on the aperture blades. Basically this would be the first thing that I would try. However, before opening up the whole thing, I would still need to try the lens out more thoroughly, as I need to analyze under which conditions I get overexposed images. Last edited by JimKnopf; 07-30-2013 at 08:54 AM.

07-30-2013, 08:19 AM	#5
tom1803 Veteran Member Join Date: Feb 2010 Location: Kokubunji, Tokyo Photos: Gallery Posts: 465	DO NOT APPLY ANY LUBRICANT!! The aperture blades are meant to be run dry. Acetone will certainly clean the blades of oil but it should not be applied in situ. The aperture assembly should be taken apart and then cleaned. Acetone has a nasty habit of attacking any plastic parts, paint, etc. I have no experience with the Tokina lens, so I can't help you there. However, the M28/2.8 is fairly easy to take apart and it's more or less the same procedure as disassembling the M50/1.4 or similar. There are plenty of articles on this forum and the web on how to do it.