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02-06-2017, 04:35 PM   #1
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The 60-250 screw drive mystery

The last thread on this was apparently 2013, so I'm not resurrecting it. After attempting this myself I found the hack that is being applied to the 16-50/50-135 obviously does not work on the 60-250 as everyone else has found. Replacing all the values of C8 or whatever it was, does actually make the screw drive work *briefly* as the camera starts up. This is really encouraging and means the hack half works. Something in the camera's firmware recognizes the 60-250 and reverts back to SDM. You can test this yourself by changing all the values laid out in the original hack, though I think there are more to replace on the 60-250 rom. So you hack the rom and put the camera on. As soon as your turn the power on and start jamming the AF button the screw drive will briefly turn. Then it seems that the camera's OS boots and it checks the lens ID and reverts back to SDM. I think if you can hack the lens to read out as another lens or an unknown lens id to the camera the screw drive should kick back in. I don't know how the camera gets information on the focal length, but it clearly passes that through the contacts somehow. I get the feeling that is a separate piece of circuitry from the actual rom itself and that a few pins along the mount are dedicated to reading out some kind of value. Speaking of which, does anyone have a pinout??

The rom may supply the camera with conversion tables so it knows the actual focal length. Just a theory. There is another reason it would be extremely useful to break apart these roms and figure out their contents: you could create new AF adjustment tables for different focal lengths. I certainly have had at least one pentax lens where the values were not correct and that stopping it down would create a focus shift that the camera adjusted poorly for. Actually flashing a rom I found here on PF out of desperation fixed my problem or at least made it somewhat more manageable. At certain fstops it would reliably misfocus every single time. This is precisely one of the reasons sigma released the USB dock and let users make adjustments to AF in different ways. Imagine if you could tweak your pentax lens rom. The data structure has to be consistent for the camera to read it, so obviously there is a table of values in the rom. There was a german pentax forum where someone had made a comment where they had largely decoded the rom. I don't speak german, but I understood the gist of what the translation read. I'm actually really curious to see how the camera reads the rom off the lens and can write it back to the lens. There is likely just a simple serial connection between the camera and the lens and someone with good sniffing gear could probably make out some of the protocol that's involved. At any rate with the modset files on a card its pretty easy to engage debug mode every time you turn the camera on. I'm thinking about making another go at this because it would A) make a perfectly good lens with dying SDM less failure prone and B) also help out a bunch of people in the same situation. I eventually need to send this lens in for service anyways but if it has to take two trips to japan like the last lens I would probably rather just have the screw drive. I tested the speed on the mz-s and its about the same on screw drive in terms of turning speed. I would imagine this is quite a bit better on the k-3 so there is possibly a performance gain to be had there if you can live with the noise.

If anything it would be useful to at least decode the pentax rom some more I think. I know there are a few people trying to hack their way into the camera's firmware. It would seem like the lens rom would be a pretty simple task in comparison. Yeah shodan, looking at you. Anyone with much better hex reading skills than me want to help? Best I can think of doing is mapping out a table of values and randomly changing them. Wash, rinse, repeat. Then you would have to look at the exif etc, to see what actually changed. There's not a tremendous amount of data really to break down. Obviously looking for values like a lens id number (what does EXIF record on the camera?), focal length values, etc.

I bought the 60-250 from KEH and brought up that the AF was sticky the day I received it. I told them that I would use it for a while since it had a 6 month warranty. At the time when I had gotten it the AF wouldn't move at all. I didn't know if it had jammed in the clutch or not. I should have just sent it right back, but then it started working and seemed ok. It then worked reliably for the most part for months. Of course it started to really fail after the warranty expired. Go figure. Lesson learned really. At any rate I am going to e-mail them and see if they can give me any kind of break on repairs since I was really doing them a favor by keeping it at that point.

02-06-2017, 04:39 PM - 1 Like   #2
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I don't like your chances, but good luck anyway!

I always thought the focal length was reported by an analogue to digital conversion of some sort of potentiometer signal (or possibly very narrow discrete steps) as the focal-length helicoid was spun up or down towards narrow or wide angle. Maybe Pentax does it differently; I don't know.
02-06-2017, 04:39 PM - 1 Like   #3
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I have the exact same issue and wound up getting a D FA* 70-200mm lens to replace it. What a shame.

See Pentax DA* 60-250mm autofocus: an exercise in frustration - PentaxForums.com

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02-06-2017, 04:43 PM   #4
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QuoteOriginally posted by pathdoc Quote
I don't like your chances, but good luck anyway!

I always thought the focal length was reported by an analogue to digital conversion of some sort of potentiometer signal (or possibly very narrow discrete steps) as the focal-length helicoid was spun up or down towards narrow or wide angle. Maybe Pentax does it differently; I don't know.
I think that is exactly how it is done with the rom supplying a table of values that correspond to focal length. There are definite discrete steps.

---------- Post added 02-06-17 at 06:46 PM ----------

If you could hack enough of the lens rom up you could really change the lens to read as anything. I see no reason why the camera would know that sdm exists if it no longer thinks the lens is a 60-250. All you need to do is preserve the focal length so that SR and any focus corrections embedded in the rom still work.

---------- Post added 02-06-17 at 06:48 PM ----------

QuoteOriginally posted by bwDraco Quote
I have the exact same issue and wound up getting a D FA* 70-200mm lens to replace it. What a shame.

See Pentax DA* 60-250mm autofocus: an exercise in frustration - PentaxForums.com

Draco
I would think that just fixing the SDM would be a lot cheaper than the cost of the 70-200 but you did at least upgrade there.


Last edited by zosxavius; 02-06-2017 at 04:48 PM.
02-06-2017, 05:26 PM   #5
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QuoteOriginally posted by zosxavius Quote
I would think that just fixing the SDM would be a lot cheaper than the cost of the 70-200 but you did at least upgrade there.
KEH will fix the SDM for $235 plus shipping...
02-06-2017, 09:37 PM   #6
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QuoteOriginally posted by boriscleto Quote
KEH will fix the SDM for $235 plus shipping..
$235 is for each one SDM fixing. If anyone can figure out how to edit the ROM to let camera using screw drive AF, that is for many many potential Pentaxians.
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