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07-05-2017, 09:02 AM   #16
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I don't know whether SDM lenses have a problem that is caused by a failing capacitor or not, so I'm not getting involved in that argument. But to be clear, electrolytic capacitors CAN fail in such a way that they will start to work better after they heat up somewhat. I can't begin to tell you how many switching power supplies I've diagnosed with a hair dryer, lol! The caps have a property called ESR- equivalent series resistance. This functions as a resistor in series with the cap. Putting a resistor in series with a cap causes it to take longer to charge. Normally, the ESR is low, and has little effect on the charge time. But when the cap starts to fail, the ESR increases, increasing the charging time. If there is fairly smooth DC across the cap, this can be unimportant. But in a timing circuit, or switching applications where the cap is charged and partially discharged thousands of times per second, the additional ESR can prevent the cap from charging to the required level in the expected amount of time.

Heating the capacitor can reduce the ESR, and the circuit starts to work. You can heat the cap with hot air, or maybe it is located close to another component that gets warm (so the circuit starts to work when power has been applied for a while). Since the ESR is real resistance, there is I^2*R power loss through the ESR, and the cap can start to work by self-heating.

With all this talk about capacitors, I just thought you might like to hear from someone who actually works with the things. There are several hundred behind my bench as I type this. Some weigh a few grams, some weigh a pound!

07-05-2017, 09:16 AM   #17
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QuoteOriginally posted by DougieD Quote
I don't know whether SDM lenses have a problem that is caused by a failing capacitor or not, so I'm not getting involved in that argument. But to be clear, electrolytic capacitors CAN fail in such a way that they will start to work better after they heat up somewhat. I can't begin to tell you how many switching power supplies I've diagnosed with a hair dryer, lol! The caps have a property called ESR- equivalent series resistance. This functions as a resistor in series with the cap. Putting a resistor in series with a cap causes it to take longer to charge. Normally, the ESR is low, and has little effect on the charge time. But when the cap starts to fail, the ESR increases, increasing the charging time. If there is fairly smooth DC across the cap, this can be unimportant. But in a timing circuit, or switching applications where the cap is charged and partially discharged thousands of times per second, the additional ESR can prevent the cap from charging to the required level in the expected amount of time.

Heating the capacitor can reduce the ESR, and the circuit starts to work. You can heat the cap with hot air, or maybe it is located close to another component that gets warm (so the circuit starts to work when power has been applied for a while). Since the ESR is real resistance, there is I^2*R power loss through the ESR, and the cap can start to work by self-heating.

With all this talk about capacitors, I just thought you might like to hear from someone who actually works with the things. There are several hundred behind my bench as I type this. Some weigh a few grams, some weigh a pound!

Although your theory about capacitors is correct.. I don't believe there is a capacitor involved in the drive. It may be involved in the frequency generation which drivers the motor's Piezo which causes the motor to rotate. Check here to understand the ultra sonic motor functions and its types.

From what I read and the diagram that depicts the working I have a stronger feeling that it is the over torqued screw that mounts the motor on the body. The friction coat and the elastic body and the space between them is easily affected by an over torqued screw reducing the gap causing the motor to struggle. that would also explain why some people who tried have had success with wiggling that screw.

I am not sold on to the bad capacitor theory at all. If it was a bad capacitor it will never work because of the nature of its function in an ultrasonic motor driver circuit. I would buy this if it was a flash unit.
07-05-2017, 09:21 AM   #18
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FTR:
https://www.pentaxforums.com/forums/attachments/114-maintenance-repair-artic...-al-if-sdm.pdf
07-05-2017, 10:00 AM   #19
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QuoteOriginally posted by shardulm Quote
I am not sold on to the bad capacitor theory at all.
I'm sure you're right, shardulm. I haven't looked into the SDM problem at all, which is why I avoided commenting about it directly. I was just concerned that people are dismissing capacitors as a problem based on their understanding of how a capacitor works when it is in good condition. As a technician, I have insight into how they behave when they fail. So, I tend not to dismiss the possibility on the say-so of someone who might not have that insight (I hope that doesn't sound arrogant!). If people are wriggling things and the lens starts to work, that certainly sounds like a mechanical fault.

07-05-2017, 10:15 AM   #20
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DougieD, what I'm rejecting is the existence of a capacitor needing minutes of charging, and somehow essential for the SDM piezo micromotor to start moving. What I'm rejecting is this claim:
"EEVIDEEENTLY!!! there's a capacitor inside the Pentax SDM lenses and it holds an electrical charge so that the ultrasonic motor has the power to move and it gets this probably small little electronic capacitor and it gets the power from the camera and if the lens and capacitors discharge over time and this lens was manufactured a year ago and put on a shelf at B you know B&H and if nobody touched this lens and fired it up in a year so the capacitor discharged it took 3-5 minutes on a camera body because it hasn't been on a camera body in a year to charge the capacitors so the autofocus works"

What I'm not rejecting is the possibility of a defect in the electronics around the SDM motor (there has to be a DC/AC converter and a controller, right?). Even if in many other cases it's the stiction.

Last edited by Kunzite; 07-05-2017 at 10:32 AM.
07-05-2017, 11:12 AM   #21
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QuoteOriginally posted by Kunzite Quote
DougieD, what I'm rejecting is the existence of a capacitor needing minutes of charging, and somehow essential for the SDM piezo micromotor to start moving. What I'm rejecting is this claim: "EEVIDEEENTLY!!! there's a capacitor inside the Pentax SDM lenses and it holds an electrical charge so that the ultrasonic motor has the power to move and it gets this probably small little electronic capacitor and it gets the power from the camera and if the lens and capacitors discharge over time and this lens was manufactured a year ago and put on a shelf at B you know B&H and if nobody touched this lens and fired it up in a year so the capacitor discharged it took 3-5 minutes on a camera body because it hasn't been on a camera body in a year to charge the capacitors so the autofocus works"
I agree with you, Kunzite. That sounds implausible. It sounds like an SDM urban legend to me, lol! If this is the basis of the claim that a cap has to charge before the lens will focus, personally I'm skeptical.

There was a time when I used to repair VCRs (a long time ago, now) among other consumer electronics gadgets. Mechanical faults were much more common than electronic faults.
07-05-2017, 02:52 PM   #22
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QuoteOriginally posted by Yoda boy Quote
I have read differing opinions about the use of SDM in DA* lenses. My experience with both DA* 16-50 and DA* 60-250 is that both have had their SDM motors replaced twice within 3 years. The first issues happened within the warranty period of 2 years ( I live in Canada ).

One comment that I have read about is that the capacitor needs to be charged but I had a lens meltdown in the middle of a far away trip where I had been using this lens extensively for days when it suddenly would not operate, even in manual mode. In all 4 instances, the authorized repair facility advised that the SDM had malfunctioned. In the last repairs to each, a new version of the SDM was installed. Replacement costs were around $300 each time.

What recent experiences have others had?

I now have added new D FA lenses: the 100 WR, the 70-200 and the 150-450 and find the last particularly wonderful. I am using these on a K3. Like many, I suspect, I have switched to the DFA designation in anticipation of sometime adding a K1 to the mix.
It used to be that when you mentioned SDM problems, the forum members would say something like "sorry but it was inevitable" and you would have to get it fixed. Now there are a number of deniers who say that is ancient history. I don't have any special knowledge of the issue except that like you, I used to own an SDM lens. In my case it was a 17-70. It was fixed the first time on warranty (also in Canada) and then in less than a year it was fixed on my tab. I was not impressed. I sold it on this site with full disclosure and the buyer got a great deal. I have no idea how it worked after that because I didn't use it after the second fix. The repair place had it cleaned up so nicely that it was like a new lens and that is how I preferred to sell it.

At a recent camera show I met with the Pentax rep who said that there were a lot of problems but the new SDMs are not affected (likely this is because they are from Tamron but he didn't say that). Personally I will never buy an SDM lens unless the price is so good that I just can't lose. I don't understand why Pentax would use the SDM on any lens going forward unless it is from Tamron and a completely different system. The DC motor is so superior in speed and durability that if there is need of updating a lens, it should be a natural choice. I have the 18-135 (a nice travel lens) and it has gone through minus 30C weather, ocean sprays, deserts, humidity, and whatever else I can throw at it and never missed a beat. If the glass was as good as the motor it would be their best zoom.

BTY I did noticed that with my old 17-70 it did seem sluggish if not used for a while. I had forgotten about that because It's been a while and one just doesn't have those sorts of issues with screw drive primes or DC motors.
07-05-2017, 03:40 PM   #23
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You sold your 17-70 in 2011
Pentax didn't release any new Pentax-SDM lenses after introducing DC. Last Pentax-SDM lenses were announced in 2008; the first DC lens, in 2010.

07-05-2017, 06:13 PM   #24
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QuoteOriginally posted by Pål Jensen Quote
It is a fact that my lens have behaved this way for the last 7 years.
My 50-135mm has done this for years. When the symptom reoccurs, I simply hold the shutter release button down for a few seconds and the lens focusing motor comes back to life and works all day. Otherwise It shows no signs of potentially failing.


My 16-50 didn't do that at all and just failed completely one day.
07-05-2017, 07:10 PM   #25
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QuoteOriginally posted by Kunzite Quote
Pentax didn't release any new Pentax-SDM lenses after introducing DC.
DFA 15-30 and DFA 24-70 are both listed as "SDM". Since they are Tamron designs it is unlikely, I would think, that the motor has any connection to the old SDM motor used in the DA*16-50 / 50-135 / etc. But Pentax does use the term "SDM" for these lenses. Considering the almost universal derision the term "SDM" has with Pentax users it is curious they would continue to use the name. Particularly if the actual hardware has no relationship to the older motor. So perhaps Pentax marketing was not informed of the SDM issues?
07-05-2017, 11:32 PM   #26
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I said Pentax-SDM
Perhaps the "universal derision" is not so universal?
07-06-2017, 06:36 PM   #27
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QuoteOriginally posted by Kunzite Quote
I said Pentax-SDM
I know, but the lens says 'SDM' which causes a negative reaction in many I think. If Ricoh understood that SDM has a bad reputation (deserved on not) they should have used a different name. Or, as you say maybe the poor reputation is not so universal.... ..... .....
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