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04-09-2022, 03:51 AM   #1
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Pentax MZ-5 autofocus unreliable/unstable

Hello,

I am a new participant to this great forum, however, I've been a happy reader of it for at least 10 years (esp. the lenses section). I own and use both MF film cameras (ME, MX and LX, since 1999) and digital K-01 and K30 ones. Having gathered some full-frame AF lenses recently for use on digital bodies, I thought that there is a reason to use these on some AF film body as well, and, therefore, got a MZ-5 for this.

Everything (metering, shutter, operation modes) seems to work in this camera except some strange behavior of the AF. The symptoms are as follows. Focusing may work straight after switching the camera on, but only for 2-3 consequent attempts. Say, you focus: (1) on infinity (focuses fine: a beep and green diamond are there), (2) change to a close distance object, 1-2m (fine again), (3) back on infinity again (fine), then the following focusing attempts fail. What I mean is not incorrect focus but no focusing at all. Half-press shutter: nothing happens, AF motor does not turn. Then, if you turn the focusing ring on the lens manually a bit, it starts to focus OK again, for a few following attempts again. Then stops reacting. Sometimes it is not focusing straight after switching on, too. Seems like partly random behavior. One, for sure, can use it in this semi-automatic mode: turn focus ring by hand, then allow it to AF precisely, but of course, it would be better to get it work as desired.

What have I checked:
- sometimes focus is not OK (no beep and to green diamond) even if the AF works, looks like AF is not complete, perhaps, lacking a few additional fine tuning steps
- contacts (body and lens) are clean
- screw drive coupler protrudes and engages fine, is easy to turn on the body without lens being mounted
- lens AF mechanics is easy to turn when the lens is not on camera
- when the lens is on camera - it's not too difficult to turn manual focusing ring when AF is coupled mechanically with the camera, too (this is what I do to bring AF temporarily alive)
- all lenses behave the same
- batteries are fine, no low battery sign, also there are enough power for the mirror motor and shutter, checked also by connecting external 4xAA batteries, all the same

What it possibly could be? Semi-dead focusing motor (which stuck half-way and then is alive when turned by hand)? Are these normal DC motors with brushes or other type? Or some circuit in the AF controller schematics, e.g. some transistor that provides a voltage to the motor? Any other reason?

I have a spare broken MZ-7 for parts and a reasonable experience with fixing mechanical film cameras of many types, but the amount of electronics and esp. wirings inside these beasts and the service manual (a bit difficult to follow) looks too scary.

If someone have any ideas or experience with similar symptoms or their remedies or have seen detailed instructions online, I would be glad to know very much. Needless to say that sending it to service (which we lack here anyway) would be an unreasonable option.

Thanks in advance,
Jaroslav

04-09-2022, 08:50 AM   #2
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The AF motor drive train may need cleaning, lubrication and adjustment (CLA).

The factory service manual is available for download. Check with your favorite search engine.

I seem to recall with all the AF K-mount cameras including DSLRs t is recommended to disengage the AF before using the MF on the lens to prevent damage to the drive train.
04-09-2022, 10:08 AM   #3
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QuoteOriginally posted by Not a Number Quote
The AF motor drive train may need cleaning, lubrication and adjustment (CLA).

The factory service manual is available for download. Check with your favorite search engine.

I seem to recall with all the AF K-mount cameras including DSLRs t is recommended to disengage the AF before using the MF on the lens to prevent damage to the drive train.
Thanks, Not a Number, appreciate your reply. Yes, I wouldn't dare to turn the focus ring with AF engaged (not to strain AF gearing) in K-01 or K-30, or even in MZ-5 if bought new at the end of 1990s for a full price. But now it's rather a choice between not using it al all, as a half-faulty, or tinker a bit to get some info on a possible problem. But, agree, not recommended.

I have a service manual, and it, as well as my experience with disassembly of MZ-7, both indicate almost complete disassembly required to get to the AF motor and gears. Need to decide on a risk, may not assemble it back correctly.
04-09-2022, 10:33 AM   #4
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I would try a second lens to see if the symptoms are the same or maybe something in the lens. I think the MZ-5 is in the series of cameras with AF problems from a cracked plastic gear on the motor shaft. I would look for more information on that in the film camera section to see if that's true and the symptoms are similar. Even if it's not the gear, you may find a guide to swapping the AF motor that's more helpful than the service manual.

04-09-2022, 11:53 AM   #5
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I'm unfamiliar with the MZ series and too lazy to unearth my broken MZ-50 to check. If the AF sensor is in the mirror box as it is with DSLRs then it may just need cleaning.
04-09-2022, 12:45 PM   #6
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QuoteOriginally posted by Just1MoreDave Quote
I would try a second lens to see if the symptoms are the same or maybe something in the lens. I think the MZ-5 is in the series of cameras with AF problems from a cracked plastic gear on the motor shaft. I would look for more information on that in the film camera section to see if that's true and the symptoms are similar. Even if it's not the gear, you may find a guide to swapping the AF motor that's more helpful than the service manual.
Typically, the problems with another motor are referred to, the one responsible for a mirror, with its tooth wheel prone to cracking.

Yes, indeed looking for some online guide for disassembling it and getting to AF motor in a correct way, haven't found yet, most are on replacing the mirror one.

Tried also cleaning the AF module lens, as suggested my Not a Number, at least of a module located where the phase detect AF is in DSLR. Unfortunately, didn't help. There are also three other, square, holes closer to the film gate.

Will look further then.

Thanks for tips.
04-10-2022, 03:49 AM   #7
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I own a MZ-5 bought brand new at the time. It still works apart from the flash, the spring is broken. Never had any problem with any lens on it or with the AF adapter from the F lens series. So the big question: what lenses do you use when the AF does not seem to work right? Did you try an original FA lens from the same era? And what was the result?

04-10-2022, 04:56 AM   #8
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QuoteOriginally posted by AfterPentax Mark II Quote
I own a MZ-5 bought brand new at the time. It still works apart from the flash, the spring is broken. Never had any problem with any lens on it or with the AF adapter from the F lens series. So the big question: what lenses do you use when the AF does not seem to work right? Did you try an original FA lens from the same era? And what was the result?
The lens is it's native kit lens, SMC Pentax-FA f4-5.6 35-80mm, silver with plastic mount. Lens works fine when mounted on K-01 digital body. Other lens of F and FA series I've tried on MZ-5, work on it with the same problem, too.

My subjective feeling is something like a lack of momentum in AF module, either because of powering of a motor, a motor itself, hardened lubrication in gears or other reason. Why, because, when I turn it a fraction, it works for a few focusing attempts again. Turning a screwdrive head in a body is smooth and monotonous, without jumps, broken tooth or cracked tooth wheel seems unlikely (but I haven't open a camera yet to have a look).
04-10-2022, 08:08 AM   #9
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I'm with NAN in looking for something obstructing the autofocus optics. It may be necessary to have the mirror in the up position to see the autofocus unit.

https://learncamerarepair.com/product.php?product=730
04-11-2022, 06:36 AM   #10
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QuoteOriginally posted by pentasonic49 Quote
I'm with NAN in looking for something obstructing the autofocus optics. It may be necessary to have the mirror in the up position to see the autofocus unit.

Product Details | Pentax MZ-5 Service Manual | Pentax | Service Manuals | Learn Camera Repair
Thanks, of course, I lifted mirror up by a toothpick and examined the mirror chamber. Everything was carefully cleaned by alcohol with, unfortunately, no effect on the problem. However, due to the angle, it's impossible to see the AF lens from the front and you do this blindly, unless one has a small mirror on a long handle (like these used by dentists). Auxillary window under the main one, which reflects image on AF sensor, seems to get in its place OK and is clean. I can see on a currently disassembled MZ-7 that the AF motor wires are soldered to two contacts on the prism circuit. If MZ-5 has similar wiring, then one can get there by removing a top cover only and check power on a motor when focusing. Need to find whether this is a DC or stepping motor. If DC, then one can unsolder these and measure current it consumes when given DC (my guess is around 3V). Might give some more info in this way. The plan for next days after finding info on a motor type
04-11-2022, 03:52 PM   #11
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Might as well do the mirror gear while you're in there...
Unless it's been done already...

-Eric
06-13-2022, 12:16 PM   #12
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To close the case, here is an update.

It was a faulty AF motor. I used instructions from ipernity: ipernity: Pentax zx/mz mirror motor repair by 5cent to disassemble MZ-5. To access two screws holding AF block (both on a bottom side of a mirror box) it is better to unsolder a ribbon cable going all the way from the shutter and located in front of a prism. The mirror box then clears higher from a body. Examined extracted AF motor block by connecting it to 1.5V AAA battery. Sometimes it spinned, other times - no reaction. Turned a toothwheel on a block - spins again, then again not. The behavior exactly the same as when driven by a camera when attempting to focus. My guess - three coils inside, one is smoked or disconnected, no start torque at certain position of a rotor, but I may be wrong. Anyway, seems like a problem found.

Then disassembled donor MZ-7 and extracted the AF block taking care to unsolder a ribbon connection cable (seems like an opto-coupler, presumably to report impulses upon motor rotation to the processor) on the other side, where it is soldered to a main board. Checked with AAA battery - works perfectly. Replaced AF motor with some equilibristic required to hold the mirror box up hanging on ribbon cables... Resolder, resolder, resolder... Getting color cables out of the way here and there. Checked camera functions - AF and the rest works.

Now, quoting Eric:
"Might as well do the mirror gear while you're in there...
Unless it's been done already...
-Eric"

Good idea. But, having another, MZ-5N camera laying around with mirror up... An in the fixed MZ-5 the mirror still works. Decisions, decisions...

Decided to fix that another MX-5N still. Took that donor MZ-7 again and extracted mirror motor from it and, a strike of luck: the bronze toothwheel. Perhaps, previous owner did the replacement? And, unlucky him - then smoked the camera... That motor assembly saved the MZ-5N and now two cameras seem to work. So far...

Now, the question. Wanted to order a few bronze toothwheels from Mikro Antriebe just in case. But before ordering contacted them to check whether or not they operate, but still no answer yet. Have anybody ordered from Mikro Antriebe recently? The webpage works. But after I filled the delivery address including destination country and selected PayPal, its bill contained a string "false" instead of a country. A bit weird, not sure whether the toothwheels will reach me then.

Thanks for reading.

Last edited by Jaroslav; 06-13-2022 at 12:26 PM.
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