Pentax 6 x 7 curtain not matching shutter

[chris_autio]

Have sent my camera twice now to get overhauled, but now my exposures from left to right are uneven. If there is a quick fix let me know.

Beginning (ever so slightly) are uneven exposures at 1/125, pronounced at 1/250 and 1/500 and not at all 1/1000 on most of frame. file shows 1/1000

[c_watson1]

IIRC, these shutters were electronically--not mechanically--timed. That means no amount of CLA or "service" will remedy an inaccurate shutter speed.

[tom_chow]

13 hours ago, c_watson1 said:

IIRC, these shutters were electronically--not mechanically--timed. That means no amount of CLA or "service" will remedy an inaccurate shutter speed.

That's not quite accurate, the timing is electronic, but the shutter curtain springs and travel is purely mechanical.

The shutter is capping, that means the second curtain is catching up to the first curtain during the exposure. So either the spring tension on the first curtain is low, or there is something obstructing/dragging on the 1st curtain that is slowing it down. Since you said it was previously serviced (and I assume it was returned working), there is something going on with the mechanism. I'd suggest that a "proper" CLA should fix it. The timing system seems fine, as the exposure starts correctly.

[c_watson1]

Good luck finding a "proper" CLA for this relic in 2023. The testing required to bring it back to spec is laborious and not cheap. Had it done on a 6x7 20+ years ago by Pentax Canada.

[tom_chow]

12 hours ago, c_watson1 said:

Good luck finding a "proper" CLA for this relic in 2023. The testing required to bring it back to spec is laborious and not cheap. Had it done on a 6x7 20+ years ago by Pentax Canada.

Yes, and that's true for any of the classic cameras from that age. Far fewer P67 were sold than Hasselbalds, and the P67 typically needed less servicing, so the end result is fewer part and service techs left over.

The P67 mechanism is actually pretty similar to most 35mm SLRs, same shutter mechanism and springs, just bigger. The main problem is parts, particularly the shutter solenoids. If I take one apart and the solenoid gets damaged, there's no replacement. No service technician wants that. The mechanical CLA is pretty strait forward, but there's always the chance that, without parts, you can't return it working.

Since the timing circuit is currently working, it's the shutter curtain travel that need adjustment, and that can be done with a CRT or scope, you are just adjusting the main springs through the bottom plate. But something made this shutter to start capping, and if the main spring is going, or fraying ribbon, then you'd probably want that replaced - which is quite a lot more work, and a difficult part to get. Or it could just need lube on the curtain spindle.

[orsetto]

The Pentax 6x7 series is known to be a bit fiddly and tricky to service in some respects. This is true of nearly all medium format system cameras, of course: they're all practically bespoke designs compared to mass-produced 35mm systems. But each MF system has its own spin on the Achilles Heel. For the big Pentax, its the shutter mechanism, which was apparently always a work-in-progress for the company engineers. 

Assuming you have had other successful dealings employing the same repair tech for other cameras, he might simply not be completely capable of fully overhauling a Pentax 6x7. This can happen with any local or non-specialist tech: they mean well, but they can screw up with low-volume cameras that rarely cross their workbench. You balance their lower fees against that risk, and sometimes lose. Here in NYC, I had a local guy do most of my Hasselblad and Mamiya servicing before he retired: he'd been in business 40 years, had the certs from Hasselblad and Mamiya on his shop wall, etc. Yet he would still goof up an arcane repair occasionally: if he couldn't correct it on a second try, he'd return some of the fee and I'd ship the item off to one of the legendary specialists who charge double to triple the going rate.

While you can get away with this kind of dice roll with common mechanically-based cameras like Hasselblad, you can't with the much less common Pentax 6x7. Far fewer techs have picked up the expertise to tune them properly, so you really do need to send them straight off to a Pentax 6x7 specialist. The big Pentax employs fairly primitive electronics on top of a slightly wonky shutter mechanism. When it develops the capping problem that yours has, it can be any one (or a combination of) three common causes: accumulated grime in the shutter tracks, drifted spring tension, or bad solenoids. Most generic techs can handle routine cleaning, but (as tom_chow notes above) they can easily misalign other sensitive parts in the course of it if they aren't well-experienced with the 6x7.

Adjustment of the 6x7 shutter is a fine art, often requiring repeated minute mechanical and electrical syncing. This goes beyond just futzing with the curtain timing springs, a quick-n-dirty hack that works for many other cameras. Theres a tech-savvy geek over on the PentaxForums who bought a beater 6x7 body and documented his DIY rebuild project: his photos and descriptions of the process were enough to send me running for the liquor cabinet. 

As with every other medium format system, there is only one elderly tech in all of North America who is well-regarded as a Pentax 6x7 guru with a supply of spare parts. Try Eric Hendrickson: his website is still up, so he may still be taking repair work (email him and ask). Otherwise, check with a few of the top generalist repair firms that receive high ratings on forums like this: get confirmation from them via email that they are indeed skilled and equipped to properly evaluate and overhaul a Pentax 6x7 shutter.  Good luck, a well-working Pentax 6x7 is a wonderful beast to shoot with!

Edited June 21, 2023 by orsetto