Replacing the mirror return motor in a Pentax MZ-5n
A while ago I bought a nice clean Pentax MZ-5n 35mm SLR body for £18 from ebay which was described as being ‘fully functional’. Unfortunately when I received it it turned out to have a problem with the mirror return motor which stops the shutter from working. I did some Internet searching and found that this is a common fault with all the Pentax MZ series – the motor which returns the mirror has a small plastic cog which splits and won’t drive the mirror anymore.
These pictures are also available in full size here
Obviously the first thing I did was try to get hold of the seller and return the camera but, strangely, I didn’t receive any response from him. After a couple of attempts I considered raising a resolution case with ebay, but in my experience this is not really worth it for £18. I would be required to return the camera via a tracked and signed for postal service, and although I would get my £18 back, I wouldn’t be reimbursed for the return postage so I would end up paying about £7 to have nothing.
So, I put that down to experience and a while later found another MZ-5n with a Sigma 24-70mm auto focus zoom for about the same price and bought that instead. Again the camera was described as fully functional and this time was supplied with new batteries so I felt there was a good chance it would be ok.
Once I’d received the camera and turned it on it looked ok – the autofocus was working, the metering was responding to light and the viewfinder display looked fine. However, as I pressed the shutter the camera fired, the viewfinder went black and a whirring in the body of the camera told me that this one had gone the same way as the last. The mirror was locked up and wouldn’t come back down.
Again I contacted the seller asked him about a return. In this instance I was more successful and this seller would accept the camera back, but I’d actually be in the same situation as before because I’d have to pay for the return postage and I’d end up out of pocket with nothing to show for it. In this case, since the lens was quite good and useful I decided I would keep the camera and see if I could find a replacement motor to try to get one good camera out of my two faulty MZ-5n bodies.
Replacement motors are actually not easy to find these days and the ones I could locate from past eBay sales seem to cost around the same price that I paid for the cameras, so in the end I decided to buy another camera body to act as a donor. I know any motor I fit which I’ve pulled from another camera has the potential to fail at any time, but I thought it would be interesting to see how easy or difficult the process would be. Fortunately the motor was fitted to all the MZ series (or possibly unfortunately since it seems to be a failure point) so I didn’t have to get another 5n to cannibalize, and I found a nice working MZ-50 body for £3 to supply the part (interestingly this was also described as fully functional and had a fault with the pop-up flash spring – another common fault).
The first thing I had to do was remove the motor from the donor body, and in doing that I basically destroyed the MZ50. Because I knew I wouldn’t be reassembling this body I didn’t take too much care and the ribbon cables on the bottom of the camera tore as I removed the bottom metal plate. There was so little room to maneuver the plate that it was just impossible to get it out without damage. Although that didn’t matter much with the MZ-50, I knew I was going to have to be a lot more careful disassembling the MZ-5n to have a chance of fitting the new motor.
Fortunately, with the replacement motor out of the MZ-50, I found a really great set of photos detailing the job from another photographer who had done the same job before me on ipernity, so I decided to follow those guidelines and rely on the fact that I had two bodies to help me if I came to a situation where I didn’t know how something fitted together in re-assembly.
These are my notes from the dismantling and reassembly of this quite complex camera:
When removing the metal bottom plate I found it is possible to get some more slack in the ribbon cable by removing the plastic surround from the data back contacts and push the exposed ribbon cable into the body of the camera. This is detailed in the pictures I followed so I was glad I’d followed the method I’ve linked to above because without doing that I don’t think the plate would come out.
I got through to the point of disassembly of the top printed circuit without too much incident, but when it came to de-soldering the pins which keep the top board away from the bottom (flash) board I had trouble. It’s important to make sure all the solder is removed from the top board so that it can freely lift from the pins and I used a de-soldering pump to extract the molten solder. I found that the suction of the pump was too much however, and it actually removed the pin from the board! At that point I had to stop and get some de-soldering braid instead. With the braid I could continue and remove the solder from all four pins and lift the top board.
I found it convenient at this point to tape the board up so that it remained out of the way whilst I removed the flash board.
The next problem point was removing the flash board. Although it was easy enough to get the screws out and de-solder the wires I had trouble removing the board because the two large capacitors under the board had a printed cable stuck to them with double sided tape and I was worried it would break as I pulled it away. Fortunately just removing the sticky with some tweezers seemed to do the trick and I could free it.
The most difficult part of all was the loosening of the mirror box and getting the actual mirror motor free. It was tricky getting the mirror box free enough to get to the screws to release the motor panel, but when I eventually did that I found that the plastic cogs which drive the mirror are only held in place by the motor panel being attached. Once I removed it and started to replace the motor I found cogs falling out of the box! Fortunately I was able to fit them back and then took a picture of the orientation to try to ensure I could refit everything if necessary.
One thing that was good at this point was that I was able to confirm that the problem was due to the cracked cog. I found on the motor fitted that I could move the cog freely off the spindle so it wouldn’t drive the cogs if there was any resistance so I was fairly sure I would fix the camera if I could get it all back together.
The re-assembly is the reverse of the dismantling and I found it went reasonably easily. There are a few hairy moments when you need to bend the printed wiring but all in all it went back together quite quickly.
So what were the results? Well mixed.
When I inserted the batteries I was pleased to hear the camera whirred and sprang to life and the mirror flipped down, so the mirror motor problem is fixed but when I tried pressing the shutter the camera won’t actually fire the shutter. At first I wondered if it could just be faulty, but after a bit of investigation I found that the shutter blades are actually held by the panorama blinds that are fitted to the MZ-5n.
This is obviously a problem I have caused during re-assembly. I’ve managed to get the blinds caught up with the shutter blades so they can’t move. I also can’t move the blinds because the switch on the back panel which moves them out of the way is stuck. So I need to take the camera all apart, loosen the mirror box again and get it re-seated properly. Although it’s not a job I’m looking forward to it shouldn’t take as long as the first time I did it and I always have the option to fix my other MZ-5n if I completely fail to get it working.
One other thing – I found a company in Poland that will make cogs to order for a very reasonable 3 Euros each so I think I’m going to have a few sent to me so I can fix the motor I’ve taken out of my MZ-5n and then work through my other MZ series cameras and get them fixed before they fail.
All in all it’s a fiddly repair job mostly because a lot of delicate parts need to be held in odd positions whilst you work on the camera and it’s time consuming, but I wouldn’t say it’s difficult – at least it didn’t appear to be too difficult but balance that with the fact that I re-assembled it incorrectly! Possibly I’ll have a different view once I’ve fixed the shutter.
After writing this piece I placed an order for 5 of the new cogs so I can replace the split cog on the motor I removed from the MZ-5n and use that to fix my other MZ-5n. I decided to follow that approach because the flexible wiring gets more flimsy as the camera is worked on and I don’t want to get it all apart again and end up breaking it.
My plan is to fix the other MZ-5n and make sure the shutter can move before I put it all together, then use the motor from that unit to fix an MZ-10 I have with the same issue. Hopefully it will work because I have an MZ-3 on it’s way to me and I don’t know if that will also need the ‘treatment’.
The cogs I ordered are actually made of a white plastic material rather than brass which was also an available option. That may seem an odd choice considering the fact that the original cog is plastic and it splits but the reasons were:
- the new cog will mesh against existing plastic cogs
- the existing cog is only failing after 10 years use
- I can push the new cog over the motor shaft but I’m not sure that I would be able to fit a brass cog to the shaft
The new cogs turned up only a few days after I ordered them and look like a very close matched replacement. I’m going to try fixing my other MZ-5n followed by the MZ-10.