Canon A-1: Repair and overhaul Finale
Hello friends of tiny screws and frustration tolerance,
EDIT: Deleted and reposted since a picture was not displaying right
here is finally the third and last update to the A-1 restoration project, my first experience with camera repair and CLA.
Last time I left you with the completed CLA and the lessons I learned along the way. This time I will address the two leftover tasks/problems:
- The viewfinder display being extremely dim (almost invisible to the naked eye), and was missing the G segment on all the individual displays.
- Since disassembling the camera implies pulling the leatherette, I wanted to re-leather the camera in a colour. After all the work done, I didn't want it to look like a stock A-1 :)
To start tackling the viewfinder display problem, I first resorted to the repair manual. The 7-segment miniature displays are controlled by a BCD decoder/driver IC, which in this case is a custom part from Canon. The only advice from the manual to change the brightness of the display is to modify a resistor, RIADJ, which configures the internal constant current source of the IC. Since this is a carbon resistor, I suspected (and hoped) that it had drifted upwards with age, but that was not the case. The resistor is an E96-series 3kOhm, and measured almost bang on. I also tested a 47uF decoupling tantalum capacitor that is right next to the IC, but the capacitance too was well withing tolerance. Bummer!
The BCD decoder IC also provides a trigger function for multiplexing not only the displays, but also several buttons and inputs to the camera. To prevent backcurrents during multiplexing, a large diode array is used. However, a failure of a diode in the array would not explain the missing G segment, since they control if a whole display lights up and not the individual segments, which is controlled by the BCD decoder itself. The probable cause would then be an output stage transistor in the IC not working.
Nevertheless, I wanted to experiment before I gave up with that intensity resistor. I replaced RIADJ with a 5k trimmer to vary the resistance and see how it affects the brightness. So, with a half-pressed shutter pushrod and a very uncomfortable position I looked through the viewfinder. Imagine how underwhelming it was to see absolutely nothing change when varying the resistance! This was enough to determine that the IC was at fault.
An interesting observation is that, when I tried the trimmer with the long cables I show in the picture, the camera acted weird. I tried firing the camera, but the curtains would hang often and randomly. Only after I shortened the wires it started acting normal again.
Luckily, the display unit is a flex PCB module which contains the display, the IC and the resistor and capacitor and can be easily exchanged. Although the module is mounted through press-fit connectors, desoldering some wires is necessary to remove it. I got a replacement module from eBay, which gave me a nice, bright display again, and a living G segment.
To crown this project, I decided to get some fake leather and make the camera look a bit different. I edited a picture of the camera to cut off the original leather and replaced it with pictures of different self-adhesive leathers I found online to see which one I liked the most. After bothering all of my friends for their opinion, I decided to go with a burgundy one I found in Amazon. It turned out to be quite thick (almost a mm), while the original leatherette of the A-1 is around 0.5mm thick. Nevertheless, I drew the cut template by scanning the original leatherette in a flatbed scanner and drew it in CAD. Then cut the leatherette with a laser cutter, painted the cut edges black with an edding and glued it on the camera. All in all it looks quite good, and the thickness difference is not that noticeable in my opinion.
Thank you for reading! It was a cool adventure. I am definitely hooked on camera repair now!