Repairing Sangamo Weston Master V

The old Sangamo Weston Master light meters seem to have a bit of a cult following with the most desirable being the Weston Master V and the Euro-Master (which is still being made!). These are now 40/50 years old most of them are still working fine. All versions of the meter use a selenium cell so no batteries are needed and they seem to stay in reasonable calibration. I was given a Weston Master II in which the meter mechanism was beyond repair but it seemed like such a cool little device I bough a Weston Master V second hand. This turned out to be broken too so I decided to take it apart to have a look. To take these meters apart you need some fine needle-nose pliers and some small side cutters (for gripping the odd shaped screw heads). Firstly you need to unscrew the two hex shaped screws that hold the alloy plate to the back. Once this is off the little door on the back for high/low range comes off. Next unscrew the knurled part in the middle of the dial on the front. Now you can access the two nuts that hold the front on. Once the front it off you can see how everything works.

The cell is sandwiched between a circular copper contact (attached to the single flexible wire you see running under the meter mechanism) and a large sprung metal contact. A stiff wire that runs from near the upper range limiter on the meter to near the scale mechanism completes the circuit. A sprung bar presses on the needle to prevent it moving until you press the button on the side. In the case of my meter the cell had died. To get the cell out you have to take the whole thing apart which is easier than it sounds. I forgot to take any pictures at this point but you simply remove the meter mechanism then keep unscrewing the odd nut/bolt pieces (remembering the order in which everything came out) until you get to the cell. I put in the cell out of my dead Weston Master II and this seemed to work fine, I also cleaned all the contacts etc. The calibration seems pretty good after zeroing it. Most of the internal nuts were loose when I removed them - this seem to be a common problem that stops the meters working. To complete the circuit from the sprung metal contact on the cell to the meter mechanism relies on 2 bolts being on tight. Putting it back together is pretty easy apart from locating the ends of the springs from the door on the back into the holes in the moving part of the scale mechanism.