Introduction: Pentax Spotmeter V Repair
I recently purchased an analog Pentax Spotmeter V and found out it didn`t work once I got home to test it. Reading up on them, I found that a lot of people were having a hard time fixing them and that the usual response was to send them in to have them repaired professionally. I couldn`t find any good information or even pictures inside one of these online so I decided I should document my exploratory disassembly and repair to help others fix their meters.
My issue turned out to be a corroded PCB trace and a broken wire. The wire from the top of the battery holder broke free and so the meter wasn`t getting any power. This spot isn`t really accessible to resolder the wire so I`ll show you what I did to get around this issue.
Step 1: Disassembly
To get the grey back of the unit off, only one screw and a ring must be removed. The screw is behind the serial number plate on the back of the hand grip, and the ring is behind the eyepiece adjustment ring. Pry off the serial plate with a small screwdriver to access the screw. Adjust the eyepiece ring all the way out (counter-clockwise) and then continue to gently turn it until the thread loosens. The ring will come off, revealing an aluminum ring with two holes in it. Use a retaining ring removal tool to get this off. If your tool can`t reach (like mine) try using the trick I show in the picture with some nuts or washers and a piece of paper. Watch that you don`t scratch the lens or tear up the holes. The ring is only aluminum so if you don`t have the tool far enough in the hole it can tear out.
Step 2: Inside the Meter
See the images of the inside of the meter for your reference. Note the green corroded trace on the main circuit board. It had rusted right off in my case and required a jumper wire to bridge the entire trace. The PCB is uncoated so this could happen to any of the traces. I used a heavier orange wire to do the repair so keep in mind those won`t be present inside your unit.
There are 3 PCBs, one on the back of the head (for the silicon diode) one near the trigger buttons, and one running the length of the grip with an op amp and some calibration potentiometers.
The bottom plate on the meter is held on by a strange threaded sleeve which is almost impossible to remove. Mine had threadlocker on it and it looks like a special tool was used to put it in place originally, so I gave up on trying to remove it. To replace the positive battery connection I took a 1/16th drill bit and drilled through the fibre insulated top of the battery holder, then soldered a loop of wire and threaded it up through the hole and into the body of the meter. I then wrapped the loop around the spring at the top. I trimmed the wire and soldered it to the PCB pad where the old wire went.
I don`t know how to calibrate this meter but there are several potentiometers inside, one on the back of the silicon diode PCB probably adjusts the sensitivity of the diode, or possibly compensates for the diode dark current. The ones on the op amp board probably adjust the DC bias and gain of the op amp circuit, adjusting the range the meter shows. The meter itself is a simple coil-based analog gauge which deflects a needle by a varying amount depending on the amount of current passing through the coil.
I hope this guide helps someone repair their Spotmeter V so these great devices can get back into service. I`ve only had mine a short time and I am already very happy with it for the extremely reasonable price I paid for it versus a modern digital spot meter. Feel free to ask me any questions and I will try my best to help.