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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.

Conclusion

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.

<p>Matt,</p><p>Have you run across any information on how the meter can be (re)calibrated? At least until fairly recently, spot meters I've looked at have three adjustment potentiometers. Although I have experimented with these, I have not been very successful making one meter match a second (same brand and model). One adjustment pretty clearly manipulates low sensitivity, but I haven't settled on how to manipulate the other two. High value(s) and midpoint? I suspect that this will suddenly become blazingly obvious to me, but I haven't yet had the Ah Ha! moment. :-) </p>
<p>I attached some PDFs I found, might be useful for you.</p>
<p>Interesting, but I think Cds cells might be quite different in the adjustments they require. My meter uses a silicon cell (Soligor digital). My favourite spot meter is the little Pentax Digital as modified by Zone VI. </p>
<p>Looking at my pictures, I count 4 potentiometers total. I imagine there could be high/low sensitivity or limit setting pots, but I suspect the one near the metal can (which I believe is an operational amplifier) could be a gain-adjust. I suggest tuning that one until you have linearity in readings, then adjust the others to try to get the right values at high and low EVs. If this is just for you and your linearity is fine but the reading is simply off, maybe identify the error and mark it on the unit so you can do quick math in your head? ie. adding 1/2 an EV to each reading to correct for the error.</p>
<p>Thanks. The meters I'm working with are Soligor digital. On the side there is a small label that peels off to reveal three potentiometers. (There is a 4th space, but it is empty.) If the engineers were reasonable, the two end ones would be high and low limits (left-to-right? right-to-left? Easy enough to find out) and the middle one would do something with linearity.</p><p>Ideally, there would be factory tuned electronics for the specific photocell, leaving only low or high point plus slope to adjust. Of even better, leaving only low and high points adjustable, the slope being inherently linear or rendered linear by electronics.</p>
<p>Or just adjust the ISO to compensate.</p>
Do you know any links that shows the optic parts of a spotmeter in detail? I want to know where is the light sensor and how it's covered to see a specific angle of view
I haven't been able to find any. Nobody apparently has ever taken apart a broken one and posted pictures. I can tell you how this one works, its very simple. It just focuses an image from the front lens onto a 45 degree mirror with a hole in the silvering in the centre, the mirror reflects the majority of the scene into a prism and ultimately to the viewfinder, but the middle hole allows the middle 1 degree of light to pass through to a photodiode.<br><br>I can still see an image inside that middle 1 degree spot though, so I don't know how they manage to do that.
<p>Thank you very much for your information, i was thinking about making a hole in center of mirror, so you say that they just removed the silver(or whatever) coating in the middle? it can be a semi transparent mirror also, and a tube to control the angle of measurement. can you say approximately the diameter of the hole? Thanks again </p>
<p>The pentax spotmeter V uses a standard SLR pentaprism and full frame ground glass for the viewfinder. Optically it is almost identical to a SLR. The difference is the hole in the mirror and the placement of the photodiode. The hole is probably 3/16&quot; diameter but the photodiode is quite large, its window is probably 1/8&quot; around. </p><p>I built a spotmeter which has very good rejection of off-angle light using just a simple laser pointer lens and a tiny light sensor placed at the focal point behind the lens, where the laser used to go. It works great, 1.5 degree tightness and very accurate. When I aim it at a light source it drops off very quickly, as well as this Pentax works.</p>
Thanks, i want to make one with tsl2561 lux meter, trying to collect more information. Thanks, it helped a lot. But i got confused about the hole, is it a hole through the glass or just removing the coating?
<p>Just the coating</p>
<p>Just the coating</p>
And another question about the lens,<br>does the lens of these kind of spotmeters have focusing ring or it's focused on infinity?<br>how is the depth of field and aperture of the lens.<br>Regards<br>
<p>Its wide open, no focus control. It's a very simple lens since it isn't intended to make a high quality image so it is imprecise about image quality, just getting light to where it needs to be for the measurement. No clue about DOF, seems to be fine at infinity even for measurements of only a few feet.</p>
<p>Matt: Thanks for posting this. I have four of these. Two have dead bright light circuits but the low light circuits work just fine. I use a variable ND filter to stop the low light circuit down to bright light range and calibrate it against a reliable new digital meter. That works fine but the I have one that works intermittently on the bright light circuit... maddening! This post will help me get over the qualms I have about opening up the good meter. Oh, the fourth meter? it's a basket case. I've been using it to learn on. I know how to get into the Spot Meter V, I just didn't want to screw up the one I have that works... mostly. thanks again.</p>
<p>So, I helped you? I think.</p><p>I recently got a second one and it has a 9V for the low circuit and a coin cell for the high, I believe. It seemed like an odd setup. I'm not sure if its older or newer than the one shown here. I'm debating about which one I want to keep.</p>
<p>Nice. This kind of project is great. It might not get thousands of views, but when someone goes looking for this kind of help, now they will find it. Very glad you shared this!</p>
That was my intention. Glad you liked it.

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