Pentax Spotmeter V Repair

Intro: 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.

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.

Comments

[Christophe_Rameau]

Hello Matt, I noticed in the drawing of the old spotmeter 1°/21°, that there is a 5 Watt resistor from the switch going to the ground if I'm not mistaken. Is this a 'pull down' resistor by any chance?

[mattthegamer463]

It's a current limiting resistor for the lamp. The old 1/21 used a bigger 1.5V cell but the LR44s in the Spotmeter V can't even supply more than 150-200mA in a dead short, so the 5W power rating is absurdly oversized.

This resistor is only to control the brightness of the bulb. Maybe the 1/21 used a 1.5v bulb and they wanted to dim it. Another option they could use would be to use a higher voltage bulb so that it would be naturally much dimmer by being undervolted.

I doubt this resistor has failed, if there is a similar or equivalent one in the V. There is no power available to burn it out, only physical damage or vibration could cause it to fail.

[Christophe_Rameau]

Yes I know, the 1/21 used even less, only 1.3v for the lamp. Unless it is in series with the 9v battery. But still.

[mattthegamer463]

Yes but that's not what the circuit shows in the image you posted. It shows it cannot connect to the 9v as they are on opposite sides of a SPDT switch.

This should not be hard to solve by tracking the resistance along the circuit using your meter.

[Christophe_Rameau]

Yes, concerning the voltage that is correct. But I'm confused in your response, do you mean the meter I have? Because that is Spotmeter V. And that is 4.5v.

[Christophe_Rameau]

Anyway, I'm still stuck in trying to replace the contact with a micro switch, because the available space under the casing is so limited that I don't really see an option. And one day I think I'm going to try to draw a diagram of what I see on the pcb's. What reminds that there is a man in the US who worked for Honeywell Photo Division and still repairs Pentax equipment, so maybe he has a repair manual of this meter, but I don't dare to ask him. It is a little bit odd. :-)

[Christophe_Rameau]

Hi, thank you for this project.
I have a spotmeter V but the 'illuminator'-light doesn't function anymore. So I want to replace it, but I wonder where it is located and which kind of light bulb this is (voltage for instance)? Any information on that?Thank you in advance.

[mattthegamer463]

Hi Christophe, I think it is a 6v bulb. I suspect the failure is the small button contacts. My own unit worked only sporatically. The bulb is under the scale, slightly above and behind the trigger.

It would probably be easier to replace with a LED, easy to run on 6v with a resistor.

[Christophe_Rameau]

Hello Matt, thank you for your reply, I will look into it.
Actually one more question: the 'fastening' ring with the tiny holes, does it work like a screw, does it has a screw thread? Because I'm looking for pliers, but I don't understand the principle.
Thanks again, Christophe.

[mattthegamer463]

It's a threaded ring, basically a large diameter smooth nut. I didn't have a lens wrench or snap ring pliers big enough so I showed a trick to undo it without being able to bridge both of the holes in the ring. If you have a lens wrench with pointed tips that would work well.

[Christophe_Rameau]

Hello Matt, a brief update. I've managed to open the spotmeter but I'm currently trying to figure out where the bulb is located, apparently I stll have to remove more parts?
And another issue: is it normal that this metal element, bridge like, doesn't make any contact?Or is this the 'ground' perhaps? Best Regards, Christophe.

[mattthegamer463]

Hi Christophe, you'll have to forgive me, I wrote this 7 years ago and haven't opened the spotmeter again since. Looking at my photos, my best guess is that is a switch used for something during factory calibration. It doesn't appear to serve a purpose in the final assembled device.

I'm not exactly sure where the bulb is because I never dismantled it beyond what you see in the photos. I think the bulb wires are the twisted pair of yellow wires that run from the bulb button contact patch up to near the top. I would check the resistance across those two wires, you should expect a couple hundred ohms if the bulb is good, and infinity if it is burned out.

[Christophe_Rameau]

Hi Matt, no problem, I understand. You mean, beneath the copper circle under the top?

[mattthegamer463]

I think so. That kind of looks like the bottom of a light bulb with the centre pin soldered to that wire. Test the resistance before dismantling as the bulb may be fine and your button contacts may just need adjustment or something. The battery power is so low it would be hard to burn out that bulb, I would think.

[Christophe_Rameau]

Hello Matt, I disassembled the socket and soldered two new wires on and put it on a battery pack of 4.5 volt, and the light bulb is still going strong apparently ,see image. But a reading of the contacts is dead in the water, so it is the dreaded scenario that there's something wrong with the contact connecting to the battery, because my light meter gives a reading, so that can't be the problem. The question is: where does the contact connects to the battery and how to access it, because the bottom plate doesn't seem to move. But I believe you already encountered that problem?

[mattthegamer463]

I think you will need to use a volt meter to trace the voltage from the button contact area to the bulb, to see what is going on. You should be able to see where the battery + connection comes in on the long PCB that is near the bottom of the meter, and follow the traces and wires to the button contacts, and see what is going on using the volt meter.

[Christophe_Rameau]

Yes, I think that is the trouble here. I see a red wire going in the device towards the battery (when I point a flashlight in the inside) and I see two yellow wires connected with the contacts what is rather odd, because on the drawing in the repair manual that I've found someplace online of a previous model of this meter, the Honeywell 1/21° , I see one of the contact wires going inwards to the battery instead. And I'm far from an expert in electronics but this seems strange.Shouldn't a contact button have the same polarity? Because it breaks the circuit on one side of the circuit if I'm not mistaken?

[mattthegamer463]

Not necessarily, those connections shown in the older model repair manual may be a more faithful representation of the circuit, but the two yellow wires going to that circuit board are probably an equivalent circuit, done in a way that makes more sense from a manufacturing perspective. the wires are the same colour because the switch is not polarity-sensitive, so reversing their position doesn't impact the operation. I think you'll find one of those wires is connected to the bulb, the other to the battery. a resistance meter will help you find the connections without manually tracing by eye. I suspect the circuit is just battery -> switch -> bulb -> back to battery.

[Christophe_Rameau]

Ok, I'm close to a solution by pure coincidence. It seems that the 'receiving' (left)part of the contact doesn't let through current, or perhaps the wiring (or some other element) doesn't function properly. If I hook up the right part of the contact straight to the light bulb, it works. So luckily it is not the connection from the battery with the contact. :-)

[mattthegamer463]

Maybe the bottom side of the contact is corroded and needs cleaning.