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01-23-2015, 03:04 PM   #1
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MX meter stopped working

Hi, my first time here, in need of some help. I've acquired a black Pentax MX which looks to be in good condition. I cleaned out the battery cover and put a new pair of cells in, and shot a roll of film just fine, seems to meter under about a top, but pretty happy with it. A few months later I tried it again with no film and the meter LEDs are dead. I think there was a flicker of life then nothing. Tried new cells, checked the switch under the bottom cover (on the end of the shutter button), cleaned the battery cover again (getting 3.1V showing between + and earth). The + voltage gets to that switch and back across. I opened up the top cover but I couldn't see anything amiss. I've got a service manual but really it just tells you how to strip it into a million bits then put it back together, not really fix it. So, any thoughts? Not an easy one I know, to start on! It's a lovely camera, I really want to get it going again. Thanks, John

01-23-2015, 07:28 PM   #2
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Send it to Eric Hendrickson for repair and a cleaning / lubrication / adjustment (i.e., a "CLA"). He fixes MX meters all the time. He will give your MX a glorious new life. The camera is definitely worth it.
01-24-2015, 12:25 AM   #3
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Another possibility. If you don't lock the shutter and store the camera in a way that puts pressure on the shutter button you can drain the batteries.
01-24-2015, 07:18 AM   #4
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Forgot to say I'm in the UK, so postage to the US will be a bit pricey! Thanks anyway. The batteries are new, and I checked the voltage in the camera (3.0-3.1V) to be sure it wasn't something like the battery cap being corroded. As I said I checked out the shutter switch (assuming there is only one - the one in the underside of the camera) and it opens and closes fine. I also tried leaving it switched on today for an hour, but that didn't seem to do anything either. It's mindless optimism I know but worth a shot.

There was one switch I couldn't find, referred to in the manual, and the details on wiring are very sketchy. I did however find the other end of its wire, and I earthed this with a flying lead to replicate the switch operating, but this didn't help either.

01-24-2015, 07:50 AM   #5
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See the other thread on this for quite a few suggestions. This is not an unusual problem with this body.
01-25-2015, 03:29 AM   #6
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So it seems, however I spanked the search facility and tried my own investigations before posting as it isn't an easy one. I may have to bite the bullet in the end and pay someone to fix it but I'd rather try to understand things first.
01-25-2015, 07:24 AM   #7
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QuoteOriginally posted by Johnny Rod Quote
So it seems, however I spanked the search facility and tried my own investigations before posting as it isn't an easy one. I may have to bite the bullet in the end and pay someone to fix it but I'd rather try to understand things first.
Check to see if you have the 3 volt supply on the PCB at the top of the camera (make sure it's turned on), also check the connections and resistance of the ASA control (J300) and the lens aperture resistor (J400). If they are all ok then it's probably a fault on the PCB.

Here is a slightly better wiring diagram than is shown in the service manual.


Last edited by Ray-uk; 01-25-2015 at 09:29 AM. Reason: diagram added
01-25-2015, 10:06 AM   #8
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The UK equivalent of Eric is Robin at Harrow Technical. K.

01-25-2015, 02:28 PM   #9
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Nice oen Ray, will check it out. Any idea what the physical location of O-J532 is? I tried shorting out the terminal of its blue wire from the main PCB on the top to simulate it closing, but it made no odds, so it may be a moot point. I could only see the wire disappearing into the front of the camera somewhere and didn't want to get into peeling the leatherette off.
01-26-2015, 05:30 AM   #10
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O-J532 is on the mirror box, see Fig 2 of service manual. The purpose of this switch is to turn off the LEDs when the mirror is up.
You don't normally notice the LEDs turning off during short exposures but you can see it easily with the shutter on B, the idea is that it stops the light from the LEDs from fogging the film, obviously important during long exposures.
If this switch is stuck in the closed position then it would turn of the LEDs permanently giving the impression that the metering is not working (could be your problem).
The mirror box mechanism can be prone to problems because of bits of old sticky foam falling off the prism housing.
Obviously shorting this switch as you have done will not help to find the fault and you will need to disconnect it, probably easiest on the top PCB to determine if it is the problem before you start digging further into the mechanism.
01-26-2015, 01:53 PM   #11
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Ah many thanks, the service manual is a random collection of info but is quite esoteric. I can check for continuity to earth in case it is jammed closed when I have the top off next; could be a while though. Will report back if I am successful!
01-29-2015, 03:17 PM   #12
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Kris - I forgot to say thanks.

Ray, you're a star, it's fixed. After all that it was the switch under the shutter button. I swear I was getting continuity across it when I started the fault-finding but maybe it was intermittent, anyway I went looking for voltage under the top cover (aka inside Terminator's skull, as it looks when in bits) and this led me back to the switch again. I certainly looked by eye to be closing too, but the ohmmeter said no. Adjusted now and LEDs back on. I checked the ASA and aperture resistors too and they are fine, unsurprisingly. So many thanks, I can stop worrying about this one now! It comes under two of my fault finding rules which I should have followed: never miss the obvious, and always double check if you're getting nowhere.
01-30-2015, 02:54 AM   #13
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HaHa, I'm sure we have all done things like that.
I remember the time I installed a brand new electric cooker, got it all connected, checked it out and everything worked except the oven. I took the back off traced the wiring, couldn't find any problem.
Took the front control panel off, traced all the wiring through and then it hit me, the ovens time clock was in the off position ...... a lesson that has never been forgotten.

Anyway, pleased to hear that it's all working, another MX saved from the scrap heap.
01-30-2015, 03:05 AM   #14
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I forgot to say, there is a really easy way to check the switch, as I now know, without any disassembly. you can use a multimeter (or 3V bulb) on the connector under the camera - mine has the combined one pictured in the bottom right of your diagram. The stud closest to the battery compartment is the battery + - you can check for 3V between this and the battery compartment cover. The other side of the switch is the third stud - if the switch is working then you will see 3V between this and the battery compartment cover.

In a fit of pique I even had a go at the rangefinder in one of my other cameras. Still got the really tricky one - a different one needs all lens elements cleaning and the shutter sticks a little. Might save that for after Dryathlon (dry January)!
01-31-2015, 10:03 AM   #15
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That's a useful tip, I can see that would also check the motordrive switch as well.

Dry January ?? ...... is that a new years resolution or have you just bought a new umbrella.
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