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Jonathan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, August 03, 2005 - 05:39 pm: | 
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Hi
I'd like to get my Pen FT's meter working properly. It's spot on in sunlight but about four stops down in low light - eg. wrong when I might actually need it.
I've located the little circuit with resistors under the top cover, but can't quite figure out where the wires lead to. Hopefully I can change the resistors to recalibrate the meter. I did wonder about making a tiny SMT replacement circuit, but that's probably a silly fantasy, especially since I haven't even worked out the existing circuit.
Can anyone describe the circuit or send a diagram?
Thanks,
Jonathan |
Tony Duell
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 12, 2005 - 04:40 am: |
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I had some spare time, so I took my Pen FT apart and extracted the meter. First odd thing, the LDR (CdS Cell) plate, under the semi-slivered mirror, has 3 connections to it. It's tapped. The rear preset is in parallel with the main part of the LDR, the front one is in series with the meter coil.
It appears the rear preset has more effect in low light levels, the front one is the high light level adjustment.
Anyway, I have produced a text file containing the notes I made when removing the meter assembly, and an ascii-art circuit diagram. I'd be happy to send this to anyone that needs it, or who wants to put it on a suitable web site (here?) |
Jonathan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 12, 2005 - 06:07 am: |
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Good grief! Are you the same Tony Duell as the one on the HP museum forum??? If so, thanks for the advice about CD-roms back in May.
Wow, that's amazing, thanks very much Tony. The diagram is very helpful. I wasn't brave enough to disassemble my Pen that much, and anyway I value mechanisms more than circuits.
It appears that the rear preset corresponds to what I've heard called a linearity adjustment, and the front to the overall sensitivity adjustment. It wouldn't be surprising that most cameras used standard circuits, but I didn't know the standard circuit might be.
It'll take some fiddling to get the meter back in line. Maybe I'll wire some pots in temporarily...
Is your camera back together? Good luck!
Jonathan |
Tony Duell
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 12, 2005 - 12:33 pm: |
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To answer your questions...
Yes, I'm the same 'Tony Duell' that appears on the hp museum forum, and for that matter on the classic computers list. I tinker with all sorts of machinery :-)
I grew up taking things apart (and often getting them back together). Mechanical cameras are not much worse than some of the other things I've worked on over the years. I do keep away from electronic cameras, though (for all I have a lot of rather high-end electronic test gear), too many custom parts to get very far.
I've not worked on enough cameras to know the standard circuits yet. But heck, the Pen FT meter only has 5 components in it (meter movement, mercury cell, LDR plate, 2 presets), compared to many of the things I work on (HP computers with 500 chips and no offical schematics!), it's simple. The main problem, as you discovered, was getting to it. What you didn't get in the notes (and anyone else who wants them, just e-mail me) was the 'false starts', like removing the meter module screws before removing the shutter speed dial. The meter seemed to 'hang up' on something, I correctly guessed that was the coupling to the speed dial assembly, so I decided to remove that to take a look. It was then fairly obvious what to do. You probably read the bit that basically said 'don't mess up the springs and levers on the front of the self-timer' That was the voice of experience, I did knock a spring off one of the trigger levers and spent a good few minutes working out why the self-timer didn't trigger, and where the spring had to be hooked.
But from what I've seen, the Pen FT circuit is not that conventional. I've never seen another camera with this tapped LDR plate.
Why do you need to adjust it? Are you trying to use a 1.5V battery, or what? If, as you imply above, it's just the low-light setting that's out, try a cautious tweak of the rear preset.
Given the contents of my bench (good temperature controlled soldering iron, reasonable multimeter), I'd desolder the wire(s) from one terminal of that preset and measure the resistance as accurately as I could (desolder the wires so that the parallel path through the LDR doesn't affect the reading). That way I could go back to the original setting if I made things worse.
And yes, my Pen FT is back together. It still has a sticking shutter/mirror (which I need to strip down _sometime_), and I need to re-set the position of the meter coupling gears (which is not hard, now I know what to do. |
Josh Levine
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 02, 2005 - 02:44 am: |
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Hi,
I recently purchased a Pen FT and am also having problems with the meter.
The film I have just developed appears rather under-exposed and when I checked the meter against a digital meter, found that it is around 3 stops out.
I am using an MR-9 battery adapter in place of a Mercury cell, but I have not yet measured this with some load to see what voltage the camera is getting (it uses a diode to drop about 0.2v).
After looking at some meter schematics yesterday it looks like the voltage would have to be too high for the camera to under-expose (since the meter would think that the resistance of the LDR is lower then it really is).
The needle also seems to move if the camera is jerked or turned upside down.
Does anyone know what this could be due too, I'm suspecting that I will have to open it and have a look!
Thanks,
Josh Levine |
Henry
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 02, 2005 - 09:45 am: |
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Depending upon the Pen FT circuitboard in your camera (there were at least two, one with variable resistors and the other with regular ones), you may be able to dial in the proper EV with a 1.55v battery. I have done it with the variable pot board.
One can do it by substituting standard fixed value resistors on the other boards, but it is a lot more tedious. I have done it on a couple rangefinders.
A variable resistor switching setup would help.
Henry |
Josh Levine
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 02, 2005 - 02:39 pm: |
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Hi Henry
Thanks for the quick reply.
Do you think that the meter has somehow gone out of calibration, or could it be damaged in some other way?
Josh |
Henry
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 03, 2005 - 09:12 am: |
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Of course, it might be something as simple as your MR-9 adapter is defective. Have you checked it?
There aren't many components in the entire circuit. And no sophisticated components at all. If I was you I'd remove the top cover and resolder the wires. You seem to be getting extra resistance from something.
Mercury battery circuits are notorious for "black wire corrosion", so the wire leading from the battery to the circuitboard may have become a resistor itself. My standard operating procedure these days is to change that wire regardless of how it looks. This isn't easy on an FT. The mirrorbox must be removed.
Henry |
Josh Levine
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, December 03, 2005 - 09:51 am: |
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Thanks for the advice.
I have tested the MR-9 with no load, and it appears to be working (battery without MR-9 = 1.58v, with MR-9 = 1.42v).
I will have a go removing the top in the next we weeks and check the connections, resistances etc.
I suspect it could be something like black wire corrosion, since the meter is always on, and even with small currents after 30 years I'm sure some damage will be done.
Although I have never done any work with cameras, I am fairly used to tinkering with small things, and soldering very fine wire so I should be able to handle it.
Thanks again,
Josh |
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