International teamwork: Fixing Spotmatics

Nesster · Expire: 2014-02-20

Brilliant, a service to humanity, really - or at least OMists and Spotmen and women

Shiladitya · Joined: 31 May 2008 Posts: 306 Location: New Delhi, India

rnbc · Joined: 29 Nov 2010 Posts: 4 Location: Lisboa

Hello! This is my first post in this forum.

I just did the meter modification in my own Spotmatic, since the V40PX battery I was using is now quite old (but still operating). I actually did it before finding this forum...

I must stress that after calibration, but before modification, with the Silver battery the meter values at normal light levels (ISO-100 down to 1/8s or so) were accurate, only becoming inaccurate at the lowest light levels, with the meter becoming oversensitive by 2/3 EV or so at ISO-100 1s (the lowest light level it can measure).

With the mercury oxide cell the values were extremely accurate at all light levels though, so I decided I should proceed with the modification to retain that excellent performance.

Anyway, I used standard resistor values since they are quite close to what we want, and if cherry-picked they might actually result in a better meter performance than with the original mercury oxide battery. Since were I get them they only sell by the dozen (0.3€ each resistor pack) I could cherry-pick them.

I recommend reading more info about the modification in this site...

www.mypentax.com/Spotmatic_meter_mods.html

Where not only is the procedure very well detailed but there are also links for Spotmatic service manuals, explaining exactly how to disassemble and assemble the camera:

www.mypentax.com/Repair_manuals.html

www.pentax-manuals.com/manuals/service/servicemanuals.htm

I found them most illustrative to understand which wires are which and the detailed procedures gave me extra confidence to proceed.

Now, back to the modification, the values I measured in my camera where the following:

-- 2890 Ohm for the meter coil itself (not 3k as they say!)

-- 1.58V Silver Oxide battery (Renata 394, 9.5x3.6mm), mildly used (1.61V when fresh, 1.55V "nominal", but 1.57V for most of the time according to the discharge graphs in the manufacturer's site).

-- 1.36V Mercury Oxide battery (Varta V400PX), mildly used but old (1.35V nominal).

-- 463 Ohm for the cherry-picked series resistor (470 Ohm nominal).

-- 21.3 kOhm for the cherry-picked parallel resistor (22 kOhm nominal).

The modification would be accurate for (463+2890)/2890*1.36=1.578V, which is close enough to 1.58V or 1.57V in my humble opinion, since batteries are not that exact anyway.

The final meter resistance after modification is 1/(1/(463+2890)+1/21300)=2897 Ohm, instead of 2890 Ohm. This is within 0.24% of the original meter coil, much better than factory tolerances!

The meter now gives exactly the same values with the Silver cell as before with the Mercury cell, as expected anyway. I also calibrated it for about 1/5 EV overexposure, since my shutter is calibrated a bit faster also -- exactly how much varies a bit with the exact speed selected, but it's about 1/5 EV in average. That way they both tend to match, and it's much easier to calibrate the meter than the shutter.

Just a small note to say that the total current flowing through the meter circuit is a lot larger than through the coil. In fact the coil is centered at 3uA but the total current varies between 24uA (no light, meter set to 1s) and 260uA (lots of light, meter at 1s). With the needle centered, which is what matters, I measured ~60uA at low light and ~90uA with lots of light. So the circuit is close to a balanced bridge, and the current flowing through the coil is quite irrelevant. I didn't tried without it, but probably the 22 kOhm resistor is irrelevant for the modification accuracy.

PS: I calibrated the meter using the same lens in the same settings in 2 digital bodies (Sigma SD14, Canon 1000D) and the Spotmatic and comparing the readings of all 3 cameras. Modern cameras tend to agree and are very accurate, I even compared with a Nikon D700 and it came spot on.

PPS: I found how much the different speeds of the Spotmatic were "off" by using a digital camera behind the spotmatic shutter curtains (the digital with a longer exposure) and comparing the results with the digital camera's own shutter (modern shutters are extremely accurate). Just out of curiosity the values I obtained were +/- the following:

set -- measured
1 -- 1
0.5 -- 0.42
1/4 -- 1/5
1/8 -- 1/8
1/15 -- 1/15
1/30 -- 1/30
1/60 -- 1/60
1/125 -- 1/125
1/250 -- 1/250
1/500 -- 1/450
1/1000 -- 1/1000

peterqd · Expire: 2014-01-04

Thanks for this extra info! You measured the voltages and resistances a lot more accurately than me. Smile

On my cameras I found it was not quite possible to calibrate the camera correctly using a 1.55v battery and simply adjusting the calibration resistor, but it was probably near enough that the difference in light reading would have been unnoticable. But probably every camera has slight differences in the resistance of the circuit, so this cannot be a general rule. That's why I share your view about the modification being worthwhile.

PS I fixed your links - "http:\\" should be omitted.

magkelly · Joined: 06 Jul 2010 Posts: 182

I had problems with this with my SP but I've never had any problems with using non-Weins with my SPII. I use that camera with the Energizer batteries all the time. Maybe a bit overexposed or under sometimes, but not by much. Usually it's just me doing it, not the camera. Nothing I can't attribute to user error and me being new to SLR's. It's nothing a quick trip through Camera Raw can't handle.

My only problem with using them is that no one seems to carry them locally so I have to special order them all the time from Amazon. Queenie, my SPII, seems to do just fine with the bunny batteries though. I like them better than the Wein cells myself. I kind of wish she used the LR44 clones though. They're all over the place, unlike the S387's.

I'd never try something like this. It is just way beyond me, but you guys, I am very impressed. I don't think that it's a problem with every Spottie. But for those who need an adjustment, why not? I say go for it if you can do it.

peterqd · Expire: 2014-01-04

rnbc · Joined: 29 Nov 2010 Posts: 4 Location: Lisboa

peterqd, as you said it's not possible to correctly calibrate the meter for 1.57V before the modification, but the difference at high and medium light levels is so low as to be meaningless. The real problem starts at low light levels, where it becomes oversensitive, and is compounded with the fact that film starts to have reciprocity failure at 1/4s or so... not a good prospect. You end up with underexposed negatives.

I find the meter is a lot more accurate than shutter speeds. BTW, my shutter is correctly calibrated, since you calibrate at 1s, 1/60 and 1/1000, with other speeds supposedly "falling in". Despite that some intermediate low speeds are shorter by almost 1/3 EV, as you can see in the table, and one of the high speeds is a bit lazy. Anyway, by factory specs the meter and shutter only have to match each other better than 1/2 EV over the whole range, so mine is officially approved, so to say Smile

We must not demand to much accuracy from these old beasts Wink

magkelly, the modification is easy, the real problem is disassembling/assembling the camera. Some tools are also not readily available: I used a modified (sharpened) needle-nose pliers and a caliper as tools... but I think if you study the manuals you won't have much trouble. Just don't try it before you understand exactly what you are doing!

Apart from the fact a LR44 has a sloping discharge curve I don't think a SR44 (silver oxide LR44) will fit the Spotmatic's compartment: it's 11.6 x 5.4 mm while the original battery is 9.5 x 3.6 mm (plus plastic gasket, but only around the top, not on the bottom). The closer you can get is, of course, the 387S, but I never saw them for sale over here. Otherwise a SR936 (AG9, Renata 394...) will do, since it's the same battery minus the gasket. To avoid a short circuit, which is what the gasket does, I recommend isolating the interior of the battery compartment (except for the + contact and the metallic lid) with a bit of tape. Also be careful when you start to screw in the lid to avoid short-circuiting the battery: check the battery is leveled, and put the lid leveled from the beginning.

A smaller cell will also work, albeit with some rubber ring to center it, and since the meter uses so little energy will probably last for the same time: cell self-discharge is probably more important than meter drain. For example, the cell I mentioned is 84mAh and will take 4 months to discharge (in the dark) even if you forget to shut down the meter, and 13 days under maximum the maximum consumption I could measure. If you don't forget the meter switched on, it would probably last >10 years, except it will self-discharge before.

magkelly · Joined: 06 Jul 2010 Posts: 182

I use the 387S from Energizer. Fits perfectly, no ring needed, and works just fine. LR44's are too big for Spotmatics.

peterqd · Expire: 2014-01-04

graffixjones · Joined: 14 Feb 2022 Posts: 1

I realize this is necroâ€™ing an old thread, but I wanted to thank you guys if you happen to still be around. I found some Vishay resistors on eBay rated at 442ohm 1/8 watt 1%, and 23.2K 1/4 watt 1%. That was close enough for me. Mounting the resistors was a fun experience, and I ended up using their legs and soldered them in â€œugly bugâ€ style, with one bridging across the circuit board, and one branching off the board and then connecting to the red lead for the galvanometer.

I converted two Spotmatic SPs, and while I havenâ€™t ran any film through them since the conversion, theyâ€™re responding to light just like they did prior to the conversion. I take that as a good sign. The meter seems to be more sensitive in the lower range, but Iâ€™ll know once I get a couple rolls of film through each of them.

Iâ€™m just getting back into B&W film after 20 years of shooting digital, so Iâ€™m getting all this old gear back into shape. Threads like this have been a godsend, even 10+ years after the last reply was posted.

Thanks again.