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12-22-2013, 01:38 PM   #16
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QuoteOriginally posted by stevebrot Quote
Ok...here are some numbers based on a blank wall reading in my home.
Looks like the Spotmatic could be OK with the silver oxide. Do you know if it was ever recaibrated after the change to silver oxide? -there is a trim resistor on top of the cds ldrs.

12-22-2013, 02:24 PM   #17
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QuoteOriginally posted by wombat2go Quote
I have seen some reference to a shottkey diode being put in a container with a AgO cell but I don't know if that will fit in a Spotmatic.
That would be a more accurate option if it is possible.
There is a C.R.I.S. adapter for the Spotmatic and Spotmatic F.

Before editing the wiki it would be good to know the actual sensitivity of the circuit to a 19% increase in voltage. I understand what you are saying in regards to the bridge and the small current that the meter needle is biased against. I thought that a possible clue might be found in the specs for the KM and K1000 cameras. Both have 1.5v circuits driven by CdS sensors. I could not find a service manual for the KM and the K1000 manual was of little help. That camera uses a "balanced" galvanometer rather than a bridged circuit .


Steve

Last edited by stevebrot; 12-22-2013 at 02:46 PM.
12-22-2013, 02:30 PM   #18
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QuoteOriginally posted by wombat2go Quote
Looks like the Spotmatic could be OK with the silver oxide. Do you know if it was ever recaibrated after the change to silver oxide? -there is a trim resistor on top of the cds ldrs.
I have no knowledge of any CLA having been done on the camera. I know about the trim resistor. It is on the National Camera schematic and is used for meter calibration against a constant voltage source. This might be a good question for Eric since he services these all the time. I asked my local tech a few years back if he calibrates Spotmatic meters to other than 1.3v and his response was that it was not needed.


Steve
12-22-2013, 04:19 PM   #19
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I have used a silver oxide battery since the 80s, with no modification to the camera
A couple of Spoties I have gotten from ebay have worked as well with the solver and the weill cell (yes I was stupid enough to try one)
and I could not see a difference.
Maybe I have just been lucky

QuoteOriginally posted by wombat2go Quote
Looks like the Spotmatic could be OK with the silver oxide. Do you know if it was ever recaibrated after the change to silver oxide? -there is a trim resistor on top of the cds ldrs.
Years ago in the spotmatic discussion group a seires of tests were carried out, let me see if I can dig those files....
QuoteOriginally posted by wombat2go Quote
Titrisol, That is very good.

About the use of silver oxide batteries, they have a plateau of 1.55V compared to the 1.32 V of the original mercury cells.
Ref 5 is not citing silver oxide as a replacement as far I I read, and the diagram P23 shows it is not a balanced bridge, so will be affected by battery variation.
Perhaps the use could be qualified with a comment that accuracy might be affected especially at low apertures/low light.

I have seen some reference to a shottkey diode being put in a container with a AgO cell but I don't know if that will fit in a Spotmatic.
That would be a more accurate option if it is possible.

By the way I don't have direct experience with the Spotmatic because don't have one here. Perhaps I need one now!
However I restored a 1960's HoneyWell Pentax Exposure Meter by re-designing its electronics to slightly more modern tech - I put article on Apug DIY about that.
Unencumbered by an aperture, it works from EV18 to EV3; I just took a EV4 reading in the office, it said , at iso 400, f/8 and 1 second, and the exposure was very good on the K-01 at that setting


12-22-2013, 04:29 PM   #20
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It could even be, in certain cameras that the silver oxide actually makes the exposure more accurate.
I saw in the 231-02 service manual that color coded trim resistors were being shipped to match select-on-test color coded Cds.
An indication that there might have been some problems back in the day.

Here is a comment by PF member woodywesty on another Spotmatic thread which is relevant so I take liberty to paste woodywesty's comment here: ( not knowing how to quote across)

"
I always used a penny as the tool to open the battery cap. As far as the meter not working.. back in 1992 or so when mercury batteries were no longer available and I and everybody else in the US had to start using the non mercury batteries in our cameras. From that point on the meter in my Spotmatic underexposed pictures. I never found a solution to this problem but I bet there are plenty of members on this forum who might have some ideas. I was told back then that the "new" batteries had slightly less voltage than the original mercury batteries and as a result effected the cameras metering. The newer batteries also didn't last very long. After a while I just gave up on the Spotmatic....

Read more at: https://www.pentaxforums.com/forums/8-pentax-film-slr-discussion/243986-spotm...#ixzz2oFU91Qgj "
12-22-2013, 06:56 PM   #21
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I wonder what batteries he was using as a replacement? Partially depleted alkaline cells? Less voltage could definitely do it.


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12-22-2013, 07:56 PM   #22
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Alkalines did not last long (I used cheap batteries a few times), and silver oxide also start underexposing at the end of life.
Howerver SR batteries tend to last longer

QuoteOriginally posted by stevebrot Quote
I wonder what batteries he was using as a replacement? Partially depleted alkaline cells? Less voltage could definitely do it.


Steve
12-22-2013, 09:35 PM   #23
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If anything, my Spotmatics over expose with A and S 1.5v cells

01-05-2014, 04:15 PM - 1 Like   #24
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I haven't used a Pentax Spotmatic much, but have used one with a watch sized silver oxide battery. At least for normal daytime lighting I got slides that look properly exposed.
The battery was smaller than the compartment of the Spotmatic, I used aluminum to take up the extra space. The biggest operation problem was that I was using the battery
in reverse polarity compared to the Hg battery. The needle than moved in opposite direction of the aperture, that slowed me up a little as I adjusted it.

dsturgis
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