Spotmatic light meter showing overexposure

Sergei Lapp · 03-31-2022, 03:40 PM

Hello Forum, this is my first time posting here. I've acquired an Asahi Pentax Spotmatic and was originally pleased to find the light metered working right away. It has one of the old mercury batteries inside which I found surprising. I've done the battery test(Bulb, Asa 100, switch on light meter) and it passed the test.
Though everything seemed to be all good to go, the camera seems to be telling me to underexpose frames quite dramatically. In a very well lit(natural light) shot it is telling me to use f22 and 1/1000. I have to point at the sun for it to suggest f22 and 1/500.

From what I have read, it could be the circuitry or maybe even a bad cell.

Should I first try replacing the battery before troubleshooting anything else?

I'd appreciate any thoughts you have.

-Sergei

**pentaxus** · 04-01-2022, 11:18 PM

What ASA is it set at and are you metering a scene in normal daylight?

Sergei Lapp · 04-03-2022, 10:30 PM

My ASA is set to 400 and I am metering in normal daylight. I have been using an external light meter on my phone for now until I sort out what is going on with the TTL metering.

Sergei Lapp · 04-14-2022, 03:28 AM

I have also discovered that changing the shutter speed has a greater impact on the meter needle than an adjustment of the aperture. This leads me to think that it is the CdS cells that are bad. Would that be a reasonable deduction?

**rogerstg** · 04-14-2022, 03:54 PM

Originally posted by Sergei Lapp

Should I first try replacing the battery before troubleshooting anything else?

That's always my first move when something electronic is acting funky.

Sergei Lapp · 04-19-2022, 04:31 AM

I have now replaced the battery with a brand new one(this one is a 1.55 volt). It is doing the exact same thing so now I'm really starting to believe that it could be the cells.

Wheatfield · 05-05-2022, 09:50 AM

Old post, and you might have an answer already, but the Spotmatic cameras originally used a mercury battery ti power the light meter. The nice thing about them was that they had a very consistent voltage over their life, consistent enough that a voltage regulator wasn't required in the camera.
The downside of them was that they had mercury in them, which led them to be banned in many countries.
They were, IIRC, 1.3 volts, so without voltage regulation, your 1.5 volt cell is going to cause inaccurate metering.
The CDS cells in the meter are getting old, and are probably losing some linearity and will likely not be happy with stopped down metering.
Unfortunately with these old cameras, the mechanicals tend to outlast the electronics.

Sergei Lapp · 05-06-2022, 12:05 AM

I am aware of all of this information, yes. But the one thing that I was informed otherwise was that the Spotmatic has a bridged circuit which means that the meter isn't influenced by the different voltage. Apparently the SP F doesn't have this bridged circuit so it is influenced by the incorrect voltage. Here is a video I found about it on this forum: https://youtu.be/Xcn1f231UyU?t=109

With this in mind I continue to believe that either the Cells are going bad or there is some bad wiring in the camera.

Thoughts?

03-31-2022, 03:40 PM	#1
Sergei Lapp New Member Join Date: Mar 2022 Posts: 5	Spotmatic light meter showing overexposure Hello Forum, this is my first time posting here. I've acquired an Asahi Pentax Spotmatic and was originally pleased to find the light metered working right away. It has one of the old mercury batteries inside which I found surprising. I've done the battery test(Bulb, Asa 100, switch on light meter) and it passed the test. Though everything seemed to be all good to go, the camera seems to be telling me to underexpose frames quite dramatically. In a very well lit(natural light) shot it is telling me to use f22 and 1/1000. I have to point at the sun for it to suggest f22 and 1/500. From what I have read, it could be the circuitry or maybe even a bad cell. Should I first try replacing the battery before troubleshooting anything else? I'd appreciate any thoughts you have. -Sergei

04-01-2022, 11:18 PM	#2
pentaxus Site Supporter Join Date: Jun 2010 Location: Central Coast, CA Posts: 1,313	What ASA is it set at and are you metering a scene in normal daylight? Last edited by pentaxus; 04-01-2022 at 11:20 PM. Reason: clarity

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04-03-2022, 10:30 PM	#3
Sergei Lapp New Member Join Date: Mar 2022 Posts: 5 Original Poster	My ASA is set to 400 and I am metering in normal daylight. I have been using an external light meter on my phone for now until I sort out what is going on with the TTL metering.

04-14-2022, 03:28 AM	#4
Sergei Lapp New Member Join Date: Mar 2022 Posts: 5 Original Poster	I have also discovered that changing the shutter speed has a greater impact on the meter needle than an adjustment of the aperture. This leads me to think that it is the CdS cells that are bad. Would that be a reasonable deduction?

04-14-2022, 03:54 PM	#5
rogerstg Loyal Site Supporter Join Date: Oct 2007 Location: Rhode Island Photos: Albums Posts: 1,168	Originally posted by Sergei Lapp Should I first try replacing the battery before troubleshooting anything else? That's always my first move when something electronic is acting funky.

04-19-2022, 04:31 AM	#6
Sergei Lapp New Member Join Date: Mar 2022 Posts: 5 Original Poster	I have now replaced the battery with a brand new one(this one is a 1.55 volt). It is doing the exact same thing so now I'm really starting to believe that it could be the cells.

05-05-2022, 09:50 AM	#7
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,981	Old post, and you might have an answer already, but the Spotmatic cameras originally used a mercury battery ti power the light meter. The nice thing about them was that they had a very consistent voltage over their life, consistent enough that a voltage regulator wasn't required in the camera. The downside of them was that they had mercury in them, which led them to be banned in many countries. They were, IIRC, 1.3 volts, so without voltage regulation, your 1.5 volt cell is going to cause inaccurate metering. The CDS cells in the meter are getting old, and are probably losing some linearity and will likely not be happy with stopped down metering. Unfortunately with these old cameras, the mechanicals tend to outlast the electronics.

05-06-2022, 12:05 AM	#8
Sergei Lapp New Member Join Date: Mar 2022 Posts: 5 Original Poster	I am aware of all of this information, yes. But the one thing that I was informed otherwise was that the Spotmatic has a bridged circuit which means that the meter isn't influenced by the different voltage. Apparently the SP F doesn't have this bridged circuit so it is influenced by the incorrect voltage. Here is a video I found about it on this forum: https://youtu.be/Xcn1f231UyU?t=109 With this in mind I continue to believe that either the Cells are going bad or there is some bad wiring in the camera. Thoughts?