Pentax Spotmatic F: How to adjust TTL meter

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Thomas78

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Hello,

I recently bought a Pentax Spotmatic F.
Since its TTL meter was designed for a PX625 mercury battery I bought a MR-9 adapter with a 386 silver cell from small battery company to emulate it.

Now my Spotmatic F TTL meter would give me about 1 stop over development (e.g. it gives me a 1/500s where a 1/1000 s would be right) compared to both with my gossem digisix and my Bessa R3A.

Can I adjust the TTL meter myself ?

Best Regards,
Thomas
 
You could just set the ISO dial to read a stop faster. That would mean, for example, setting 800 ISO (or ASA as it will probably be on your camera) when you have 400 ISO film in the camera. I don't know how to adjust the meter.
 
I'm afraid its not as simple as that. The potentiometer that selects the aperture has probably deteriated and I found the need to modify it with a 4K7 ohm resistor in series. Sorted the problem but does require extensive surgery to sort. IE mirror box out!

Best,

normclarke.
 
Thomas78 said:
...
Can I adjust the TTL meter myself ?
...
Click to expand...

It's not an easy task.

I suggest you redo your meter comparisons at a wide range of lighting levels. If the 1 stop "error" is present across a wide range then simply follow Koolzakukumba's recommendation and lie to the camera when setting the ISO.
 
Spotmatics have bridge circuits. Assuming the meter is in proper working condition, they adjust themselves to the differing voltage. No adapters are needed. If you're having a problem it's not because of the battery voltage.
 
I also have a Spotmatic and Spotmatic F... the battery adapters work fine and give good meter readings... (note: different adapters for SP and SPF). Just to confirm it's the meter... have you checked the shutter speeds. It's common for them to be off, I know that mine were.... but rather than have the shutter speeds adjusted, I just compensated at certain speeds - not all were incorrect ;-).

If you are unsure about the shutter speeds, try the slower speeds against the meter... just to eliminate that possibility... I understand that you checked against another meter(s), but how do the negatives look...

Note: it is a spot meter so you have take care how you meter. I have the Gossen Luna-Pro that also need the adapters... good meter.

I think I might have a service manual around somewhere that talks about the meter adjustment... you can also find it online...

Casey
 
I agree with normclarke, the battery voltage doesn't control the meter directly. I would send it out to jon Hendrickson or some name like that. There is an adjustment meter screw under the rewind knob which you will easily find on the web. But if the meter isn't consistent across the EV range this will not help.
 
Thank you all for your replies!

shimokita said:
I also have a Spotmatic and Spotmatic F... the battery adapters work fine and give good meter readings... (note: different adapters for SP and SPF). Just to confirm it's the meter... have you checked the shutter speeds. It's common for them to be off, I know that mine were.... but rather than have the shutter speeds adjusted, I just compensated at certain speeds - not all were incorrect ;-).

If you are unsure about the shutter speeds, try the slower speeds against the meter... just to eliminate that possibility... I understand that you checked against another meter(s), but how do the negatives look...
Click to expand...

I finished the first roll tody (Delta 100) and will develop it tomorrow evening. I used my external meter for it.


Sometimes the needel seems to jump up and down while metering and also sometimes giving different values for open aperture metering and stopped down, perhaps I have a problem with bad contacs or corrosion...
 
You should note that there are two classes of adjustments to make on this type of meter:

  • Mechanical - There will be a zeroing adjustment screw and some additional adjustable "weights" on the meter movement that are used for the first adjustments. The zeroing adjustment changes the spring tension. You adjust it so that with no battery the needle floats to a proper position. This position varies with the meter design. The "weights" are used to balance the movement so that it zeros the same regardless of camera orientation (horizontal, vertical, and lens straight down are the usual tested orientations).
  • Electrical - The adjustments will vary with the electrical design. There is generally a "master" adjustment resistor and sometimes a second "trim" resistor. The master is used for the basic adjustment, generally for some standard bright light test. The trim, if present, is then use to correct the reading at some standard low light level.
The mechanical adjustments must be made before the electrical if you want the system to work well. Over the years, I've seen many cameras that would read correctly in one orientation and 1-3 stops off when rotated 90 degrees. When the meter movement is out of balance as the result of some shock gravity will pull differently when the orientation changes.
 
Honeywell Pentax Spotmatic (operating manual)

Built-in meter measures the brightness of the ground glass, and couples directly to shutter and film speed settings. Film speed (ASA) setting ranges from 20 to 1600 (LCV 1-18 for ASA 100 film with standard lens.) Meter is powered with a mercury battery.

Asahi Pentax Spotmatic F (operating manual)

TTL CdS meter measures the average brightness of the ground glass at full lens aperture and couples directly to shutter and film speed settings. Film speed (ASA) from 20 to 3200. LCV 3 - 18 for ASA 100 film with 50mm lens. Powered with one 1.3V mercury battery.

From the internet:

The camera had originally been designed to use spot metering, and this feature was included in prototype models exhibited at Photokina 1960. Shortly before production Asahi decided that spot metering would be too difficult to use, and so the metering system was altered to use average metering, and with later models a center-weighted system. The change took place too close to production to change the name, and so Spotmatic stuck.

Note: I must have one of the prototype models ;-)

@ronnies... Thanks for that correction.

Casey
 
The first roll came out quite well (exposeure determined by hand held meter), so the shutter seems to work OK (at least for b&w negatives).

When I take the battery out, the needle is almost in the middle position as it would indicate correct exposure.

I already took of the base plate, no corrosion visible and the wiring looks OK.
 
Ben Z said:
Spotmatics have bridge circuits. Assuming the meter is in proper working condition, they adjust themselves to the differing voltage. No adapters are needed. If you're having a problem it's not because of the battery voltage.
Click to expand...

Whilst it is a bridge circuit, it's not quite as simple as that as this article (with a proper Pentax circuit diagram) explains:

http://forum.mflenses.com/international-teamwork-fixing-spotmatics-t7047.html

For what it's worth, my SP1000 gives at least a one-stop difference in reading between a 1.35 volt and 1.5 volt battery, the latter leading to underexposure if you believe the readings. I'd be surprised if that has a different circuit from the Spotmatic F.

Zinc/air hearing aid batteries seem to be a perfect substitute for the originals. They might not last as long as a mercury battery but they have the right voltage (and characteristics) and they aren't particularly expensive. Much easier than trying to make modifications to the camera.
 
Thank you all for your contributions!


I managed to get access to the variable resistor and I am going to try to calibrate it tomorrow.
 
According to my information - conveyed to me by a camera technician, the Spotmatics are amongst the few cameras that do not require recalibaration for different batteries. The meter needle is centred when the current flowing across the circuit is balanced. Apparently the way it works is that even if a different battery is used the circuit compensates automatically and hence no adjustment is required when a battery of slightly different voltage is used.
 
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