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Pentax Spotmeter

Mag Miksch , Sep 27, 2007; 04:34 a.m.

I own an old Pentax Spotmeter, it looks similar to the Spotmeter V but is a little different as it has 2 different batteries, 1x 9V block for reading and a smaller one fot the lightning. Now it stopped reading in the upper scale, when I press the L button for low light it works. A dude of mine is willing to repair it but he didnt get clear with it without service manual. So I am looking for a service manual or factory repair manual.
Thanks and Kind Regards
Martin

Responses

John Gleason , Sep 27, 2007; 09:26 a.m.

This might be the manual you need:

http://www.butkus.org/chinon/pentax/spotmeter_iii/spotmeter_iii-splash.htm

but check around on the Butkus site, because he has manuals for several versions of the Spotmeter.

Mag Miksch , Sep 27, 2007; 09:57 a.m.

John thank you, I will give it a try

Richard Fateman , Sep 27, 2007; 12:03 p.m.

If this is in fact your meter, note that it uses a Mercury 1.3v battery, hard to get since selling it is illegal in most places (e.g. USA).

The symptom you are experiencing is, I think, consistent with the mercury battery being dead.

Alternatives to a mercury battery are discussed at great length elesewhere.

Mag Miksch , Sep 27, 2007; 01:37 p.m.

Thanks to all of you, big shame, I just forgot that the two batteries are used for low light or normal light, so the one is dead and the according metering doesnt work. What a tragedy getting old^^
Thanks and Kind Regards
Martin

Phil Eide , Dec 21, 2007; 08:39 p.m.

Martin--

It is possible to successfully modify the first Pentax/Asahi 1-degree Spotmeter ( Asahi model K circa 1968 ), for use with a modern 1.55Vdc silver oxide cell. The model K is the early Spotmeter that uses a 9 volt transistor radio battery and a 640 size mercury cell. I obtained the diagram by tracing the circuit inside the meter. I will e-mail you the .gif file if you are interested.

Since mercury batteries are now un-available for this meter, I substituted a silver-oxide cell in its place ( 392 size ). Silver Oxide potential is 1.55 volts DC compared to the 1.35 vdc ( mercury cell ), so the needle will read about a stop-and-a-half high compared to the original voltage.

Once the new battery is in place, the meter can be recalibrated by adjusting R1 to a higher resistance, (refer to circuit diagram) but problem is, that factory potentiometer R1 is only 2200 ohms, and runs out of adjustment range ( with the new battery voltage). R1 needs to be about 3500 ohms, so I substituted a little 10k ten-turn pot in its place, and re-calibrated the meter with a grey card and three other TTL meters I have here. Close enough. The silver oxide discharge profile is nearly as flat as that of mercury cells insuring accurate readings until end-of-life.

In order to fit the much-smaller silver-oxide cell into the battery compartment, one has to fabricate an aluminum adapter about the same size as the original battery, that the new cell lives in. We did it on a metal lathe.

I will e-mail you photos if you are interested.

Also the meter has a number of metal-to-metal switch contacts that easily oxidize over time and cause the meter reading to either become erratic or fail completely. They oxidize from battery outgassing. Without question, these un-reliable electro-mechanical connections are the weak point of the design. The way that Asahi engineered the Low/High switch, the Battery-Check reeds and the battery tab is really poor.

( Especially the battery tab. I gotta clean that one with my pocket knife about once a week.)

I wonder how many of these Spotmeters have been tossed into the dumpster due to this deficit. Other than that, it is a marvelously precise instrument, and is extremely useful for zone-type photography.

I bought mine in 1969 and did not find much use for it until last year. Now it is the only meter I use.

- Phil Eide

Undisplayable photo attachment:
Pentax K spotmeter circuit diagram -- Asahi Spotmeter K circa 1968.tif)

Phil Eide , Dec 22, 2007; 10:36 a.m.

Attached photo shows the newly installed potentiometer, R1.

Phil Eide , Dec 22, 2007; 10:39 a.m.

And this is the machined adapter to allow the use of a 392 sized silver oxide battery.


392 battery adapter

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