The metering circuit of the Spotmatic range is a little sensitive to the battery voltage. This is because when correct exposure is indicated, a current of 3uA flows through the ammeter. That’s because Pentax decided that a good way of indicating that the meter was turned off, would be to have the needle dipped low when there was no current flowing through it.
I guess they thought it was a good idea at the time, but the current flowing through the meter with the meter centered is always a fraction of the total current in the metering circuit, for a given light level. The total current will be higher if the battery voltage is higher, and so will the meter current. This means that the needle indicates a higher exposure if the battery voltage is also higher. In this case, you will end up underexposing the negative.
In fact, the error is not actually huge, and it actually depends on the light conditions. If you conduct a fair test in different lighting conditions, you will find that under most outdoor daytime lighting situations the error is less that 1/10 stop, but this rises to 1/4 to 1/3 of a stop under dim indoor conditions. This in itself is no cause for concern, but it is worth remembering that the factory calibration of the meter and shutter speeds were considered OK if within half a ‘stop’ each. So the 1/3 stop could be 1/3 stop on top of that!
Despite many offering this as a camera repair, my own view is that the error is generally acceptable, (provided the shutter is properly adjusted), and not really worth worrying about, but there may be situations where this would not be the case.
(In case you were wondering, the later K’ series does not suffer this problem. That’s because it uses a meter with two coils, one drives the needle up, the other down. As long as the currents are equal, the needle is centered irrespective of the size of the currents involved. All that happens if the battery voltage changes are that both currents change by the same amount, and the needle stays centered.)
This means of course that whatever power source is used, it needs to provide a stable voltage that does not change much with time, temperature, or the amount of current required by the circuit. While this can be provided these days using special circuitry to provide a stable fixed voltage, back then most electronics was still being designed using vacuum tubes, and stable voltage references were in any case specially manufactured batteries.
These requirements could only really be provided in a compact and convenient form by a mercury battery.
Unfortunately, mercury is quite toxic, and this has led to the banning of these cells in most of the world. While this is no bad thing, it leaves the owners of many older cameras with a problem.
In fact two problems.
First, most mercury batteries came in different shapes from almost all modern batteries, and the battery used by the Spotmatics (Mallory RM640), is no exception.
Secondly, there are no modern cells which provide precisely the 1.35 volts provided by the mercury cell.
The first problem can be solved by using a smaller cell and providing a means of centering and stabilizing the cell. My favorite method for doing this is described in the link below.
The second is more difficult. In the following pages, I will describe three different ways of solving this problem. The methods described cover a range of accuracies and difficulty, so you should be able to find one which suits you.
After making these modifications, the metering circuits may need re-calibrating. To do this it is useful if you have a handheld meter of another reference.
How to fit a smaller cell.
This describes my preferred method of fitting a smaller cell. All the modifications described use a cell of the same size. The cell dimensions are 3.6mm high, and 7.9mm diameter. Depending who made the cell, and what standard they use you will find these marked 392 or PR41, but it’s best to use the dimensions as a guide, there are lots of standards.
Zinc-air and other cells.
Once you know how to fit the smaller cell the first thing you may want to try is using a zinc-air cell. This is often touted as the ideal solution as the zinc-air cell has a nominal cell voltage quite close to the ideal, but there are a number of reasons why this may not actually be the case. Let’s take a look.
Note: The ‘Wein cell’ is a zinc-air solution, using selected zinc-air cells to provide a closer match to the original mercury battery. They are essentially a standard zinc-air cell with a plastic ring already fitted in a similar fashion to my method described in ‘How to fit a smaller cell’. They are however much more expensive, but you might want to buy one in order to save the trouble of making your own adapter.
Germanium diode modification
This is the quickest, cheapest and easiest modification, which is probably the reason that it is the preferred method employed by professional camera repairers. Essentially the diode is used to reduce the battery voltage to a more acceptable level by inserting it into the battery supply. It is not, however, the most accurate modification. The reasons for this are explained in the technical description of the modification.
Germanium diode technical description
How to modify the metering circuit using a germanium diode
Meter sensitivity modification
This modifies the metering circuit to reduce the sensitivity of the meter used in the Spotmatic metering circuit using two small resistors.
This modification requires that the top casings of the camera be removed to access the circuitry, which requires the use of some special tools. However, these tools are not difficult to make for yourself.
The modification instructions are based on the values given for the meter properties given in the Pentax service manual, but as these cameras were made over a period exceeding ten years, your particular example may be slightly different. The technical description tells you how to calculate the precise values required by your meter if you wish to get this exactly right. However, the difference is not likely to amount to much in practice.
Meter modification technical description.
How to modify the meter circuit sensitivity to use silver oxide cells.
If you have read all the above pages, you should no know how to modify your Pentax Spotmatic to meter correctly. These modifications should work with all the Spotmatic range of cameras (SP500, SP1000, etc.), and I hope that reading the descriptions has given you some insight into how to go about modifying the circuitry of other cameras designed to use mercury cells.