This page documents the checking and re-calibration of an Pentax K1000. The prism is also removed and the space between the screen and the prism cleaned.

This camera is an early model first bought in 1976. How do I know this? Well, that's when I bought it! The very early ones are easy to spot, the serial numbers are on the top cover, and there is a dimple to the left of the pentaprism where the battery-check button would be on a KX. (This would have saved them the cost of new dies for the cover on what was after all intended to be a cut down basic model for the 'K' range of cameras). It's perhaps ironic that early K1000's in good condition frequently command a higher price than other, more highly featured models in the range.

The camera has given excellent service over the years, and has never until now had any maintenance at all. Only very recently I noticed that the metering was tending towards over-exposure, and decided that it was probably about time to pop the lid and check this out. While I was doing this I took the opportunity to dust out the viewfinder which had accumulated some camera-mite droppings due to an infestation some years ago.

(To deal with camera mites, wrap the camera in a freezer bag, with a sachet of silica gel. Wrap the whole thing in several layers of newspaper, to slow down temperature changes, and put it in the freezer for a few days!)

The camera also needs re-lubrication, as the shutter release sticks at low temperatures, but I'll leave that for another day.

You will need some special camera repair tools. The tools described in the Pentax service manuals are ideal, but not readily available. These tools make dismantling one of these cameras easier and quicker, but are not really necessary. While anyone with a good drill press, and handy with a file could probably fabricate these from scratch, it's probably not worthwhile unless you intend repairing these cameras on a regular basis professionally.

In any case it is much easier to modify readily available tools, and this has at least the advantage that a single tool can be used for many different jobs. The tools used here, and how to make the more unusual ones can be viewed on the tools page.

The removal of the top cover is virtually identical for all the 'K' range of cameras, and for the Spotmatic range. The main difference is that the Spotmatics tend to have slot-headed screws as opposed to cross-heads.

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Opening the Camera Top

The first thing to do is remove top cover.

To do this first remove the counter dial cap. This is held on with three small grubscrews.Just loosen the screws one or two turns, so as to keep them in the holes in the cap. This will help keep them from being lost.

The screw in the centre of the dial is left-handed. It unscrews in a clockwise direction.

After removing the dial, the first 'special' tool is used to remove the ring-nut holding the winder-cap.This is another left-handed component.

This tool can be made by grinding the jaws of a vernier caliper to fit the nut.

Notice the groove on the cap rim. This is where the grubscrews locate to retain the counter dial cap.

Before removing the advance arm, remove the speed dial. Again a special tool is used, but I have simply re-ground the other side of the same caliper to two narrow points. This screw is the normal right-handed type.

With the screw removed, the speed dial and ISO film speed selector dial can be removed.

 The spring is narrower at the top than the bottom, and there is a washer in the bottom of this that can't be seen here. Note that setting the dial to 'bulb', and noting the position of the components at this point is a good idea. It's a good reference point, as it is easy to find.

The shutter speed selector ring should lift off now.

Next remove the three screws locking the advance lever retaining spring.

This spring serves as a friction clutch, as well as a bayonet retainer.To remove it, it should be rotated until the lugs in the spring line-up with the cut away segments of the advance driver.This is most easily done by pressing behind a raised section of the spring, and waggling the advance lever.

Now remove the rewind crank. Simply lock the film dog, and turn the crank anti-clockwise. (As viewed from above.)

Don't remove the rewind shaft, or push it down into the body. If the back is closed without the rewind shaft, it is difficult to release the latch!

At this point, the removal of the cover differs from the Spotmatic.On the K1000, this is just a large flat nut, not a film data indicator. It can be removed using the back of the modified vernier. Also, some later models seem to use a plastic part for this component. So take care that it is not damaged in the attempt to remove it.

There is a spacer underneath. Remove it. In case you are wondering why it is the odd shape it is. It's to allow access to a trimmer used on the KX to calibrate the stop-down metering mode. The other are there for similar reasons. Note the dimple next to the visible screw, and the location of the serial number. You will only see this on early examples.

Now simply remove the screws in the top plate. Note which goes where, they are not the same. Also there is a pin which sits inside the shutter release button. This release pin ensures reliable operation with release cables, and also keeps dust out. It is also easy to forget when re-assembling.

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Explanation of the Meter Circuit

OK now we have the top off, we should have a look at the electronics of the meter circuit to get an understanding of what we are looking at under the bonnet. Study the circuit closely, it will be useful if you accidentally break a wire! The circuit employs two CDS LDR's in the metering circuit, and a third cell is used in the battery circuit.

The first thing to note is the absence of any power switch. Switching the circuit on and off is achieved using the transistor Q1, via the LDR, R12, and the associated resistors R10, and R4. The resistance of an LDR is extremely high when not illuminated, and falls to a lower value depending on the amount of light falling on it. When the lens cover is removed light falling on R12 conducts enough current to switch Q1 into conduction, allowing current to pass to the metering circuit proper.

Should the meter fail to switch on with a good battery, and the screw grounding the PCB is making good contact, the component to check first is R12, and it's connections, Q1 is less likely to have failed.

Also notice that the components R12, R10, and R4 are always connected to the battery. The current consumed by these components is very small when R12 is not illuminated. The battery would probably self-discharge in less time than it would take for these to run it flat, but it probably accelerates the process somewhat.

Next have a look at the components in the dotted 'Sensor' box. This is the light sensing circuit. The two variable resistors R1, and R2, are set at the factory, and serve to set the linearity of the sensors. They should only be disturbed as a last resort. R7, and R8, LDR's are rather odd, as they have an extra connection tapping into the middle of each cell. It is unlikely that these are at fault, but if they are it's probably better to replace the whole sensor assembly, than the individual cells. You should be able to pick up a scrap camera for a few pounds on eBay or  somewhere like that. (See my comments on buying cameras on eBay for some observations on this source.)

A word of warning. The LDR's in the camera are not encapsulated in resin, as is usually common, and the sensitivity is dependent on a number of factors including the exact chemical composition of the photosensitive material. The chemicals in sweat can cause significant changes to this. Consequently you should take pains to avoid touching the sensitive area of these components.

Before looking at the speed and diaphragm resistors, take a look at M1. This has two coils, and that is the key to the circuit operation. Normally a meter only has one coil (or winding) which interacts with a magnet to indicate the current flowing through it. The two coils here are perfectly matched, and operate in opposition to each other, so that if they both have the same current flowing through them they cancel each other out. If on the other hand, the currents are different, the meter needle moves one way or the other depending on which carries more current.

Now look at the speed and diaphragm resistors R5 and R6. The values of these is much higher than the resistance of the meter coils, so that the current flowing through each is almost completely independent of the other, determined by the voltage across the meter circuit, and their resistance. The sum of the two currents passes through one coil of the meter.

If a lot of light falls on the sensors, a large current will flow into the sensor coil in the meter, causing the meter needle to rise in the viewfinder. To counter this the current through the other coil must be increased to match it. This can be accomplished by turning the shutter speed dial to a higher speed, which reduces the value of R5, or by selecting a smaller aperture having the same effect on R6, or possibly both. It should be clear that a balance can be obtained with many combinations of settings for a wide range of illumination of the sensors.

In a nutshell, that's how the metering circuit works. There's no real magic, except that the way the resistance changes in R5 and R6 has got to be properly matched to the way the LDR resistance changes to get proper exposure. This is not a difficult thing to do when manufacturing variable resistors.

In order to calibrate the meter circuit, should it be required, Pentax have provided terminals for adding a calibration resistor to either the sensor side, or the setting side of the circuit. These I have labeled TP1, TP2, and TP3 in the diagram. RC1 and two are the possible calibration resistors.

The value of the calibration resistors provided by Pentax range from 110K Ohms, corresponding to one EV, to 1.1M Ohms, equal to 1/10 EV. Ten resistors are available in 10% increments. It should therefore be possible to calibrate the circuit to within 1/10 EV.

Note that there is nothing particularly special about the Pentax resistors, except perhaps their shape. The values supplied are electronics industry standard values. There is plenty of room under the cover, and many resistor types will be small enough to fit.

Connection between TP1 and TP2, increased the current through the sensor leg, increasing the reading for a given light level, and so a slower shutter or reduced aperture is required to match it . Connecting TP1 to TP3 has the opposite effect.

In the case of this camera, it has been over-exposing, and the resistor should therefore be connected between TP1 and TP2.

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Checking the CDS LDR's

Before diving in and adding calibration resistors it's worthwhile checking the LDR's cells, especially if the camera is overexposing by exactly one stop! Before we actually do that let's have a look at the camera circuit board, so that we know where everything goes.

This is how the camera metering circuit should look with the top removed.

The PCB on the left provides the connections for the meter, Q1, and it's associated resistors, and the pads for the calibration resistors. The small black screw on the right, with it's washer provide the ground connection for the circuit. It's worth removing and cleaning both of these to ensure that the connection is good.

Just behind the viewfinder is the sensor board. It's held in place by two screws accessible from the top of the camera. The LDR's are on the opposite side. You can see the two factory set resistors, R1 and R9, on either side of the viewfinder aperture.

However, there are three LDR's on the board. The middle one is R12. The outer ones are R7 and R8.

A calibration error could be due to the failure of one of the these two, so it is worth checking them. The resistance value in the dark should be around 30M Ohm each, and a few K Ohm when illuminated.

It may take some time to reach the maximum dark resistance, and the value is quite variable, anything above 1M Ohm is probably fine. All we need to know is that the LDR's are working, and have a marked response to light. (It's already been pointed out that there can be quite a spread of actual values for these components.) Carefully unsolder the leads and check that they are about the same, they should have started their career as a reasonably well matched pair, and they should not be too much different now.

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Removing the Prism

Since I have the top off, and the sensor PCB is out of the way, I'll take the opportunity to clean the camera-mite poo from the focusing screen.

First unhook the pentaprism retaining springs using some tweezers, and remove the retaining cover.
Now locate the two prism adjusting screws, at the front and either side of the pentaprism frame.

These screws bear on the sides of the prism itself. Loosen one of these only, and lift out the prism. The prism sits in a plastic dust seal, but in my experience, this is often the source of contamination. Clean as much of it away as you can with a wooden scraper, taking care to avoid touching the focusing screen.  Replace it with light seal foam, and when replacing the mirror only tighten the screw you originally disturbed.

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Cleaning the screen

To clean the screen, blow away any particles with a hand blower. (Don't use a aerosol can, the jet from one of these is powerful enough to cause damage to the delicate meter mechanism.) At most use a soft dry sable brush to remove any stubborn particles. Anything stiffer risks scratching the screen. Don't under any circumstances use any kind of fluid or solvent. I have yet to find a cleaning agent which does not either dissolve the plastic, or leave a deposit behind. The surface structure on focusing screens is very fine, and almost any residual deposit will completely ruin it.

A safer method of cleaning is to use an ultrasonic cleaning tank. Proffessional tanks cost hundreds of pounds, but a number of places sell smaller 'domestic' type tanks which are up to the task for prices in the tens of pounds. Make sure that it is an ultrasonic bath before you bu. Some vendors sell cheap cleaning baths described as 'sonic' baths or suchlike which are not ultrasonic at all, and totally useless for this kind of work!

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Fitting Calibration Resistors

OK, the viewfinder is clean, and I have re-fitted the prism. Now I need to calibrate the meter circuit.

The PCB connections are as shown on the left.

To calibrate you need a reference meter, and an evenly lit subject. You can use a large white card, or even the kitchen or bathroom wall. For bright readings, an overcast sky works as well. But it is also possible to use the computer screen, displaying large white area. (This has the advantage that you can vary the luminosity.)

As a reference, I use the meter shown above. It is accurate to1/10 stop, which is as accurate as we need. I have to say however that I have a Sekonic with a selenium photocell that was old when I bought it thirty years ago which agrees very well with this one, and was a lot cheaper!

Re-assembly Notes

All that remains to do now is re-assemble the camera, which is a relatively simple operation, simply the reverse of assembly.

There are no special problems with re-assembly. Provided you haven't taken anything apart beyond what has been covered here, there is only one way everything goes together so you can't get it wrong!

Conclusion

That covers all you really need to do to re-calibrate the meter of a K1000 camera yourself. The same should apply to the other manual exposure K' series cameras with little, if any changes.

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