There's been a bit of discussion lately on the flickr Praktica group about light meters, batteries, accuracy, repair etc. If you've got an old Praktica SLR - a TL3, MTL3, MTL5B, TL5 in fact pretty much anything with a TL in the name and you want to know more about the metering, read on...

A while back I compared the light meter performance on one of my old Prakticas with a hand held meter and a couple of other cameras and found it was quite a long way out. So I took the old East German beast apart to find out if it was possible to adjust it. Getting access to all the metering components is very easy.  First, remove the battery cover and battery. Then unscrew the four small screws on the base plate and carefully remove the plate.  The meter circuit PCB is exposed in all its glory!

On the PCB are the battery contacts, the switch contacts and three variable resistors.  Wires go from the PCB to the LDR (light dependant resistor) sensor, the meter and the combined film/shutter speed control.  All three variable resistors affect calibration, so it's not just a matter of tweaking a single adjuster until the meter reads correctly!  I traced the schematic from the PCB......

The circuit is a Wheatstone Bridge.  Basically, two voltage dividers are placed across the battery.  The meter connects between the two dividers and indicates the voltage difference between them.  The film speed/shutter speed selector forms one voltage divider, the other is a resistor and a light dependant resistor (LDR).  When the two dividers are balanced, the voltage difference between them will be zero, so the meter needle will stay at the centre point on the meter.

The wiper contact of the speed selector switches out of circuit at the extremes of the dial.  This explains why the meter doesn't operate at certain ISO settings and extreme shutter speeds.  With the ISO set to 50, the wiper travels the full distance of the selector between B and 1/1000s.

If your meter isn't working at all, first check the meter switch contacts actually close when the button is pressed.  The PCB may be loose (1 screw holds it down) or the contacts dirty, either would lead to poor performance.

I'll edit this again soon when I've worked out how to do the actual calibration !