Re your initial post. The MTL5B is a mechanical camera. The battery is only needed for the light metering function. So it is actually perfectly usable as it is, providing you meter separately.
I wouldn't be particularly inclined to reference the adjustment of the meter based on a digital camera. I prefer, if necessary, to make comparisons against a constant light source with a trustworthy film camera and ideally one using the same type of meter cell (Ie. selenium, CdS, silicon, etc). This helps prevent deviations based on cell sensitivity to different colour temperatures, something that would definitely be an issue, if, for instance, you were using a selenium meter late in the day to try to calibrate a CdS meter.
I strongly recommend doing some basic checks, prior to attempting any adjustments. Ie. battery voltage to spec. Battery contacts present, clean and in good condition. Lens aperture functioning correctly, and stop down/metering switch working properly.
What is the condition of the viewfinder like? Is it bright and clear? The MTL5B actually has a very good finder in my opinion. But if it has darkened, this may suggest that deterioration of the jointing surfaces could be affecting the meter accuracy.
You have assumed that the meter is completely functional, and simply badly adjusted. This may in fact, not be the case at all. There are many other reasons a meter will go out of range. Some of these include:
- Deterioration of the CdS cells altering their resistance values;
- Old cement becoming opaque and changing the amount of light reaching the cells where they're attached to the pentaprism;
- Deterioration of the prism itself (as mentioned above);
- Contamination by prism mounting foam, dust etc. impacting the light reaching the cells (depending on how they're installed);
- Wiring continuity faults in the meter circuit affecting current flow;
- Galvonometer faults, such as damaged or detached windings, needle off its perch or obstructed by debris of various sorts, bad electrical connections.
If the meter is likely to respond well to adjustment, it should already be reacting to various light levels in a consistent and proportionate manner but simply be out by a certain value, and out consistently. If it isn't, adjusting potentiometers is unlikely to improve matters at all (and will probably make things worse).
Assuming the meter is basically functioning correctly and is simply out by a stop or three across the range, it is possible (and I stress the word "possible") that you can dial it back to spec. But it's not a 5 minute job. At least, not without a manufacturer's test facilities and setting instructions. Meters often used a tiered, or primary/secondary, adjustment process, that facilitates accuracy across the full range of EV to be obtained. So making an adjustment to one pot won't necessarily get it ideal at all EV and probably won't. It may simply Eg. improve accuracy at higher EV, but not at lower EV (or vice-versa), and even make accuracy at the opposite end of the range worse. Hence, the presence of two or three pots, each of which may need to be balanced against the other and adjusted in a particular order (as they would have been at the factory no doubt) to get best performance. One of them may well be a calibration pot for the circuit that gets input voltage to spec. Many older cameras had one such in series with the battery check circuit (which, as I recall, the Praktica lacks). So I assume you have a multi meter, and know how to use it.
Before you touch anything, take a good, sharp, digital image, showing precisely where each pot is set. If I were doing the process, I would make comparison readings with another SLR using the same type of meter cell, using a prime lens of identical focal length and speed, set to the same focusing distance, and carefully take some precise daylight readings off a grey card as my initial reference. Having done this, I'd carefully reset one pot, checking and re-checking for improvement. If no improvement or a decrease in accuracy, reset pot to original setting and try next pot and so on.
Assuming you can achieve an increase in accuracy at a higher EV, you then need to repeat the process at lower EV levels. Using the same light source, so, assuming daylight, initially, this means moving into some shade and repeating the measuring process above. If low EV level accuracy is not good, you will then need to check the other pots in turn as mentioned above, to inspect the effect of adjusting these.
I'm quite fond of the MTL series and their kin. They're not perfect, but I handle a lot of second hand equipment which I unearth from various sources, and I've had much better luck with the functionality of 1970s Prakticas than I have with Eg. 1970s Pentaxes. Including their meters, which are often still working and accurate in my experience. But bear in mind that the camera is now several decades old, and that whilst you should endeavour to achieve perfect accuracy, this is a standard which may not realistically be possible with an as found vintage camera. Ultimately, you may have to settle for acceptable performance across the medium to high EV range (being the more commonly experienced imaging light range for most people) and accept that at lower EV, the meter is no longer optimum.
Also bear in mind that MTL5Bs, whilst worthy cameras, are not exactly rare or expensively priced, and, if having a 100% functional light meter is a must for you, you may be better served investing in another, fully functional example.
Cheers,
Brett
