Pictures from the Leica MR4 operating room

[drpops]

Guys,
I have a couple of non-functional mr4 meters. Having studied the circuit diagram and with nothing to loose.
I began my exploratory surgery. I have included some pics of the patient along the way.

I now know more about the intricacies of the Leica meter than I thought possible. The galvanometer that moves the needle is a very delicate with gorgeous minute watch springs to move the needle to the rest position.

With the exception of the galvanometer, this is a very robust design. It can be recalibrated to use 1.5 silver oxide cells.....but you then need to surgically change the 2 resistors that are in the battery check loop.

I would really love to change to a bar graph output that would indicate exposure.....that galvanometer is just too fussy.

I will keep you guys abreast of my progress.

David

 

[robklurfield]

you are brave. of course, you did say, nothing to lose. let us know your patient's progress.

 

[t.s.k.]

....or your patience 🙂

 

[Beemermark]

I used to repair these a long time ago. I do not remember the particulars but generally the problem was dirt on the contacts or galvanometer. Gentle cleaning with Radio Shack Contact cleaner usually restored them to life. The circuit has a Rosetta bridge which automatically compensated for the changing voltage in a silver oxide battery. Often they do not even need to be re-calibrated. Newer MR-4 meters show that either the PX-625 (silver oxide) or PX-13 (mercury) can be used. I think they are the best meters available for the M series with out a meter.

 

[drpops]

Well....as a great engineer once told me...they didn't work this morning....the worst that can happen is that they don't work tonight either. I also asked the patient if I could do the surgery and no complaint was lodged, so away I went.

But now I have measured data for the resistor ranges , those that are adjustable, the resistor values for those that are fixed. I also have the resistance value for the Cds cells when exposed to a wide bandwidth of lighting and intensity. The 2 Cds cells read nearly the same value with surprising consistency. I had frequently been told these cells fail when they get old. From my exploration I cannot confirm this parable.

Having this data and measured values, I can now calculate the expected current to be fed to the galvanometer. This now gives me the values should I wish to look for a substitute output metering system. i.e. digital bar graph in the window.

And yes my patience has reached a new height.

thanks for the input,
Dave

 

[jamato8]

I still have some mercury NOS for my meter. The one I have in the meter still works after 25 years, which I find hard to believe but it does. I keep the batteries about 6 in the frig. I don't know when I will ever use them.

 

[dof]

As a confirmed geek fanboy I have to say, "This thread is dope."

Please, please continue...

Back to your regularly scheduled doldrums

 

[drpops]

Well, now I have a great electrical schematic for the meter along with the values and ranges for the resistors, Cds cell and galvanometer. Now I know too much...the galvanometer is THE weak link in the system. I would really love to replace it with a simple Integrated chip with digital display.