Tesa Modul - old swiss lenght measuring instrument

[asd99]

I got this Tesa Modul electronic lenght measuring instrument and 2 inductive probes to go with it. However it didn't work at all after powering it up. After disassembly I found a charred resistor inside, which actually seemed to still function, however the tantalum cap next to it was shorted to ground. I replaced the cap, but left the resistor be, since it seemed to have quite an exact resistance of 130 ohms. The instrument now somewhat works with both channels, but the ranges aren't quite right. Even with the smallest range the scale only moves a single interval (in this range 0.1um) after totally pressing the probe in, which is half a cm or so. There's a knob per channel for +/-, which is supposed to change the direction to which the scale should move, for maths with the two probe inputs. This works, so the signal must be getting atleast somewhere properly. The offset is also pretty random, and the knob for it doesn't seem to work.

The weird thing is that I couldn't really find any symmetry across the pcb to differentiate the channels, which means most of the stuff there is probably common for both the channels. This might explain why the channels behave the same.

I went around checking the transistors' voltage drops and didn't really find anything out of the ordinary. Furthermore, there's a 741 opamp and MC1439G opamp, the rest is mostly caps and resistors.

Is it possible to somehow find out what the charred resistor's value is supposed to be? What else is there to look for?

[tggzzz]

Is it possible to somehow find out what the charred resistor's value is supposed to be? What else is there to look for?

That's a thoughtful post, with decent photos, so it is worth trying to help you

My next step would be to draw the schematic for the components connected to the resistor. Go as far as power supply rails, whatever is driving this part of the circuit, and whatever is being driven by this part of the circuit. Keep an eye out for the "design patterns" involving individual transistors, e.g. common emitter amplifier, long-tailed pair, inverter, constant current source etc.

It looks like the resistor is connected to the BC108's base, in which case it is probably not critical, but 130ohms would be rather low for many configurations.

[MadTux]

I have this tesa box, looks different but the inside might be similar:
http://img.directindustry.com/images_di/photo-g/24600-3130255.jpg

BTW, whatever you do, don't attempt to disassemble the probes. I got a broken probe (ferrite rod was broken) along with several working ones and tried to dissasemble it. The distance measurement rod has a linear bearing on the front and if you decide to pull that rod out, all those small 0.3mm or so balls will fall out. You will have a hard time putting that back together, even if you manage to catch all the small balls

[asd99]

Do you by any chance have a manual for your instrument, since there's not much out there for mine? They seem similair so it would likely be relevant for both. If not, mind simply explaining the workflow for using it, as in what would you do setting it up to get some measurements out of it.