The instrument was simply dead, so I took it home to try to fix it. Accompanying the DMM was an excellent manual, with schematics and service instructions.
Taking a look at the motherboard showed some blackness around a LM317 linear regulator, so I took that out and measured the resistance at the mounting wholes of the regulator, to see if anything had shorted out. Resistance was as expected. I replaced the regulator with one I found in my junk pile. No difference. The regulator got really hot.
|Replaced regulator is the TO package located just below transformer.|
I measured some test voltage points that where described in the manual, and they all showed a somewhat lower voltage than expected. So the problem is probably with the power delivery.
I looked around the net and found some tips about modifying the 8050A so that it works without batteries. Hm, hadn't thought about it not working without it's batteries. Of course, Ni Cd batteries from the late 70's, early 80's, won't work properly today. Because of this, I had already taken the batteries out, thinking that it would work without them, as many modern pieces of equipment does.
To test if this was the problem i connected an external 4.8V power supply to the battery terminals, to emulate a working battery. Voila! The display lights up, but is missing some segments.
The segments are fixed by removing the LCD display and cleaning the contact surfaces. Here's the result:
|Fluke 8050A, with emulated battery, measuring the resistance of a resistor.|
So the question is now, do I replace the old batteries? Four new Ni Cd C batteries cost about 40€, which I should think is very close to the value of a working instrument... It would be nice to have the old instrument in original condition though. Modifying the DMM to work without batteries is pretty easy and is described here. I will have to think about what to do, and will return in part two to tell what I ended up doing.