Monday, July 22, 2013

Fluke 8050A Repair, Part 1

We scrapped an old Fluke 8050A, Digital Multimeter. It's and old bench multimeter with 4.5 digits. This one has the built in rechargable battery option.

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.

Saturday, July 20, 2013

Finally, a proper workbench

i finally got a proper workbench installed at home! This is really good for me, since I usually only have an hour or so on my projects, after my kids are put to bed. Before I had all my equipment in a cupboard. So to start working, I needed to set everything up on the kitchen table. When I'd finished working, everything needed to be put back. So, not a lot of time for working...
 Now I've got a workbench where I can leave projects lying and work on them for five minutes, if that's what I have.

The metal detector is yet to be finished, I'll do that soon. Right now, as can be seen in the picture, I'm trying to fix a broken Fluke 8050A, which I salvaged from work. That will be in my next post.