| Electronics > Repair |
| Fluke 8845A 6.5 digit Multimeter smoking transformer |
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| Kleinstein:
The main purpose of the primay fuse is to prevent a fire after the transformer is blown or when used with the wrong mains voltage setting. The normal cheap fuses are not very accurate: they may blow just above the nomunal current, but could also last long at twice nominal. The tolerace is too large to really protect the transformer without-over sizing the transformer and use a relatively small fuse. It may help with the wrong mains setting, but subtile over load, like just 1 winding seeing a near short (e.g. Ta capacitor causing a short) can over load it. With frequent hot - cold cycled the transformer should also be the primary source of failure, like getting some windings shorted out. |
| strawberry:
fast /slow /delayed and cheap made out of ill-proper materials like aluminum |
| murta79:
Answering and closing my old thread... I was cleaning out some storage at home and found this old broken instrument. I decided to have one last look at it and see if I could fix it before scrapping it. This time I decided to take out the board from the casing since I was going to throw it away anyway. On the underside I discovered a burnt component, a rectifier for the +/- 20 volt supply rail. After a closer inspection it turned out not to be the rectifier itself that was burnt but the de-coupling capacitor before the rectifier. The capacitor was not only burnt, it was completely missing, as in, it had basically exploded and had left metal residues on the board and pads. I \$\Omega\$-measured across where the capacitor had been and it showed ~500 \$\Omega\$. I cleaned the board with acetone as much as I could and ohm-measured again, this time it read >5k \$\Omega\$. I figured that some of the capacitor residues must have left a low- \$\Omega\$path between the pads and this was why the transformer had started smoking, drawing too much current on the +/-20V rail (I also de-soldered the rectifier and measured it, it was OK). After that I re-soldered the rectifier back in place and powered on the unit, ready to pull the plug. To my surprise this time the smoke did not come from the transformer but from the underside of the board (I had to flip it right side up when powering it on). I immediately turned the power off and after that ohm-measured across where the capacitor had been situated. This time it showed almost a dead short ~6 \$\Omega\$. The PCB between the capacitor pads had charred quite a bit so I started to carve off the sot. When I have carved off and cleaned I ohm-measured again and to my surprise the measurement had climbed to ~100 \$\Omega\$. I repeated the procedure, carving out more and more of the charred PCB between the pad and every time the resistance kept climbing. I was basically scratching out charcoal until I hit the ground plane ~1mm down in the PCB, after cleaning with acetone this time the measurement showed infinite resistance. I again powered on the board and this time... no smoke! And the instrument seems to work just fine! :) :) :) Now the question is... Why had the ceramic capacitor exploded? Was it just component failure? |
| factory:
Did you take any pictures of the damage & repair? I'm not familiar with this Fluke DMM, but the HP equivalent is know for surface mount tants burning up, multilayer ceramic capacitors aren't much better, I've had them burn up in older things at work, they tend to crack & short internally. David |
| drhex:
Capacitor probably leaked and the board soaked up the dielectrics and got conductive. Had that happen a few times. Also, burnt board can be conductive, too. |
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