| Electronics > Repair |
| Fluke 8845A 6.5 digit Multimeter smoking transformer |
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| murta79:
Hello, I got this Fluke 8845A that I am troubleshooting, from the scrap bin :) The symptoms thus far is as follow. When plugged in the first time the screen was showing "OVERLOAD" and the transformer got very hot, to the point that streams of smoke came out and I quickly shut off the power. My first thought was that the voltage selector was in the wrong position, but it is in 240V which is correct here in Sweden. I have done some basic troubleshooting: - I disconnected the power-plug that goes to the FPGA-side of the board and measured the voltages on the "communication/VFD-display-side", these voltages looks OK (...and also no smoke when I only have this side connected). - I then tried and connect only 1 of the secondary voltages on the FPGA-side at a time. I found that if I leave +/-20 volt disconnected (violet cables) the instrument stops showing "OVERLOAD" (The transformer still gets hot but at least no smoke this time either). - Voltages on the secondary side of the transformer: Violet_1 = 16,4VAC Red_1 = 13,9VAC Violet/White = 0V Violet_2 = 12,3VAC Red_2 = 12,6VAC Measuring resistance (on PCB) between +/-20V->0V gives Mohm readings and between +/-15V->0V gives 50-200Kohm readings (fluctuating). Grey_1 = 3,7VAC Grey/White = 0V Grey_2 = 3,7VAC This is as far as I have come. I was hoping someone here has some good ideas on how to proceed in my troubleshooting. Regarding the transformer, I´m pretty sure some of the windings on the +/-20 volt has shorted out, probably because of a faulty component on that rail, maybe also at the same time shorted windings on the +/-15V ? The question is, is it necessary for me to buy a new transformer to continue the troubleshooting or can I use the one I got until I figure out what other components are faulty on the board and then buy a new transformer? |
| murta79:
..Forgot to mention, I did look trough a bunch of other posts on this forum regarding this multimeter but I could not find any with similar problem symptoms as mine. If anyone knows of any posts that can help me (on this forum or other), please notify me. :D |
| andy3055:
Check the power supply caps and diodes first. Something is shorting. |
| strawberry:
add some load (resistor ~1k/incandescent bulb ~50W) in series with multimeter cord to save transformer from destruction observe voltage drop |
| bdunham7:
First, the 'OVERLOAD' warning is very common when the unit is turned on with nothing connected. It is just the result of noise and autoranging, a quirk to be sure but one that should be easily resolved by shorting the inputs or selecting DCI (current) immediately upon turning it on. Not having power to the guarded power supply also might cause the 'OVERLOAD' to go away, but for now I would just not worry about it. If the transformer is smoking, can you measure the amount of current the unit is drawing? And if that is excessive, what happened to the fuse? Does the unit appear to have been worked on or is there any physical damage? Your transformer has not likely been improved by smoking, but if the current draw is normal (low) when the secondaries are disconnected, then it may be OK enough for now. Possibly the unit was not set for 240V, connected to 240 volts long enough to overheat or overvoltage and damage a diode, regulator or other component in the guarded power supply and then finally set to 240 volts, but too late and without replacing the fuse--which is 1/4A slo-blo for 120V but only 1/8A slo-blo for 240V. Check which you have...if you see a 1/4A fuse in there, this is the likely scenario. If it sort of works but draws too much current, you'll have to map out the guarded power supply because AFAIK there are no schematics available. I've worked on these a bit, but don't remember the exact setup for the guarded PSU. Perhaps post some good photos? |
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