EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: jpenn on August 12, 2014, 07:03:21 pm
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Hello all.
I recently acquired an old piece of test equipment and when disassembling it I discovered something very strange. At least to me, I've never see this before. When I opened the case I saw something glistening on the ends of some resistors connected to a selector switch. When I looked at the resistors with a magnifying glass I could see they were tiny glass like and very prismatic bubbles. I took a thin wire and touched one and it cracked like thin crystal. My first thought was that it was caused by overheating of he resistors but the resistors looked fine. Does anyone have any idea what these bubbles are?
Sorry about the image quality.
Thanks
John
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Very odd. Never seen that before. Maybe an additive in the case material which leached out because of heat??? :-//
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what the ...
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Carbon composition resistors that got hot, and the wax impregnation has melted slightly and formed bubbles with the gas of the lighter volatiles evaporating. Not a problem, it just is a property of the resistor that the wax will melt when hot, and this one had some lighter volatile parts and blew bubbles. Normally the wax is there to protect the carbon composition from moisture, and will soften with heat. It just says they were run at slightly above maximum power for a while, so they dumped the wax. You can also see that the colour bands have discoloured slightly from this, the nominal 22k 10% bands have gone slightly brown from the light toasting. If you are worried you really will just replace them with higher wattage units, likely 1W units.
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Thanks SeanB
Sounds like a logical explanation. I'm not to concerned with the condition of the resistors. I bought this unit for parts anyway. I'll keep what tests good. I was just curious as to what the bubbles were.
Regards
John