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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Emitter resistor query
Those white ceramic cased ones about 3 watt in the form of a rectangular
block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. eg 6 power transistors in "parallel", one has a lot of powder around it on the pcb, one has some around it and the other 4 are clean. No fan air directed in that area to blow anything away. Both have the powder deposited not along the long sides but at the end away from any possible heat radiated from the transitor/ heatsink, perhaps just coincidence. Can anything be inferred from these deposits/ associated transistor? I've seen failed o/c ones of this type with a crack in the casing, does it mean the powdery ones have been overheated and will fail imminently or an indicator that the associated transistor is taking too much of its share of the current? |
#2
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Emitter resistor query
"Nutcase _Kook" Those white ceramic cased ones about 3 watt in the form of a rectangular block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. ** Crack heads - the lot of 'em. But not near so obnoxious as CRACKPOTS like you !! ****wit. .... Phil |
#3
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Emitter resistor query
" Nutcase _Kook"
Those white ceramic cased ones about 3 watt in the form of a rectangular block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. ** Crack heads - the lot of 'em. But not near so obnoxious as CRACKPOTS like you !! ****wit. .... Phil |
#4
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Emitter resistor query
On Dec 29, 5:09*am, "N_Cook" wrote:
Those white ceramic cased ones about 3 watt in the form of a rectangular block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. eg 6 power transistors in "parallel", one has a lot of powder around it on the pcb, one has some around it and the other 4 are clean. No fan air directed in that area to blow anything away. Both have the powder deposited not along the long sides but at the end away from any possible heat radiated from the transitor/ heatsink, perhaps just coincidence. Can anything be inferred from these deposits/ associated transistor? I've seen failed o/c ones of this type with a crack in the casing, does it mean the powdery ones have been overheated and will fail imminently or an indicator that the associated transistor is taking too much of its share of the current? How is the resistance of the various units, within specs? |
#5
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Emitter resistor query
N_Cook wrote:
Those white ceramic cased ones about 3 watt in the form of a rectangular block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. eg 6 power transistors in "parallel", one has a lot of powder around it on the pcb, one has some around it and the other 4 are clean. No fan air directed in that area to blow anything away. Both have the powder deposited not along the long sides but at the end away from any possible heat radiated from the transistor/ heat sink, perhaps just coincidence. Can anything be inferred from these deposits/ associated transistor? I've seen failed o/c ones of this type with a crack in the casing, does it mean the powdery ones have been overheated and will fail imminently or an indicator that the associated transistor is taking too much of its share of the current? The cased ones are just essentially wire wound resistors that are potted in a ceramic material. From age and heat, the potting compound has broken down and is turning to dust. This in itself does not mean the resistor is bad. measure it. if it measures in the ballpark its still usable. If you feel bad about it, replace it. Now, i have seen them in audio power amps that look burnt and have the lettering burnt off or disfigured. Usually when this happens, the outputs are shorted and the emitter resistors are open and shot. bob |
#6
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Emitter resistor query
bob urz wrote in message
... N_Cook wrote: Those white ceramic cased ones about 3 watt in the form of a rectangular block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. eg 6 power transistors in "parallel", one has a lot of powder around it on the pcb, one has some around it and the other 4 are clean. No fan air directed in that area to blow anything away. Both have the powder deposited not along the long sides but at the end away from any possible heat radiated from the transistor/ heat sink, perhaps just coincidence. Can anything be inferred from these deposits/ associated transistor? I've seen failed o/c ones of this type with a crack in the casing, does it mean the powdery ones have been overheated and will fail imminently or an indicator that the associated transistor is taking too much of its share of the current? The cased ones are just essentially wire wound resistors that are potted in a ceramic material. From age and heat, the potting compound has broken down and is turning to dust. This in itself does not mean the resistor is bad. measure it. if it measures in the ballpark its still usable. If you feel bad about it, replace it. Now, i have seen them in audio power amps that look burnt and have the lettering burnt off or disfigured. Usually when this happens, the outputs are shorted and the emitter resistors are open and shot. bob Unfortunately my m-ohm-meter is 10V pk-pk. Using Fluke DVM and "time division ratioing" of last digit I cannot see any ohms difference between them, so all about 0.3R +/-0.05R presumably. Of course if they were vitreous Re there would be no tell-tale dust and also the other 6 on the other channel that are inverted so dust would drop into case I last saw this amp a year ago and placed some 4mm diameter irreversible thermochromic labels on the output transistors and the 2 associated with the dusty Re are a bluer shade of cyan , so having been hotter. This time a preamp problem so placed new dots on the devices, tightened all bolts and will leave at that. A band that thinks its a planet-bound Disaster Area and has survived a year of intensive use with no power amp problem so if it ain't broke, don't poke |
#7
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Emitter resistor query
"N_Cook" wrote in message
... bob urz wrote in message ... N_Cook wrote: Those white ceramic cased ones about 3 watt in the form of a rectangular block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. eg 6 power transistors in "parallel", one has a lot of powder around it on the pcb, one has some around it and the other 4 are clean. No fan air directed in that area to blow anything away. Both have the powder deposited not along the long sides but at the end away from any possible heat radiated from the transistor/ heat sink, perhaps just coincidence. Can anything be inferred from these deposits/ associated transistor? I've seen failed o/c ones of this type with a crack in the casing, does it mean the powdery ones have been overheated and will fail imminently or an indicator that the associated transistor is taking too much of its share of the current? The cased ones are just essentially wire wound resistors that are potted in a ceramic material. From age and heat, the potting compound has broken down and is turning to dust. This in itself does not mean the resistor is bad. measure it. if it measures in the ballpark its still usable. If you feel bad about it, replace it. Now, i have seen them in audio power amps that look burnt and have the lettering burnt off or disfigured. Usually when this happens, the outputs are shorted and the emitter resistors are open and shot. bob Unfortunately my m-ohm-meter is 10V pk-pk. Using Fluke DVM and "time division ratioing" of last digit I cannot see any ohms difference between them, so all about 0.3R +/-0.05R presumably. Of course if they were vitreous Re there would be no tell-tale dust and also the other 6 on the other channel that are inverted so dust would drop into case I last saw this amp a year ago and placed some 4mm diameter irreversible thermochromic labels on the output transistors and the 2 associated with the dusty Re are a bluer shade of cyan , so having been hotter. This time a preamp problem so placed new dots on the devices, tightened all bolts and will leave at that. A band that thinks its a planet-bound Disaster Area and has survived a year of intensive use with no power amp problem so if it ain't broke, don't poke if it ain't broke, don't poke I like it! Mark Z. |
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