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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?

Reply to
N_Cook
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"Nutcase _Kook"

** Crack heads - the lot of 'em.

But not near so obnoxious as CRACKPOTS like you !!

Fuckwit.

... Phil

Reply to
Phil Allison

" Nutcase _Kook"

** Crack heads - the lot of 'em.

But not near so obnoxious as CRACKPOTS like you !!

Fuckwit.

... Phil

Reply to
Phil Allison

Isn't the "powdery" stuff just cement? I've worked with those things,

3 / 5 watts ,33 ohms, .22 ohms etc... in BGW, Soundcraftsman, Peavey and several others and can honestly say that I don't connect the cement decomposing over a period of time with the device running out its design parameters.
Reply to
Meat Plow

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How is the resistance of the various units, within specs?

Reply to
hrhofmann

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

Reply to
bob urz

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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

Reply to
N_Cook

I like it!

Mark Z.

Reply to
Mark Zacharias

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