I posted this for opinions on S.E.R so apologies to those like Jeff who frequent both groups. SED's got a wider readership, so I thought I might as well ask here as well. I'm just wondering if this damage was due to over-current or just a manufacturing defect. It still reads its advertised value of 1200 ohms. What does the Panel think?
Its probably a bubble in the coating of the resistor. The resin seems to rise up close to the crater without being cracked or charred. John
If we assume a relatively small overload of 10 W was sufficient to caue the damage, that corresponds to over 100 v across the resistor. Where would a voltage like that come from in that machine?
In practice I would expect is would take an even greater instantaneous overload than 100v to instantly blow a hole, rather than a sustained overload which would gradually cook the whole thing.
The highest voltage in that thing is 90V p-p at 100kHz from the bias oscillator, but it's nowhere near the AGC.
If it is a carbon film resistor, you could be seeing the result of a massive over-voltage that created a hot channel through the carbon film.
The hot channel could get very hot and boil the paint directly above the channel, but once the volts went away the carbon film would cool down again, and the carbon in the hot channel would have exactly the same resistance as it had had before.
This is the reason that carbon film and other negative temperature resistance materials aren't used in intrinsically safe circuits.
You want something that will get blown away and create an open circuit, rather than than something that can dissipate an unlimited amount of energy after it has been overloaded.
...and the oscillator would stop if it were loaded with 1200 ohms. There just isn't that sort of instantaneous power avaialble in the machine to blow a resistor like that, so you need to look for a different type of fault altogether.
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