[R-390] Carbon comp resistors

[Jay Rusgrove]

2 cents...

Several years ago, while working on a military antenna development project that used carbon composition resistors (yes, an antenna with resistors!), I ordered in a number of different value carbon comps. These were new old stock resistors from Allen Bradley. The majority of resistors, of all values, were out of spec - all on the high side. A call to the supplier yielded a single page paper, MIL-R-39008C and paragraph 6.9 reads:

>6.9 Out-of -tolerance resistors. Resistance shifts due to absorption of moisture are inherent in carbon >composition resistors. Before being considered failures, out-of-tolerance resistors should be conditioned in a >dry oven at temperatures of 100 deg. C +/- 5 deg. C for the duration shown below prior to conducting >resistance measurements.

>Style RCR05: 25 +/- 4 hours
>Style RCR42: 130 +/- 4 hours
>All other styles: 96 +/- 4 hours   

>Resistors which continue to be out of tolerance after the above conditioning process shall be considered >failuires.

So before changing out resistors that are only mildly out of tolerance it might be a good idea run the radio for a period of time and let the resistors experience a good 'heat soak'. While you won't achieve 100 deg. C (hopefully!) inside the radio a several day power on stint may prove worthwhile. I wouldn't expect resistors that are wildly out of tolerance to work their way back in but those that are out 5 to 10% may. 

Conditioning resistors for the antenna project caused most to come back into tolerance. Values changes were noted at as much as 15% but most changes were in the 2 - 7.5 % range.

Jay W1VD