[HBR] HBR-11; lessons learned to date

[Walt Hutchens]

Tom commented:
>> I think I read where the carbon comp resistors drift up in value
>> over time?? Well, last night, I pulled out a 100 Ohm 10% 1/2-watt
>> resistor from my junk box.? After cleaning off the gunk, my DMM
>> measured the unit at about 148 Ohms!

Marvin said:
> Yes they drift upward. Sometimes you can bring them back down by cooking 
> them in your wifes oven for an hour or two at 350 when she's off visiting her 
> Mom.
> 
> Another point: If you're using carbon comp resistors, even the NOS ones are 
> probably old and should be checked and possibly cooked.

Carbon composition resistors DO generally drift upward.  Here are a
few examples.   All data is 'marked'
followed by 'measured.'

NOS

1/2W

100k 113.6k
100k 106.8k
82k 87.4k
390k 404k
50k 60.8k
50k 54.5k
39k 41.8k
2.2M 2.4M

1w
18k 20k
30k 32.8k
2.2M 2.37M
2.7M 2.9M

Used -- National NCX 5

1/2W
47k 50.6k
470k 484k
10k 10.34k
150k 156.1k

Used -- Collins military subassembly 1950's

1/2W
100k 113.7k
220k 239k
220k 241.5k
68ohms 78.9ohms

Used -- 1960's military test set

1/2W
39k 42.1k
24ohms 29.2ohms
10k 11.22k
1.5k 1.607k
1 meg 1.15 meg

In other words, most carbon composition resistors would be perfectly
okay.  Two footnotes:

1. I have in the past measured resistors that were half again or
double the marked value, even though they looked okay.  The 1940's and
earlier resistors were substantially less dependable than 1960 and
after.

2. If overloaded, cc resistors will go way off value rather quickly.

3. I've measured dozens of cheap (Radio Shack) carbon film resistors,
including some with the paint partly burned off.  They generally
measure within 1-2% of marked value unless the overheating is so
severe that the carbon itself starts to burn.

For new construction I use the modern parts.  It's tough enough
getting anything to work without having to keep parts value drift in
the back of your mind as a possibility.

Walt Hutchens
KJ4KV