[Milsurplus] Carbon Comp Resistors: The Darkness Gathers?

[Bruce Gentry]

The worst, but maybe not only, resistors that rise in resistance look 
like ordinary carbon resistors with a few distinguishing 
characteristics. First, the ends of the body "cylinder" are rounded 
instead of squared off. Next, they are of somewhat smaller size than 
others. They have a "woody" textured surface instead of shiny. National 
used them by the train load in their post-war rigs like the HRO 50 and 
60, as well as NC-183 and plenty others. I have never seen a lower 
resistance one in a cathode circuit go bad, or as a control grid 
resistor. Thet sometimes fail as plate resistros, but are almost always 
bad if they are the screen resistor.  On my HROs, the IF amplifiers had 
about fifteen volts on the screens, and the IF gain was aweful. After 
finding over half of them bad, I changed them all out. I agree, 47K is 
one of the worst for going high in resitance. Regardless of value, they 
also have a way above average chance of being noisy.  These style or 
resistors were used in British rigs such as Eddystone as well, and 
resistors in ceramic tube bodies with color bands are even worse- 
British, Japanese, or USA.  I test all the older carbon resistors I use, 
they are almost always within tolerence and work fine. There is no doubt 
the new ones are immensely better, and a 1/2 watt today is only a little 
bigger than a 1/4 watt carbon resistor, so you can order a 1 watt to 
look more like the original.   I haven't gotten any Rhode & Schwarz tube 
rigs yet, so I don't know how good late 40s -60s German resistors are 
today. If anyone has a tube  Rhode and Schwarz or Telefunken receiver 
-1950 or later ONLY!!!- they are willing to part with, please contact me 
off list.

       Bruce Gentry, KA2IVY


On 7/2/18 4:15 PM, David Stinson wrote:
> I've always heard that the carbon comp resistors
> that fail Hi-Z most often are those in the
> Mega-Ohm range ("green band").  Oddly, over the
> last four major projects:  TCS, ATD, RAX and now
> SCR-522, haven't found that to be the case.  The
> resistors most often failed-High are in the
> 15K-500K range, with the #1 "bad guy" being 47K
> Ohms, followed by 470K.  I remember changing one
> or two Mega-Ohm value resistor in the last three
> projects.  Every 47K resistor in the last TCS
> receiver and in the ATD had to go.
>
> Which brings me to this:
> I've got piles of NOS Carbon Comps that have been
> following me around for eons.  The majority of
> these have started reading Hi-Z by 5% or more.
> Many by much more.  I just threw a half dozen NOS
> 100K away because they read from 107K- to
> 122K-Ohms.  Other values are showing similar
> drift.
>
> Has twilight come for Carbon Comp resistors?  I've
> stopped buying them at hamfests and use only the
> newer, ceramic looking thingies for projects now.
> What do you think?
>
>
> GL OM ES 73 DE Dave AB5S
>
>
> ______________________________________________________________
> Milsurplus mailing list
> Home: http://mailman.qth.net/mailman/listinfo/milsurplus
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Milsurplus at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>