[R-390] When to replace carbon composition resistors (CCR)

Fri Nov 15 11:50:31 EST 2024

Hi

The resistors in the picture are carbon film rather than carbon comp. That is the main
reason they look odd. It also is why the size to power rating is a bit different than what 
you are expecting to see.

Cost and availability wise, they are “what you use”. Authenticity wise, no they don’t look
like the original parts. Assuming they came from reasonable sources and the part number
is correct, they should meet their stated power ratings. 

My guess is that the ones on the right in the photo are not the correct part. That could
be confirmed by digging into the data sheets and looking at the dimensions on the various
power ratings. A vernier caliper comes in handy for those sort of checks ….

Bob

> On Nov 15, 2024, at 11:35 AM, Barry Scott <72volkswagon at gmail.com> wrote:
> 
> Regarding replacement resistors, what are folks using these days?
> Lately, the resistors I've been getting from Mouser haven't been
> exactly what I'd like to use.  I'm attaching an example of some I
> ordered this week.  Both are 150-ohm, 1/2W, metal film; however, the
> ones on the right side are SO TINY!  It's hard to imagine those can
> actually dissipate that much heat but I presume they can.  My issue is
> just the overall look of them.  If those are placed in the circuit
> where the old ones were, they would look so much out of place.  I
> ordered two different types and will be using the ones on the left as
> they're at least somewhat larger.  I hesitatingly used one much like
> it in a Tek scope and it worked fine.  It just looks weird.  I
> hesitatingly used one much like those smaller ones in a Tek scope and
> it worked fine.  It just looks weird but I wasn't finding much else
> for the specs I needed so I used it.
> 
> I know I can order from sources like eBay and get packages that are
> more in line with the sizes of their older counterparts, but I don't
> like to get those as it's questionable as to where they're sourced,
> their reliability (which is probably better than I'm thinking they
> are), etc.  Just wondering what others might be using.
> 
> Thanks,
> Barry - N4BUQ
> 
> On Mon, Nov 11, 2024 at 2:04 PM Jim Whartenby via R-390
> <r-390 at mailman.qth.net> wrote:
>> 
>> It is my understanding that after the CCR was manufactured, it went through a sorting for tolerance.  All resistors that were within 5% of the mean value got the gold stripe.  The silver stripe was given to all that were above +/- 5% but at or below +/- 10% of the mean value.  The remaining were given no stripe if they were within 20% of the mean value.  So there are no 10% resistors that are within 5% of the mean value unless they have drifted to this value over time and temperature.
>> 
>> Jacques makes a reasonable suggestion that resistors that are 20% or more above or below the mean value are suspect.  The accumulative effect of component tolerance, "the tyranny of numbers," as it was described in the mid 1950s, was the downfall of tube based electronics. The more complex the circuit, the more prone it was to a failure.  This led to the military specifications for all types of circuit components from vacuum tubes to resistors.
>> 
>> I believe that all of these military component specifications have been rescinded such is the quality of modern components.  So if one is inclined to replace a CCR, the better resistor to use is the carbon film resistor.  They are more stable in value and less affected by time and temperature.
>> 
>> One last thing, the least reliable component is the vacuum tube followed by the capacitor.  I will always check tubes first, follower by a visual inspection.  Capacitors are eyeballed next followed by resistors.  Then it is time to troubleshoot.
>> 
>> Regards, Jim
>> Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy
>> 
>>    On Sunday, November 10, 2024 at 09:05:07 PM CST, Jacques Fortin <jacques.f at videotron.ca> wrote:
>> 
>> Hello Barry,
>> 
>> IMHO, if all the resistors in your RF deck are at ~20% higher than their
>> marked value, it can be well the cumulative tolerance of all those making
>> trouble more than only one individual value.
>> Sorry to deliver the same message again, but every part which is not 5%
>> close to the stocklist value should be changed.
>> The Solid Carbon resistors may also develop a funny behavior in the sense
>> that they measure a given value when they are cold which drifts to a higher
>> one when the current passes thru.
>> Nothing that was used in a R-390 at the build time was expected to be still
>> OK 70 years later...
>> 
>> 73, Jacques, VE2JFE in Montreal
>> 
>> -----Message d'origine-----
>> De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
>> part de Barry Scott
>> Envoyé : 10 novembre 2024 20:56
>> À : R-390 Mailing List <r-390 at mailman.qth.net>
>> Objet : [R-390] Cathode Resistors in 1st and 2nd Oscillators in R-390/URR
>> 
>> I mentioned that the screen/plate resistors in my R-390/URR are a bit high.
>> Today, I removed the oscillator deck and measured those.  The 68k is about
>> 71k so not too bad.  The 82k is at 87k so a bit worse but I'm still not sure
>> how much that affects the performance.
>> 
>> What I did find is the 150-ohm cathode resistors are both at 190 ohms.
>> I presume that may be the primary cause of the low output for both of the
>> oscillators.  The 2nd mixer has a bit lower voltage at the test point than
>> does the 1st mixer so that 82k may still need to be replaced although it is
>> still in tolerance.
>> 
>> Thanks,
>> Barry - N4BUQ
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