[R-390] Resistors

[Stan Gammons]

Thanks for the comments on the resistors.  Sounds like it's Ok to use metal film anywhere but the RF deck. Perhaps it would be "best" to replace any that need replacing with the Ohmite  carbon composition resistors? The OD/OF/OY series depending on what wattage the resistor is.


73

Stan
KM4HQE

On September 25, 2017 12:53:31 PM CDT, Charles Steinmetz <csteinmetz at yandex.com> wrote:
>Bob wrote:
>
>> The gotcha is that the 270K resistor in the circuit at 30 MHz is
>actually presenting 150K. The circuit was designed
>> for 150K rather than the 270K. It shows 270K since that’s what is
>marked on the carbon comp resistor. If you are
>> going to replace them, you need to take that into account.
>
>That differential is more than 10x greater than any reactance effect 
>I've ever observed in an actual, new-condition CC resistor.  There may 
>be CCs that measure like that, but not quality resistors in new
>condition.
>
>In the actual CC resistors I've tested, a 270k CC will operate within
>5% 
>of nominal through HF into VHF.  Note that most CCs in boatanchors were
>
>originally rated +/- 10%, with a few rated +/-5%.  Also, note that a 
>390/390A will work just fine (and meet spec) with resistors in most 
>positions at +/- 50% (which is even further from nominal than the 
>270/150 example given, so even if someone did happen to get a 270k CC 
>that exhibited 150k at 30MHz, the radio would almost certainly still 
>meet spec.  But, as noted above, things are not *nearly* that bad in 
>real life.)
>
>Best regards,
>
>Charles
>
>
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