[R-390] Resistors

[Bob kb8tq]

Hi

Here’s a table that shows the effect:

http://g3ynh.info/zdocs/comps/refs/EPD_Botos.pdf <http://g3ynh.info/zdocs/comps/refs/EPD_Botos.pdf>

He does not go into the details, but the megahertz megohm stuff looks about right. Keep in mind that
I’m doing this all from memory of a paper I last read in 1973…..

Bob


> On Sep 25, 2017, at 1:53 PM, 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
> 
> 
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html