[R-390] Resistors

[kc9ieq]

Is it possible there is also high DC potential across this resistor, additionally lowering the effective value being carbon comp?  I'm not familiar enough to know where this resistor is without pulling up schematics and hunting.
Regards,Chris 


Sent from my SMRTphone
-------- Original message --------From: Charles Steinmetz <csteinmetz at yandex.com> Date: 9/25/17  12:53 PM  (GMT-06:00) To: r-390 at mailman.qth.net Subject: Re: [R-390] Resistors 
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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