[R-390] Megger and Capacitor Measuring

[Bob Camp]

Hi

There is a subtle, but well known by the 1940’s difference between carbon comps and most other resistors:

With carbon comps, they stay resistive as you go up in frequency. The resistance drops, but there is no stray C.

With a metal film, you have a stray C that parallels the resistance. The end caps make up the C, the body has the R.

The impact in a circuit is more pronounced with both types as the resistance goes up at a given frequency. 

If you are designing something like an RF circuit the difference could matter. On a DC or audio circuit - forget about it. The only place in a radio it might matter would be loading resistors across a tuned tank. They are rare enough that I would not loose a lot of sleep over the issue. 

Bob

> On Oct 25, 2014, at 7:39 PM, Perry Sandeen via R-390 <r-390 at mailman.qth.net> wrote:
> 
> List,
> 
> I agree with Charles that using a megger is generally not a
> good way to trouble shoot a radio. It is
> cumbersome and uses up an enormous amount to time. Taking
> one end out of the circuit risks parts breakage or re-soldering problems.
> 
> The true value of a megger would be in checking new caps for
> a potential
> 
> My position is if one is working on radio like the *A* or a
> SP 600 the most time and cost effective way is a whole sale replacement of all capacitors
> EXCEPT some silver mica’s. (The new type, not the old *postage stamp* units.
> 
> I do this because if one , two or five out of 50 BBOD’s
> check OK, how much longer than their current 50 + years are thy going to last?  And generally speaking the new caps are
> smaller sized.
> 
> I believe the same for all the carbon resistors.  Many are known to drift.  It is now even more cost effective to replace
> them as Mouser sells the many Vishay .5W 1% odd value metal film resistors that
> can be used for 24 cents for single unit with no minimum quantity. Xcion 1%
> metal film .5W are 50ppm and 15 cents each but one has to buy them in units of
> 10. However they are 100 for $6.50 which is ideal if you have several radios to
> rehab.
> 
> Several less obvious benefits  comes with the MF resistors. Circuits using
> them will maintain a more stable condition. I’m a great believer of removing
> all variables possible. Also as a general rule they are smaller which makes for
> easier
> 
> I readily concede that the process I do takes far more time upfront.  However in the long run-years and years I’m
> way ahead.
> 
> Regards,
> 
> Perrier
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