Parts replacement

[Gerald C. Lemay]

I don't have any specific data to present to the list, however while at
Measurements we experimented with various carbon composition resistors to
find useful upper frequency limits. Whatever "useful" meant I think was
SWR of less than 1.5 to 1 as obtained with a return loss bridge while
using the resistor as a termination. Two watt resistors we good up to
about 30 mHz, while 1/8 watt were useful up to about 150 mHz. I probalby
have the data stored away somewhere but it's been over 30 years. As for
metal film, they're not necessary all of spiral construction. Some look
like metal deposited cylinders with longitudenal (spelling?) slits cut
along the body of the resistors. Those resistors, I imagine, have very low
inductance. There is another type in which the metal coating thickness
varies. There is no way to tell which it is unless you have the
manufacturer's data sheet before you for inspection. The majority of
components today are surface mount and I would imagine that as time goes
on leaded components will all but disappear. Maybe the place to look for
information on what I would call "RF friendly" components is in the
various trade publications often available for free to those "qualified"
individuals. I used to get a ton of them until I deceided to cancel most
of them. I hadn't done any RF work professionally for a couple of years
and the pile of unread magazines was getting pretty high.

> My opinion only:
>
> Carbon composition resistors are getting harder to find. Replacing them
> with carbon film or metal film is fine if you're an audio guy and want
> lower noise, etc. But if you're an RF guy (HW-12...yup, RF gear), the
> modern film resistors are usually laser trimmed in a spiral pattern,
> which creates a more inductive resistor than the blob of carbon
> 'composition'. Indeed, (how literary sounding, eh?) a concern in the
> 'old days' at high frequency was the capacitance between the ends of
> carbon composition resistor terminals. I can't visualize CC resistors
> having more capacitance
> than a spiral cut film one, but I guess the issue was that at some
> frequency, every component begins to look less ideal.
>
> Spiral-cut resistors probably have higher inductance and thus might
> affect an RF circuit...not enough to make it work incorrectly, but
> enough to make some measurable difference.
>
> If your CC resistors are drifting, you may not have much choice. A
> little inductance added is better than 150% extra resistance!
>
> Capacitors...you have to be careful making changes...you have to know
> what you're doing...like not trying to use hi-k (or most other) ceramic
> caps where frequency stable silvered micas were, not using polyester in
> place of polypropylene if high ac voltages are present, and other
> things.
>
> In frequency-stable stages, there might be a mix of ceramic discs with
> various temperature coefficients...changing the mix might give you
> different drift characteristics.
>
> Enough already....
>
> Murray
>
>
>
> ----- Original Message -----
> From: "Hunt, Keith" <keith.hunt at SFCC.EDU>
> To: <HEATH at LISTSERV.TEMPE.GOV>
> Sent: Wednesday, November 20, 2002 12:06 PM
> Subject: Parts replacement
>
>
>> Is there any advantage on using higher quality caps and resistors in a
>> HW 12A rebuild?
>> Many Thanks !
>>
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