Parts replacement

Malcolm Leonard radiorepair at MINDSPRING.COM
Thu Nov 21 02:33:46 EST 2002


So,polyester is not as suitable where high AC volts exist compared to
polypropylene?I am never going to get my head on straight about the
differences in these plastic dielectrics!Is there a good web site on this?
Thanks,
Malcolm Leonard
----- Original Message -----
From: <multi-volti at MULTI-VOLTI.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Wednesday, November 20, 2002 5:28 PM
Subject: Re: Parts replacement


> 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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