Parts replacement

[Malcolm Leonard]

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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