David wrote:
>> From: Ian White GM3SEK <gm3sek@ifwtech.co.uk>
>> Date: 2008/12/09 Tue AM 08:39:07 GMT
>> To: amps@contesting.com
>> Subject: Re: [Amps] Fwd: rx noise?
>>
>> David Robbins K1TTT wrote:
>> >I think the noise in that one is solved... take a look at the scope pic
>> >for a 12v regulator with a 2v p-p oscillation on it at:
>> >http://wiki.k1ttt.net/2008%20Maintenance%20and%20Upgrade%20Blog.ashx?NoR
>> >edir ect=1#Bottom That's one repaired, another one tested clean for
>> >now, and 5 more to go.
>> >
>> Glad you found the problem, Dave. By coincidence(?) someone recently
>> found a similar problem with a 78xx in a GaAsFET preamp that had been OK
>> for years, but then decided to become unstable.
>>
>> Any thoughts about what changed to trigger the instability? (Aside from
>> it being CQWW weekend, and the regulator having poor stability margins.)
>>
>Did the regulator have proper bypass in the first place or was this a
>problem just waiting to pop out? Capacitor-to-regulator distance is
>critical. I've never known a regulator go funny if it was properly set
>up in the first place.
This came from a source I would trust - an experienced moonbouncer who
keeps a very close watch on his noise level, and immediately noticed
when that level increased.
The e-mail exchange was only copied to me, so I didn't keep it, but
could easily enough ask for repeats.
>As Pease would say "do what the manufacturer tells you or all bets are
>off."
Yes, absolutely. Different types of regulators require different values
of input and output capacitors. Values recommended for one type may
cause another to oscillate, or at least move closer to the edge of
stability.
The interesting question is why instability might suddenly appear much
later, maybe after several years. One possibility is electrolytic
capacitors drying out and losing capacitance, tipping a marginally
stable circuit over the edge.
Dave's experience showed that temperature may be a factor as well.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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