Well, its not an epidemic anyway. Just finished checking the other 6 of
those amps in the shack and none of them had any oscillation, at least when
first turned on cold. Perhaps as someone suggested something happened to
the input cap on that one.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: David Cutter [mailto:d.cutter@ntlworld.com]
> Sent: Wednesday, December 10, 2008 07:42
> To: k1ttt@arrl.net; Amps
> Subject: Re: [Amps] Fwd: rx noise?
>
> That makes it even more curious. The LM7812 is less susceptible to
> instability compared to some others and according to a NS application note
> does not need an output cap for stability, though I would always fit one
> to
> supply pulse currents. The only thing left is the input cap which went
> bad
> and your new ones corrected that.
>
> Curiouser and curiouser
>
> David
> G3UNA
>
> ----- Original Message -----
> From: "David Robbins K1TTT" <k1ttt@arrl.net>
> To: "Amps" <amps@contesting.com>
> Sent: Tuesday, December 09, 2008 11:38 PM
> Subject: Re: [Amps] Fwd: rx noise?
>
>
> > It's an LM7812 on a decent size heat sink standing up on the board.
> >
> >
> > David Robbins K1TTT
> > e-mail: mailto:k1ttt@arrl.net
> > web: http://www.k1ttt.net
> > AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> >
> >
> >> -----Original Message-----
> >> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On
> >> Behalf Of David Cutter
> >> Sent: Tuesday, December 09, 2008 23:05
> >> To: k1ttt@arrl.net; amps@contesting.com
> >> Subject: Re: [Amps] Fwd: rx noise?
> >>
> >> Fascinating stuff. I couldn't find a reference to exactly what
> regulator
> >> is
> >> used. 7812 or variable regulator?
> >>
> >> David
> >> G3UNA
> >>
> >>
> >> ----- Original Message -----
> >> From: "David Robbins K1TTT" <k1ttt@arrl.net>
> >> To: <amps@contesting.com>
> >> Sent: Tuesday, December 09, 2008 10:50 PM
> >> Subject: Re: [Amps] Fwd: rx noise?
> >>
> >>
> >> > p.s. as I noted in my blog I have heard similar wandering birdies
> >> > before
> >> > but
> >> > never had them land on a band like that one did during cqww. I have
> >> been
> >> > doing other repairs on those amps so they have moved around from band
> >> > to
> >> > band periodically, its possible that amp has been oscillating for a
> >> while
> >> > and it just never got caught... add in temperature sensitivity and
> >> > apparently some line voltage sensitivity and it may have just been
> hot
> >> > enough and the voltage at just the right spot that day. As I said,
> >> > when
> >> I
> >> > tried to reproduce it I could only get it in the 20m band for a short
> >> > time,
> >> > most likely it was drifting all over the place.
> >> >
> >> >
> >> > David Robbins K1TTT
> >> > e-mail: mailto:k1ttt@arrl.net
> >> > web: http://www.k1ttt.net
> >> > AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> >> >
> >> >
> >> >> -----Original Message-----
> >> >> From: d.cutter@ntlworld.com [mailto:d.cutter@ntlworld.com]
> >> >> Sent: Tuesday, December 09, 2008 10:33
> >> >> To: Ian White GM3SEK; amps@contesting.com
> >> >> Subject: Re: [Amps] Fwd: rx noise?
> >> >>
> >> >> 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. As Pease would say "do what the manufacturer
> >> >> tells
> >> >> you or all bets are off."
> >> >>
> >> >> David
> >> >> G3UNA
> >> >>
> >> >> >
> >> >> > 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.)
> >> >> >
> >> >> >
>
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