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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