Take a look at the choke in these photos.
http://s863.photobucket.com/albums/ab196/knicely1/LK%20500/
That is what my choke looked like when I purchased my LK500.
I would say you should replace the choke first. I rewound mine. I counted
135 turns of #26 enameled wire when I unwound the damaged choke. That did
not work out so well for me. The tube arced on 160 after the rewind and
with all new tubes. I then replaced the choke with am MFJ choke and all has
been well since. Before I put the MFJ choke in, I scanned the rewound choke
and the MFJ choke with my AIM 4170 antenna analyzer and found that the choke
I rewound was passing rf on 160 where the MFJ choke was not.
Ken
On Sat, Dec 11, 2010 at 1:53 PM, Gary Hinson <Gary@isect.com> wrote:
> Thanks Ken - and sorry for the long pause.
>
> I've noticed that the anode choke is cooked so that will be replaced "soon
> as" as we say in ZL. I have one on order from Ameritron, now MFJ.
>
> Meanwhile, I have swapped out the tubes one-by-one, and replaced all the
> 10nF coupling and decoupling caps under the tube bases but still have the
> problem, so I agree that it might be taking off.
>
> Aside from the -ve grid current episodes, I'm getting no output on 160m and
> on 80m the load and plate caps both max out at 100. I have changed the
> padder capacitors previously. I've checked the bandswitch with my
> multimeter thinking that it might be not selecting the padders but it
> appears the switch contacts are OK - they look normal with no obvious signs
> of arcing or melting. Maybe this issue is related to the anode choke
> problem as well - it could just be way off tune I guess. I've even idly
> thought of removing and dismantling the switch, turning the wafers to use
> the contacts that are currently unused, but it's quite a lot of work to get
> it out of there.
>
> I presume the choke was originally a nice, even winding on the ceramic
> former. It is now bundled up in the blackened area, so my guess is that
> the
> wire has been getting hot enough in that area to expand and so sag.
>
> I couldn't find much about this problem on the web so I must remember to
> make some notes and take a few photos for my website to help others who
> come
> across the same issues.
>
> Cheers for now and thanks again for the tip, much appreciated.
>
> 73
> Gary ZL2iFB
> www.G4iFB.com
>
>
> > -----Original Message-----
> > From: Ken Nicely [mailto:ken@nicelyweb.com]
> > Sent: Friday, 26 November 2010 3:06 PM
> > To: Gary Hinson
> > Cc: amps@contesting.com
> > Subject: Re: [Amps] LK550 neg grid ... and bandswitch woes
> >
> > If the plate current is 400 ma, then at least one tube is
> > conducting. Mine registers slightly negative grid current
> > when it is not transmitting, so that is not the issue.
> >
> > I would say that it is oscillating. I had a similar problem,
> > but I was actually getting arching in the tubes. I would
> > suspect that you will find that either the plate choke has
> > partially melted and shorting out between windings.
> >
> > If not that, then I would suspect a bad tube. Take one tube
> > out at a time and see if the problem goes away.
> >
> > Ken
> >
> >
> > On Thu, Nov 25, 2010 at 12:46 AM, Gary Hinson <Gary@isect.com> wrote:
> >
> >
> > Gents,
> >
> > Any idea what fault would cause my old and previously
> > faithful Amp Supply Co
> > LK550 sometimes to register ~400mA of plate current and
> > negative grid
> > current, even with no PTT and no drive?
> >
> > First time it happened it took me a while to notice
> > and, judging by the hot
> > smell, something inside very nearly let out the smoke.
> > After leaving it off
> > for a few days to cool off while I mulled over what
> > might be wrong (without
> > any inspiration :-( ), I plucked up courage to switch
> > it back on and it's
> > working OK now, most of the time, but I've noticed the
> > neg current thing
> > again, including one time immediately after switched it
> > on, even before the
> > ALO trip circuit would normally let me transmit. This
> > time it cleared
> > itself.
> >
> > [I know I should pop the lid and look for charcoal but
> > first I'm trying to
> > comprehend what the fault would be. It's part of my
> > 'self training in
> > amateur radio'!]
> >
> > 73
> > Gary ZL2iFB
> >
> > PS Any idea where I might find a new bandswitch for
> > the old amp? I changed
> > the padder caps recently but it still maxes-out on tune
> > and or plate C on
> > the low bands, so I strongly suspect the switch is no
> > longer connecting the
> > padders.
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> >
> >
> >
>
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|