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.
>
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>
>
>
>
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