The best thing about a forum like this, is reading the happy ending
stories and learning just that little bit more about symptoms,
troubleshooting and fixes.
Thanks for the follow up Paul.
73 from the left coast downunder.
Alek. VK6APK
On 12/12/2016 1:08 AM, Paul Kraemer wrote:
The L7 amplifier with the negative grid current on standby has been repaired
and tested.
The problem was not the transformer I was suspecting but rather one of the 0.05mfd
disc ceramic filament bypass capacitors was leaky. I don’t know how I missed
it but after so many suggestions to check the feedthrough capacitors (like the L4B has
but not the L7) I cut them loose and the problem went away. The capacitors have been
replaced and operation verified.
I also installed the recommended diodes across the meters and I installed a
string of (8) 1N5408 diodes in the filament ct to the antenna relay contacts
and placed a 100K 5W resistor across the contacts so the resistor is the cutoff
mechanism in standby and shorted by the relay in transmit. No more of the 120v
going to the ct as Drake had it and now the amp runs a lot cooler with the
small operating bias---you really have to be long winded on ssb to get the fan
to kick into high now.
As far as 10m conversion here is what I encountered. The amp already went to
the extra position when I got it, there was not stop to remove or even any sign
of one having been there. I removed the 10m trap on the input and ran new
input coax direct to the ts in the input compartment. Not looking forward to
what one would have to do to add or change the mica cap buried in the bottom of
that compartment I cautiously tried the operation on 10m and found the match to
my FT-950 as good or better than any band and the output was good. With 50w
drive, swr was very low, output was over 550w on 10m whereas it is approaching
650w on other bands. I left it as is---it works.
Thanks to all for the advice and suggestions. Got it repaired and I couldn’t
be happier with the outcome.
Paul K0UYA
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