Amps
[Top] [All Lists]

[Amps] L7 FIXED (was L7 problems)

To: <amps@contesting.com>
Subject: [Amps] L7 FIXED (was L7 problems)
From: "Paul Kraemer" <elespe@lisco.com>
Date: Sun, 11 Dec 2016 11:08:53 -0600
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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

_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
<Prev in Thread] Current Thread [Next in Thread>