HEATH Digest - 24 Dec 2001 to 25 Dec 2001 (#2001-346)

[John Q. Doe]

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Date:    Tue, 25 Dec 2001 09:30:43 -0500
From:    Mark Graalman WB8JKR
Subject: Re: SB-401 low output on 10 meters

> Before I start tearing out the tuned circuit
> with the plate inductor and 50 pf cap, is there
> any other place I should look for another problem?
>
> Chuck WD4HXG

  Chuck, have you tried a new C64 the 50 pf cap? Sounds
like its too high in value or leaky.

Mark  WB8JKR

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I had been hesitant to tear down that far (lazy) but it
looks like that is the magic spot.  Heath's alignment
procedure specifies setting the driver control one mark
up from total counterclockwise. I went back and moved it
2 marks and then went through the alignment procedure again.
That time I was able to locate a peak in the coil adjustment.

I will remove the coil & cap assembly and measure each on the
Q meter. One or the other is off. If it "AIN'T" then I  have to
read Heath's instructions closer.

The next pain is the neutralization. Went through the instructions
for alignment and found I could not find a sweet spot for it. There is a minima in the adjustment while the trimmer plates are still partially meshed but after connecting the screen's and plate's  supply there is no correlation with the max power output and plate tune dip.

The odd thing is that there is not a pronounced dip in the Plate
Tune on 10 meters and the other bands are kind of shallow. I am
use to more of a quick dip in plate current. Tried the bypass
caps around the 6146's, 6CL6 and checked the values of the 430
pF cap off the driver plate leg. Finals are new 6146A's and the
6CL6 is new also.

I pulled the neutralization cap from the trimmer and tried the
amp without feedback. Max power of course occurred off any dip
but again I could not find a definitive dip in the plate tune.
Will check the resistors next. A quick check previously did not
reveal any that were seriously off value. The only thing left
that rings a bell at this point is to verify that the bandswitch
is really switching the final coil. I doubt it is a problem as
the rig produces 100 watts out easily on 80, 40, 20 and 15.
Output on 10 is about 80 watts. I don't see any signs of a
higher amplitude parasitic in the scope trace. The output sine wave
is nice and clean on a 100 MHz scope.

I did not build the rig. SO if all else fails I guess I need to go
back and verify that everything is wired properly.

Thanks for the reply

Chuck WD4HXG

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