[Hallicrafters] SX28 help

[John Hurst]

Guido.
It sounds like the 1st RF stage is oscillating, possibly because the gain
was increased by a better quality (lower ESR) catode bypass cap. A poorer
screen bypass cap might also be the cause. You might be able to test out
this theory by using a lower gain tube like a 6SK7 in place of the 6AB7. If
the problem is a VHF parasitic oscillation then you might need a grid or
plate "stopper" resistor, maybe 47 ohms directly in series with the grid (or
plate) lead.
73, John, KU6X
----- Original Message ----- 
From: "guido" <laffitte at prtc.net>
To: <hallicrafters at mailman.qth.net>
Sent: Wednesday, November 19, 2003 2:24 PM
Subject: [Hallicrafters] SX28 help


Hi gang,

  I just finished the whole recap of my SX28. Well, this means that I
finally took the plunge into the RF section to replace the remaining paper
caps there. I still have some problems like a scratching noise in certain
positions of the antenna tuner cap. I have checked and see no plates
touching. I also have some howling feedback in the lower portions of bands
2,3,4 when it gets into strong signals. You can tune it but it also depends
on the position of the antenna tuner cap. I have checked all tubes and some
resistors of the 100K value need replacement but they are not that far off
as to be the cause of the problem. I have also noticed that even after the
recap the sensitivity is not as good as what is expected. I pass to the
combined wisdom of this list the problems for your recommendations. Maybe
someone has experienced something similar.

Many thanks!
Guido KP4FAR

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