[Hallicrafters] SX-28A "deaf" on higher bands

[Roger Basford]

Hi All,

Just an update on my "deaf SX-28A".

Firstly, thanks to all who came through with ideas. I have tried feeding the 
receiver with my doublet TX antenna and this brought the noise levels up 
over my normal active loop RX antenna but HF signals were not significantly 
stronger and feeding the coax output from the loop to terminals A1 & A2 with 
no ground gave similar results. I've done as much tube-swapping as I can and 
checked the existing tubes on my Heathkit TC-2. All seem to be acceptable, 
within the limits of the TC-2. Medium Wave BC stations produce a good range 
of S-Meter readings, up to S9+40dB, but on SW the strongest BC stations give 
only S3-4 at most.

One thing I did do is check the tube base voltages with an extender and all 
those in the RF/IF region appear in spec; I can also see AGC action on the 
grids. It's almost as though the RF stage that is switched in on the 4 
higher bands is acting as an attenuator, so I will be looking at that area 
more closely,  may disable it and jump the antenna circuit to the next 
stage, perhaps.

Any further suggestions welcome. One thing I have noticed is that few of the 
resistors show much ageing, possible because the receiver hasn't run for 
more than a few hours since it left the factory so perhaps a soak-test will 
reveal any developing problems.

73,

Roger Basford, G3VKM - Norfolk, England. 



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