[Hallicrafters] SX-28A issues

[Nick]

Maybe you got a shorted power transformer(shorted secondary) ,happened to me once on an  SX-28. Did the dial lights go dim when the loud hum happened ?


Nick








-----Original Message-----
From: Waldo Magnuson <magnuson at mac.com>
To: hallicrafters <hallicrafters at mailman.qth.net>
Sent: Mon, Apr 14, 2014 5:42 pm
Subject: [Hallicrafters] SX-28A issues


Hi,  Earlier I posted:
-------------
Hi, My SX-28A was playing okay (after a long rest). Then the
 BCB station went away and the audio became a loud hum. I've
 tried: pulling the 1st audio amplifier (v12, 6SC7), hum
 remains, Substituted both 6V6GT audio output tubes - hum
 still present, paralleled C48 & C49 (power supply output
 caps) and C47 (cathode bypass for 6V6s) - no change, B+
 measures maybe 150v (not schematic 250v). Any suggestions
 welcome.
 > Skip W7WGM
-------------
   And I got several responses, mostly to replace the filter capacitors in the 
power supply section.  The caps were about 5 years old but I replaced all of the 
audio and power supply electrolytic caps.  I tested the capacitors I put in and 
the capacitors I removed - all tested good with no shorts or leakage.  I also 
tested all the tubes (except for the 6B8) and detected no shorts, all tested 
okay.  When I bring the SX-28A up with a Variac, when the voltage reaches about 
35 vac, the hum becomes pronounced out of the speaker (like before).  Previously 
I was listening to a BCB station and the station went away and the hum (loud) 
began.  I hesitate increasing the line voltage further.  Are there any 
suggestions on what to check next?  I would appreciate the help.  Thanks.
73s, Skip W7WGM
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