[Hallicrafters] SX-28A issues

[Richard Knoppow]

----- Original Message ----- 
From: "Waldo Magnuson" <magnuson at mac.com>
To: <hallicrafters at mailman.qth.net>
Sent: Monday, April 14, 2014 5:20 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

     If you get loud hum with the first audio tube removed 
it almost has to be the power supply. Since you replaced the 
filter caps the only thing left is the choke. If its shorted 
it would pull down the supply voltage and increase hum. If 
you have a scope or an AC voltmeter check the amount of 
ripple coming out the filter.  The cathode bypass on the 
output will not affect hum.
     I am not sure from what you wrote, did you completely 
replace the filter caps or only bridge them?  If the latter 
it won't help if they are leaky but I think you said you had 
replaced them. Try pulling the output tubes and see if the 
hum remains. Even with the push-pull transformer there may 
be some of the filter is not working.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com