[Hallicrafters] SX-28A issues

[Mike Everette]

If I remember right (don't have my manual handy), the B plus for the push pull audio stage in the SX-28 comes from the rectifier tube side of the choke; so the only filtering for the audio B plus is the filter's input capacitor.

The choke itself is only rated for 110 milliamps.  The audio section pulls close to an additional 100 milliamps.  If everything went through the choke, that would be one huge hunk of iron (as if it ain't already big).

Symptoms sound like the input filter cap may be open.

73

Mike
WA4DLF
--------------------------------------------
On Tue, 4/15/14, Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:

 Subject: Re: [Hallicrafters] SX-28A issues
 To: hallicrafters at mailman.qth.net, "Waldo Magnuson" <magnuson at mac.com>
 Date: Tuesday, April 15, 2014, 12:56 AM
 
 
 ----- 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
 
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