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Author Topic: AC Hum  (Read 1595 times)
N4PSE
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« on: August 11, 2004, 05:38:32 AM »

I'm redoing an old HW- 16 Heathkit. I have replaced all the electrolytics with modern caps. The xmit signal quality is now fine, but the rig now has a pretty bad ac hum on receive. What next- replace all the by-pass caps or look at possible tube shorts in audio section? I don't have a tube tester (who does anymore??). Obviously I'm not much of a technician, but I'm having a lot of fun learning. Thanks for your input.
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W8JI
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« Reply #1 on: August 14, 2004, 04:00:32 AM »

Look for tube H/K shorts and for bad ground connections near tube sockets for heater grounds.

Have fun, even if it isn't a 160 rig!

73 Tom
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N4PSE
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« Reply #2 on: August 15, 2004, 06:23:41 PM »

Thanks Tom. Looking forward to 160 again this winter, but had to find something to play with during the summer. The lil HW-16 works super except for the hum. Tested all the tubes for HK shorts. That wasn't it so I'll keep trying. Probably something very simple. At least the rig is very useable as is.  73 Jim N4PSE
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KA5N
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« Reply #3 on: August 15, 2004, 08:21:49 PM »

You said that you replaced all the electrolytics, however you might double check by bridging the caps for points A and B (on schematic)using a 40-60 ufd 400 volt electrolytic.  Do this one at a time discharging the cap after each test.  A 60 Hertz hum means a heater to cathode short in a tube or stray pickup.  A 120 Hertz hum indicates a power supply problem.  Did you check all the tubes in the receiver section?  Checking with an ohmeter (or DVM) with the tubes cold may not show a short that occurs when the tube is hot.
Good luck Allen
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N2EY
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« Reply #4 on: September 09, 2004, 01:43:10 PM »

HW-16 hums on receive...

Of course the power supply electrolytics are the first thing to check - in fact check all the electrolytics in the set. But you're probably way beyond that by now.

Then try these tests:

1) Turn set off, pull the 6GE5 final tube. Does hum go away? If so, you have heater-to-cathode leakage in the 6GE5 and the cure is a new tube. (Found that one the hard way - all the received signals had hum on them, even though everything checked out perfect. The HW-16 TR system involves the 6GE5 cathode, and any AC on it modulates the incoming signals)

2) Turn down AF gain - does hum go away? If so, it's getting into the set before the AF gain control

3) Turn down RF gain (with AF gain up) - does hum go away? If so, it's getting in somewhere in the RF or IF section. Try removing tubes one at a time to see what kills the hum.

73 de Jim, N2EY

btw, the 6GE5 is just a compactron 6DQ6. A reversible socket swap permits using a 'DQ6 if you can't find a 6GE5.

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N4PSE
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« Reply #5 on: September 10, 2004, 06:25:15 AM »

Thanks- haven't had time to mess with it lately as we are running from hurricanes almost weekly here in S Fla! Will let you know how your suggestions work out. Thanks again. Jim N4PSE
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