Re: SX-28A Hum
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I am not sure that the problem is resolved.? CH2 was replaced with a similar value choke with less winding resistance so it will have higher Q.? Higher Q will affect the audio filter curve so there will most likely be more ripple in the audio bandpass in the bass out switch position.? This may not be that noticeable to the ear but the scope will see it. In Tom's first video, at about the 45 second mark, the scope shows a strange waveform.? The main peaks are of a 60 cycle waveform.? The center peak is at 120 cycle but it's peak is negative, the two remaining peaks are at 240 hertz and positive.? There must be some nonlinearity to cause these harmonics to be generated.? This nonlinearity is acting like a diode and causing the second and forth harmonics of the 60 cycle waveform. Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Wednesday, February 19, 2025 at 02:13:41 PM CST, Richard Knoppow via groups.io <1oldlens1@...> wrote:
Thank you. Since its a different choke it doesn't explain why the original was humming. Doesn't matter if the problem is solved but would be interesting to know. Its possible there is a leak between the winding and core or frame in the original. A sensitive ohm meter would probably tell. I had thought of suggesting putting the old choke on an insulator (just a sheet of paper) temporarily to see if the hum stopped. Not worth taking the new choke out to try. The SX-28 is an interesting receiver, an attempt by Hallicrafters at advanced engineering that didn't quite come off. The Lamb noise blanker was a very good idea but was not well implemented. Eventually, Drake and others, did a better job with it. This type of blanker works well on pulse type noise, like ignition noise, but hardly at all on the more common (now) power line noise. Hallicrafters eventually published a modification (on BAMA) to disable it and replace it with a conventional series limiter. I hope not many SX-28s were mutilated this way. On 2/19/2025 11:57 AM, thoyer via groups.io wrote:
Yes, I replaced the original CH2 with another 4hy one I had from an -- Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
Jim Whartenby