Re: SX-28A Hum
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Tom OK, does R35 have any affect on the hum? Can you scope the terminals of CH2 when the Bass switch is in both positions?? You most likely will have to do either a differential measurement using channel B minus channel A because of the B+ or you can disconnect one end of R37 to remove B+ from CH2.? You might also adjust R35 to see if it has any affect on the o'scope display.? Testing so far seems to indicate that CH2 is the source of the hum in the audio amplifier.?? Have you reset the mounting screws for both CH1, CH2 and the power transformer?? Somewhere there is a path for magnetic flux to affect the audio amplifier.? 120 cycle hum seems to indicate that the source is after the rectifier but the only center tap secondary winding in the power transformer is in the high voltage winding so I also suspect the power transformer. The ripple you see on the plates of the 6V6 appear to be what is expected.? 7 volts of ripple amounts to 2.5% of the B+ which indicates adequate power supply filtering so the filter cap C49 is doing it's job. Sticky wicket! Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Sunday, February 16, 2025 at 07:36:57 AM CST, thoyer via groups.io <thoyer1@...> wrote:
Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other. CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom ? From: [email protected] <[email protected]> On Behalf Of Jim Whartenby via groups.io
Sent: Sunday, February 16, 2025 3:26 AM To: [email protected] Subject: Re: [HallicraftersRadios] SX-28A Hum ? Tom When R33, the audio gain control, is set for minimum volume, do you still hear the 120 cycle hum regardless of S10, the Bass IN / OUT switch position?? If you still hear hum then the source of the hum is not before the volume control.? The hum must be coupled into the audio amplifier after the volume control.? Does R35, the tone control, have any affect on the 120 cycle hum level? ? The SC-28 manual does not show the physical locations of chokes CH1 and CH2.? Are they located next to each other?? Is it possible to swap the mounting ends of one of the chokes to see if it affects the hum level?? Could it be possible that they are now physically configured as hum aiding instead of hum bucking? ? Finally, do all the audio "ground" connections to the chassis use screws and washers or are they soldered?? Soldered is the least problematic, screws more so due to corrosion which may not be obvious to the casual observer.? Soldering would require a 100 watt American Beauty or similar soldering iron which are, unfortunately, not as common as they once were.? Nokorode is OK to use, it is not an acid flux. Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy ? ? On Saturday, February 15, 2025 at 07:54:33 PM CST, thoyer via groups.io <thoyer1@...> wrote: ? ? I unscrewed CH2 from the chassis and let it float, no change. ? From: HallicraftersRadios@groupsio <[email protected]> On Behalf Of thoyer via groups.io ? Ok, tried two different 6V6¡¯s, no change. ? On the plates of the 6V6¡¯s there is 6.8Vrms of 120hz ripple. If I remove the 6v6¡¯s it drops to 4.7Vrms and the hum is no longer audible (as expected). ? With the 6V6¡¯s back in (Russian tubes BTW), switching the bass ¡°IN¡±, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple. ? These measurements are made with my o¡¯scope. ? I did some poking through my ¡°inventory¡± of parts and do not have a suitable choke to swap in for CH2 ¨C still thinking there may be some leakage going on there¡¡? ? Fun stuff huh? ? Tom W3TA ? From: [email protected] <[email protected]> On Behalf Of don Root ? Hi Jacques, I just received 1.5 cents from you. Your proposal may well be, but how do you explain the difference ?the switch setting makes? From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io ? My two cents: If the 120Hz ¡°hum¡± is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal ?
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Jim Whartenby