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Re: SX-28A Hum
¿ªÔÆÌåÓýTime to re-evaluate where I¡¯m at and what I¡¯ve tried, yes the hum is still there¡¡. ? Tried three different 6SC7¡¯s no noticeable difference between each one. ? Confirmed the wiring of the switch agrees with the schematic. I have no reason to suspect that the wiring was changed by a PO at the switch ¨C looks original to me. ? Speaking of that switch, I found by poking around with my magic wooden stick that pressing on the switch terminals would result in noise / popping sounds ¨C could this be the ah ha moment? Scrounged my switch stock and found one that looked the same. Checked the switch action with a DVM, all good. Installed switch ¨C same results¡¡¡ Checked original switch with DVM and it definitely has issues. Must have dirty contacts inside as one direction is a solid 0.3 ohms and the other would vary from 10 ohms to over 100 by wiggling the terminal. ? I also confirmed that the hum is 60hz, not 120hz. I see 120hz on the audio xfmr primary side to gnd but see 60hz on the 500 ohm output (with the scope). ? I checked my wiring ¨C all good. ? Now here is where things get interesting. I removed the choke (measured 4.3hy on my Sencore tester) and installed the one I found last night using 8¡± or so clip leads so I could lay the choke on the bench. Seemed like the hum decreased, thought another ah ha moment. Swapped the original choke back in using the same clip leads, same result hum still present but seemingly lower in amplitude. Interesting¡¡.. ? Put original choke back into the radio, hum is easily 2x louder which is what I remembered before removing the choke. So that makes me wonder if the choke is really the culprit. ? I need to take some baseline measurements with the choke mounted, unmounted and then with the extension leads. ? Then I¡¯m going to wire in the other choke where the original one went and compare measurements. ? Right now, my brain is not up to the task so it¡¯ll have to wait for tomorrow. Been up since before 5AM and going all day. Once tiredness sets in (usually around 8:30, 9pm) I avoid playing around high voltages¡¡¡¡ ? Thanks for all of the discussion, I find it educational. ? Tom W3TA ? From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Monday, February 17, 2025 8:02 AM To: [email protected] Subject: Re: [HallicraftersRadios] SX-28A Hum ? Last night I was able to find another 4hy choke. I knew I had one around just couldn't put my finger on it. It was salvaged from an old R390A AF module.?Dawned on me last night at 8:30 to go check the shed. Bundled up headed out into the 50mph wind storm we were having (dog didn't even want to follow me out). Found the choke, swapped it into the radio - hum still there.......... ? That tube is looking more and more suspect............ ? Tom w3TA ? On Monday, February 17, 2025 at 07:56:32 AM EST, thoyer via groups.io <thoyer1@...> wrote: ? ? Could be....? ? One other tid bit, whenever I switch between in/out there is a loud thump in the speaker - annoying to say the least. ? On Monday, February 17, 2025 at 02:11:24 AM EST, Jim Whartenby via groups.io <old_radio@...> wrote: ? ? So the hum in question might just be 60 cycle leakage instead of the originally stated 120 cycle?? I guess that this is possible but what is the explanation for the loss of the hum when SW10 is in the Bass (boost) IN position? ? I believe that the SX-28 schematic is correct after all.? With SW10 in the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38 make up the plate resistance.? In the Bass (boost) OUT condition, CH2 and C43 are in circuit but only R38 is used as the plate resistance.? A lot of control is accomplished by a simple SPDT switch. ? It seems to me that the curves in Figure 11 of the manual are believable since the contribution at 1kc of CH2 and C43 are, as previously stated, clearly evident in the Bass (boost) OUT curve.? What the contribution of R35 for the midrange frequencies in either curve is not mentioned but clearly, the lower the resistance of R35, the lower the available midrange audio frequencies. ? Perhaps part of the problem is that the SX-28 was made a decade before the high fidelity craze of the 1950s.? Today, over all tone is considered to be composed of bass, midrange and treble frequencies.? Treble frequencies for the most part are not available in the SX-28 since the high audio frequencies are limited to perhaps 3 to 4 kc.? Midrange frequencies are fixed by C40 in the phase splitter and by C42 and the R35 pot in the 1st audio triode section of the 6SC7.? Since the SX-28 is, in the end, a communications receiver, the loss of treble frequency control is not an issue. Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy ? ? On Sunday, February 16, 2025 at 08:01:10 PM CST, Richard Knoppow via groups.io <1oldlens1@...> wrote: ? ? The 6SC7 is used as a "floating paraphase" phase splitter. At least
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