Re: SX-28A Hum
¿ªÔÆÌåÓýDang ! Could you measure a leakage between the winding of the ¡°original¡± choke and it¡¯s core ?? Happy that this ¡°mystery¡± is solved ! ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de thoyer via groups.io ? Ok folks, I found the culprit ¨C that dog gone choke! I replaced it with a 4hy one from an R390A AF deck and the hum is gone. I laid it in the chassis, insulated from the circuits and wired it in with short leads. Actually the hum is barely there, you have to put your ear up to the speaker to get a hint of it, but this is the level I¡¯d expect from an old radio like this. ? There is a small difference in chokes in that the original is, I believe, 220 ohms and the Collins one is 110 ohms. I¡¯ll have to try to locate one that fits the mounting holes. ? For now, I think we can put this head scratcher to rest. ? I appreciate all of the thought and input from everyone, I definitely learned something about this circuit which is always a good thing. Problem is will I remember it next week¡¡¡¡. ? ? Thank you all, ? Tom W3TA ? From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 6:08 PM To: [email protected] Subject: Re: [HallicraftersRadios] SX-28A Hum ? Jacques, ? I ¡°may¡± guilty of not returning caps to the same ground as original¡¡¡¡ I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ? From: [email protected] <[email protected]> On Behalf Of Jacques_VE2JFE via groups.io ? Let¡¯s resume the case?: Tom pulled the 6SC7 and the ¡°hum¡± remained. He shorted the two 6V6 grids to GND and the hum disappeared. No hum when the ¡°bass¡± switch is in the IN position (both plate loads of the 6SC7 equal and CH2 + C43 shorted). Hum present when the ¡°bass¡± sw in OUT position. The ¡°hum¡± is at 60Hz¡ ? Which suspects remains ?? Does the C44 is ¡°grounded¡± at the same place than the filaments of the 6V6s and this connection is resistive ?? Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum. When it is not: hum ! Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and that the C43 is practically an open circuit at 60 Hz. Could it be that the 60Hz injection point is the C44 grounding lug ? ? My two SX-28As are too far to be quickly reached. But If I manage to dig the ¡°never touched¡± one out of the storage during the next weekend, I can check how it is wired. Keep thinking¡. ? And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is ENABLED (literally, ¡°bass¡± is in). When out: bass is out (DISABLED). CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to ¡°tailor¡± an audio bandpass response for voice frequencies (300Hz ¨C 3kHz) ? ? ? ? ? 73, Jacques, VE2JFE in Montreal ? Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ??
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Jacques_VE2JFE