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 Post subject: SX-42 Wiring Question
PostPosted: Nov Fri 27, 2020 2:54 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 288
Location: Warminster, PA
Working on a new to me SX-42. Lots of parts had been replaced and the radio was "supposedly" working. Well I'm finding many parts still need to be replaced and previous workmanship left a lot to be desired - but I digress.....

For reference this SX-42 is built to the "G" schematic.

Working on the 3rd IF xfmr wiring. There is a shielded wire that comes from the BFO and ends at a terminat strip near the 2nd IF tube. The schematic shows this point to be capacitivly coupled 3rd IF xfmr pin 5. There was not a connection from the terminal strip to pin 5.

Question is where should I make this coupling? I understand what the "gimmic" cap is just need to know where to best make the connection and how many turns?

Hopefully the pics help explain what I'm asking.

Tom
W3TA


Attachments:
gimmic.jpg
gimmic.jpg [ 729.82 KiB | Viewed 349 times ]
gimmic1.jpg
gimmic1.jpg [ 1.41 MiB | Viewed 349 times ]
IMG_2745.JPG
IMG_2745.JPG [ 1.1 MiB | Viewed 349 times ]
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 Post subject: Re: SX-42 Wiring Question
PostPosted: Nov Fri 27, 2020 6:45 pm 
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Joined: Feb Fri 13, 2009 5:09 am
Posts: 712
Location: Santa Clara, CA
It looks to me that the shielded conductor in your schematic just diagrammatically crosses over the conductor going to the gimmick capacitor, and is not connected at all (unless I am misunderstanding your question...)


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 Post subject: Re: SX-42 Wiring Question
PostPosted: Nov Fri 27, 2020 7:04 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 288
Location: Warminster, PA
Bob E. wrote:
It looks to me that the shielded conductor in your schematic just diagrammatically crosses over the conductor going to the gimmick capacitor, and is not connected at all (unless I am misunderstanding your question...)


The shield is shown in the schematic on the wire in question but does not show in the snippet of the schematic I posted.

Tom
W3TA


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 Post subject: Re: SX-42 Wiring Question
PostPosted: Nov Fri 27, 2020 10:05 pm 
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Joined: Jan Fri 06, 2012 8:47 pm
Posts: 7076
Bob E. wrote:
It looks to me that the shielded conductor in your schematic just diagrammatically crosses over the conductor going to the gimmick capacitor, and is not connected at all (unless I am misunderstanding your question...)
After looking at this for a while I think the "crossover" IS the gimmick capacitor (see green and blue crossing below).
Attachment:
Gimmick.png
Gimmick.png [ 836.64 KiB | Viewed 304 times ]
In other versions of the schematic it shows up as a 7 pF capacitor (very, very small).
Attachment:
Injection.png
Injection.png [ 285.05 KiB | Viewed 304 times ]
It is just the coupling of the BFO signal into the detector.

Curtis Eickerman

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 Post subject: Re: SX-42 Wiring Question
PostPosted: Nov Fri 27, 2020 11:30 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 1792
Location: Peekskill, NY
I had the same setup on the SX-11 I was working on, the BFO never really worked correctly until I replaced the
(apparently non-existent gimmick) with a 250 pf silver mica, which I then upped to 330.

If your BFO is not working well simply start with around 100 pf and work your way up from there, I would say.


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 Post subject: Re: SX-42 Wiring Question
PostPosted: Nov Sat 28, 2020 12:44 am 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 288
Location: Warminster, PA
[/quote]After looking at this for a while I think the "crossover" IS the gimmick capacitor (see green and blue crossing below).In other versions of the schematic it shows up as a 7 pF capacitor (very, very small).It is just the coupling of the BFO signal into the detector.

Curtis Eickerman[/quote]

Interesting observation Curtis. I'll need to take a closer look at that area.

Tom


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 Post subject: Re: SX-42 Wiring Question
PostPosted: Nov Sat 28, 2020 12:45 am 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 288
Location: Warminster, PA
jim rozen wrote:
I had the same setup on the SX-11 I was working on, the BFO never really worked correctly until I replaced the
(apparently non-existent gimmick) with a 250 pf silver mica, which I then upped to 330.

If your BFO is not working well simply start with around 100 pf and work your way up from there, I would say.



I'll keep this in my back pocket, thanks!

Tom
W3TA


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