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Title: Cable Connection: Collins 75A-2 To A Heathkit SB-610 Post by: w0ng on February 13, 2008, 08:23:08 PM I'd like to monitor my received signals from the grid of the last IF stage in my 75A-2 receiver by connecting it to my Heathkit SB-610 monitor scope.
The SB-610 manual says to ONLY use RG-62 coax (93 ohm) to make the receiver-to-scope connection. Do I have to use RG-62 or can I use something I already have in the shack like RG-174/U, RG-58A/U, RG-8X ??? Would appreciate some advice here. Thanks, Bill w0ng Title: Re: Cable Connection: Collins 75A-2 To A Heathkit SB-610 Post by: KB2WIG on February 13, 2008, 08:28:13 PM I don't know offhand if it will make any difference, but i'd go fer the 93 ohm stuff.....
you can find this stuff floating around an office building that had a local computer net... no one will miss a little nip.......or a surplus computer store........... klc Title: Re: Cable Connection: Collins 75A-2 To A Heathkit SB-610 Post by: Pete, WA2CWA on February 13, 2008, 09:04:13 PM This should be taken into consideration:
RG62 - 13.5 pf nominal cap./foot RG58 - 28.5 pf nominal cap./foot RG174 - 30 pf nominal cap./foot RG8X - 33.5 pf nominal cap./foot Title: Re: Cable Connection: Collins 75A-2 To A Heathkit SB-610 Post by: w3jn on February 14, 2008, 08:35:16 AM IIRC the coass used for car radios was 92 ohms or thereabouts. Rob an old car antenner of its cable and see how it works.
What'll happen with 50 ohm cable is the coass will load down the radio, reducing sensitivity of the monitor as well as requiring re-alignment of the radio. A much better solution, rather than using a SB-610, is to use a normal, garden variety oscilloscope with a 1 or 5 -meg resistor in series with the cable (at the radio). The scope will have more than adequate sensitivity to make up for any capacitive loss in the cable, will have better (and more linear) sweep, etc. Title: Re: Cable Connection: Collins 75A-2 To A Heathkit SB-610 Post by: Jim, W5JO on February 14, 2008, 09:22:21 AM A much better solution, rather than using a SB-610, is to use a normal, garden variety oscilloscope with a 1 or 5 -meg resistor in series with the cable (at the radio). The scope will have more than adequate sensitivity to make up for any capacitive loss in the cable, will have better (and more linear) sweep, etc. Also connect to the first IF to reduce non-linearity of the 2 IF stages. You will see a better signal. Title: Re: Cable Connection: Collins 75A-2 To A Heathkit SB-610 Post by: WA1HZK on February 14, 2008, 09:48:02 AM Usually a few pf in series with the cable to a scope works fine here.
You can add the resistor if you have any loading issues. try 10 pf. & see if it works. All I have ever used is 50 ohm cable. No problems. Keith WA1HZK Title: Re: Cable Connection: Collins 75A-2 To A Heathkit SB-610 Post by: Vortex Joe - N3IBX on February 21, 2008, 08:06:51 AM Hello, AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
I've substituted 93 ohm with 72 ohm cable with Heathkits, Collins rigs, etc, and never had any difficulty with the 72/75 ohm cable if the run wasn't too long. The 93 ohm stuff is difficult to come by so I thought I'd use the next best thing, and it worked FB. Regards, Joe N3IBX |