SB-303/610 IF question

[w1id]

On Tuesday, February 06, 2001 at 09:55:09 PM, Chuck Penson wrote:

> Got this query from a fellow yesterday. Am hoping someone can help him.
> Please respond directly to him.
>
> Thanks -- Chuck / WA7ZZE
>
>
> --------- Begin forwarded message ----------
> From: "Norton, Dennis J" <Dennis.Norton at usa.xerox.com>
>
> I am setting up my SB303/401 combination at the main operating position.
> All
> is well with these 2 units--they function together just fine. I wanted to
> extract the IF signal on the 303 to a rear panel jack in order to fire it
> over to the 610 to watch the received signal. I had previously equipped
> my
> SB102 in such a way by adding the internal coax and picking up from
> somewhere in the IF through a cap that was between 1 and 10 pf in
> value--worked just fine. Now I want to do the same for the 303. The sb610
> book predates the 303 and does not discuss the hook-up. The tech
> bulletins
> available on KB9JJA's web site talk about about picking up the signal
> from
> the collector of Q503 through a 1uf cap. I tried that and the if
> oscillated
> as soon as the cap is connect top the 610. I wondered what kind of 1uf
> (wound foil, lytic, npo????),. I also wondered if the 1uf was a mistake
> and
> tried .1uf, .001uf etc--no signal at the scope and it killed the IF into
> oscillation. The co-ax that I am using between the jack, and the board
> internal is RG174, and from the 303 to the 610 is a piece of RG58. The
> 610
> is set up for an IF freq of 3395. Any ideas on what is wrong and how to
> make
> it work??
>
> Any help will be appreciated.
>
> Thanks, Dennis Norton  W2DJN
> --------- End forwarded message ----------
>
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Whatever it was I'm positive it couldn't possibly be a 1uF cap. At 3.395MHz, that would be 0.047 Ohms! From the schematic I see that the AM detector and the AGC detector both use 5pF coupling capacitors, so I would try that. The value of the resonating cap across T503, located internally inside the can is 330pF. Regardless of the value of coupling capacitor used, you'll have to retune T503. I would also use short leads and miniature coax right at T503. Myself, I don't bother with this. I installed a small FET source follower at T503 and connect the gate through a 5pF cap to the top (pin 2) of T503. Then I can use any coax of any length to the SB-610 without having to retune the transformer. If you still get oscillations, try moving your coupling cap to pin 5 of the transformer. Keep in mind that unless you have a wide bandpass filter, whatever you see on the scope will be pretty useless. The SB-310 AM filter comes closest to giving an accurate representation of the received si!
gnal. The SB line with its narrow SSB filter rounds out all the clipped peaks and smooths the rough edges off any received signal. Add the effects of the AGC and you have a pretty well "doctored" representation of the actual transmitted signal. It does look impressive though.

w1id
e-mail: w1id at fcgnetworks.net

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