[Heath] SB-610 input questions

[Robert Groh]

Well, I got a bit goofed up on what I had sent and what had bounced so I  am 
repeating my last 2 emails. If redundant, please accept my apologies.

73
Bob Groh, WA2CKY
------------------

Guess what?  On my previous email, I'm wrong.  Or at a minimum, not  entirely 
correct.  We do NOT have a capacitive divider. My bad. 


I  dug up a copy of the SB-610 manual and threw down some sketches on a  piece 
of paper. The SB-610 uses a tuned circuit on the vertical input  (for the  3 to 
6 MHz range anyways). That inductor (L1 on the schematic)  is resonated against 
the tuning capacitor (330 pF according to the  manual), all the stray capacity 
AND the shunt capacity from the coax  cable. In other words, inductor L1 is 
tuned to absorb ALL the shunt  capacity from the coax cable (as well as stray 
capacity due to  connectors,etc).

With all the capacity 'zero'ed out' by L1, you  only have your series capacitor 
from the IF amp plate connection (e.g. 5  pF which is -9,500  ohms) working as a 
voltage divider against the Vertical Gain adjustment pot in the SB-610 (100K 
ohms). 


Basically  the only limit on the choice of coax and the length of the coax is 
the  ability of L1 to tune out whatever shunt capacity the coax presents.   
Loss, inductive effects, etc of the coax should be pretty much zip at  3395 kHz 
(e.g. a wavelength is 80 meters or about 240 ft!).

One  thing that could be happening is that L1 is running out of range (i.e.  
max'ing out).  This is fairly easy to see - you should get a 'double  peak' as 
you tune the inductor.  If you don't, then you are at maximum  and your should 
reduce the shunt capacitor (e.g. from 330 pF to 270 pF). 


Now past this point, my previous post is correct.  To get more  signal, you can 
increase the value of the series cap (e.g. from 5 pF to  10 pF).  The limit is 
how much additional load you can tolerate on the  IF Amp.

Whew!  I hope that is  correct and that it helps.

73
Bob, WA2CKY


________________________________
From: Robert Groh <rgroh at swbell.net>
To: mdilli at nnwifi.com; Dick KF4NS <kf4nsradio at verizon.net>; 
ma.locksmith at juno.com
Cc: Heath puck list <heath at puck.nether.net>; heathkit at mailman.qth.net
Sent: Thu, March 8, 2012 8:36:54 AM
Subject: Re: [Heath] Fw:  Heathkit SB-610

Morris has it dead on.   If you think about it this way:

You  have a cap (5 pf in your example) connected from the source (the IF Amp  
plate) to a load (the coax which drives the SB-610 and then the input  circuit 
of the 610 which has some (assumed) input impedance - lets say  the 610 input 
looks like 20K ohms in parallel with 10 pf. 


For  all practical purposes, we can say the coax just looks like a shunt  
capacitor (x ft at y pf/ft).  This coax capacity is in parallel with the  610's 
input.

So we wind up with our source driving through a 5  pf cap to a parallel RC 
network - and the easiest way to think of this  is that the combination is a 
voltage divider (yeah, I am simplifying  it!).   As an example, assume we have 5 
pf in the series capacitor, 15  pf of  coax capacity, 10 pf of input C at the 
610 and we ignore the R  component. We would have a voltage divider of 5 pf 
working against 25 pf  or a 5/30 voltage divider.  


You can readily see that the longer  the cable or the higher the coax capacity 
per ft, the higher the  voltage division ratio. You can of course increase the 
series cap to  offset this (e.g. 10 pf --> 10/35 ratio) but the trade off is 
that  you are now affecting the tuning of the IF stage more. 


A picture is worth a thousand words but hopefully this will help.

73
Bob Groh, WA2CKY
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