[Heath] Fw: Heathkit SB-610

[Robert Groh]

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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