SB-104a transmitter if board "C" question

[Chris Iwata]

The correct version is in the schematic: What's needed is a series resonant 
 circuit. This theoretically has zero impedance at resonance.  So having 
the  caps in parallel results in an equivalent capacitance of 140pF plus the 
variable  caps value.  This network is in series with the inductor. 
 
My SB104 (not A) has the waxed combo wired in this way.  But the null  
isn't so great either.  It may be cuz of component tolerances the combo  didn't 
actually resonate at 3395. Heath should probably have used a  smaller fixed 
cap and larger variable cap to get more tuning range. Failing that  one can 
probably spread or compress the turns on the inductor, but I was too  lazy 
to melt the wax.  Read on ...
 
The source of this 3395Khz signal is poor balance in the diode-ring mixer  
on the C board.  I put in instead 4 MATCHED Schottky diodes, then I forgot  
about the trap. When I tested the original diodes they were poorly  matched. 
A true purist would substitute matched diodes in ALL of the  diode-rings. I 
noticed that newer Schottky diodes are pretty well  matched if in the same 
lot. I guess modern technology makes tighter  manufacturing tolerances easy. 
 
73, Chris KL7DM

 
 



 
In a message dated 2/4/2010 7:01:34 P.M. Pacific Standard Time,  
dsnaib at GMAIL.COM writes:

I'm  refreshing a SB-104a C board and noticed that the 3.395 trap at  the
output (pin 24) doesn't match the schematics.  The schematics  shows the
variable and the fixed 140pf capacitors in parallel and the  coil in series
with one end to ground but my board has the 140pf and the  coil flipped so
the variable capacitor and coil are in parallel and the  140 pf goes to
ground.  I have two 104a's and they are both wired  this way.  The one I'm
working on currently will not completely null  out the spur but it also has
other questionable parts that may be  contributing to that.  Anyone know 
what
is the correct way  is?



Thanks,



Dean

K4DSN





-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.aspx?list=HEATH
To post - HEATH at LISTS.TEMPE.GOV
Archives - http://lists.tempe.gov/archives/HEATH.html