heathkit part # 40-1653

[Chris Iwata]

In a message dated 2/14/2007 3:39:15 P.M. Pacific Standard Time,  
mgrandy at TELUS.NET writes:

Looking  for Heathkit part # 40-1653. This is a 6.8 micro H peaking coil used 
on the  "C" board (L323) SB-104A. Is this available anywhere or a suitable  
replacement. I have checked several sources on the internet with no  luck.
Murray (VE7MGY)



This coil is Heathkit's attempt to boost the output of Q305 on 15 and 10  
meters as the peaking coil is somewhat resonant around these freqs. But,  I've 
found mostly this makes the stage unstable.  Ever notice how if the  housing 
around this transistor is not grounded just right by those brass spring  clips 
the rig does strange things on 10 meters?!  
 
A better solution is to re-design the Q305 stage as a broadband  amp (like 
Q304).  Change L323 to 5 turns on a FB37-43 ferrite core. Add  shunt feedback by 
connecting  680 ohm resistor and 0.1uF ceramic  in series between base and 
collector of Q305  (in the same fashion as R301  and C301 on Q304). Change R309 
to 4.7 ohms to realize a little more gain to  combat the feedback and change 
C308 to 0.1uF. These mods make the stage more  stable and also provides more 
even output for ALL bands and makes Q305 a more  consistent source for the 
subsequent band pass filters. All because of the  feedback. If you can't find the 
core, swipe a ferrite bead from another board  (Counter board has way too many) 
and run 4 turns through it, that's what I  did.  These ferrite beads are 
similar to FB43-301.  If your  transmitter output is running a bit low from these 
mods, lower the value of  R318 to compensate.
 
If you are a purist who thinks Heathkit design was golden, I might  have the 
peaking coil laying around somewhere. If you want to go this route  get back 
to me, but before that try substituting a ferrite core or ferrite bead  for 
L323. This alone may be sufficient, and actually either one is better  than the 
original peaking coil anyway  ...
 
Chris, KL7DM.
 
   

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