[Heath] SB-101 Refurbishment

[G3OOU at aol.com]

Hi Ken
 
I have now resolved the 5dB filter ripple problem on my unit to less  than 
2dB for both SSB and CW filters. A replacement SSB filter produced the same  
result as the original filter.
 
I connected 3.395MHz tuned circuits to both the input and output sides  of 
the filter switch and adjusted both for minimum ripple using a signal  
generator with a narrow sweep facility driven by the X output of my scope. The  
sweep width was set to about 4KHz and a sweep speed of about three per 
second.  The AGC was disabled and the RF level at the product detector displayed 
on the  scope, making sure that the IF system was not overloaded.
 
Although both tuned circuits had an effect on ripple, the best result  was 
obtained with the tuned circuit connected to C506. I ended up with a 75uH RF 
 choke (two 150uH chokes in parallel) connected from the switch end of C506 
to  ground and no correction on the other end of the filter.
 
This is a test of one unit so other Heath owners may need to carry out  
similar tests to see what works best with their units.
 
I do not know the exact spec for terminating the Heath filters other that  
the stated 2000 ohms but it would appear to be 2000 ohms in parallel with a 
very  low value capacitance. The 75uH inductor suggests that it has 
corrected for  an excess capacitance of about 29.5pF. On other solid state units 
where I have  used the same Heath filters in a 50ohm signal environment, a 
matching L network  on each end produced an almost flat passband response so my 
47 year old  filters seem to be fine.
 
73

Bob

Bob F Burns
C Eng,  FIET, MSE
Amateur Radio Callsign: G3OOU
Registered RSGB  Instructor
G-QRP Member No: 6907; Zone: 14, QRA: IO91WH; WAB:  TQ25
Secretary of Crystal Palace Radio & Electronics Club: _www.g3oou.co.uk_ 
(http://www.g3oou.co.uk/) 
Technical web site: _www.qsl.net/g3oou_ (http://www.qsl.net/g3oou) 
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