SB400 - Low Grid Drive on 40 Meters

[Bob Burns]

Hi All
 
If you are intending to run the SB400/401 PA in class AB1 on SSB then there 
 should not be any grid current and if there is then the ALC system should 
kick  in as that is how it detects excess drive.
 
Does your transmitter behave in the same way if you run it standalone -  
disconnected from the receiver? I find that my 301/401 combination does give a 
 little less output in transceive mode on most bands than standalone, 
probably  due to lower drive from the receiver oscillators but this has never 
been a  significant problem.
 
The issue of significantly lower drive only on 7MHz should be fairly easy  
to locate as there are only a relatively few components that are specific to 
 each band. The first two that I would check are the heterodyne crystal and 
its  respective anode tuned circuit. My SB-401 circuit shows an oscillator 
test point  with which you can measure the relative excitation on each band 
- they should be  similar although dropping off a little on the higher 
bands. If the  reading on 7MHz is significantly lower then the crystal may have 
poor activity  or the anode tuned circuit be incorrectly adjusted or faulty.
 
If that does not fix the problem then look at the RF preselector tuned  
circuits and respective solder joints. Make sure that each circuit actually  
reaches a proper resonant peak - if the tuning point is very broad and the 
core  is fully inside the winding then you may not be getting to the correct 
tuning  point.
 
There will always be small differences between the bands due to a number of 
 parameters and if you are getting full output on 7MHz as well as the other 
bands  then you may not have a serious problem.
 
73

Bob 

G3OOU
G-QRP  6907
Joint site with Crystal Palace Radio & Electronics Club: _www.g3oou.co.uk_ 
(http://www.g3oou.co.uk/) 
My technical site: _http://www.qsl.net/g3oou/_ (http://www.qsl.net/g3oou/) 

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