[Heath] SB 300 help needed

[G3OOU at aol.com]

Hi
 

You need to do some measurements around the AGC and IF amplifiers to see  
what is happening at various signal levels.  The AGC line should measure as  
a high resistance with the AGC switched on, the SB300 switched off and cold. 
 Then switch on, connect an aerial and start with the AGC detector  output 
at the anode of CR1.
 
The AGC line should go more negative with increasing signal strength which  
will cause the controlled IF stages to reduce their anode current and the  
voltage drop across the cathode resistors to reduce. The change in cathode  
current in valve V7 is used to drive the S meter.
 
It has been noted on the Heathkit forums that there have been some leaky  
6AU6 valves which cause the AGC line to become positive and therefore not 
work  correctly.

 
73

Bob

Bob F Burns G3OOU
C  Eng, FIET, MSE
G-QRP 6907
Secretary of Crystal Palace Radio &  Electronics Club: www.g3oou.co.uk
Technical web site:  www.qsl.net/g3oou
@BobFBurns on Twitter:  http://twitter.com/BobFBurns
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