SB-401

[Bob Burns]

In a message dated 18/08/2010 00:14:06 GMT Standard Time, k4jrs at AOL.COM  
writes:

I have  an SB-401 that has been working perfectly. It recently decided that 
after  about an hour it would suddenly key itself up. I replaced the 6D10 , 
6EA8,  6AV11 and 6J11 with new stock I had on hand. It still does the same 
thing  after about an hour warmup. I wonder if anyone else has experienced 
the same  problem. It is probably a component that is getting hot in the 
vox-keying  circuit. Any suggestions.  ?

Tnx, Jim 



Hi Jim
 
One of the most common problems with Heathkits of the age of your SB401 is  
upwards drift in resistor values. They used carbon composition resistors 
which  can move upwards in value by 50 - 100% or more in that time frame. I 
would check  all of the resistor values with the rig cold.
 
I would also look at the hold-off bias on the cathode of the VOX relay  
control valve V12B which should be sufficient to keep the valve cut-off in the  
key up condition. Also check that C133 has not become leaky as that would  
forward bias V12B. I have not had any regular problems with the ceramic or  
silver mica capacitors that Heath used but foil or electrolytic units should 
 all be checked.
 
The manual has a table of typical voltages.
 
73

Bob

Bob F Burns
C Eng,  FIET, MSE
Amateur Radio Callsign: G3OOU
G-QRP Member No: 6907; Zone: 14,  QRA: IO91WH; WAB: TQ25
Joint web site with 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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