SB-401

[Herman N4Ch]

Hello All........
 
I built my SB-401 back in 1967 (gee, it doesn't seem like THAT  long ago, 
but it was).   It worked fine the first decade or two I had  it, but sometime 
later (probably during its 3rd or 4th decade, not sure) it  developed a 
similar problem where the rig would "key" itself almost from a cold  start (it 
didn't take it an hour to "warm up" to this problem like  yours).   The 
problem turned out to be a bad electrolytic (sorry, I  can't remember which 
one), and the mechanism for unwanted relay latchup was AC  riding on the 
supposedly filtered DC in some of the transmitter's  circuitry.   This included 
unwanted AC in the mike amp and VOX  circuitry, which was the cause of the 
problem.........the VOX "thought" that it  was receiving legitimate audio coming 
in from the mike line, when in fact  AC (from the lack of proper power 
supply filtering.....the VOX "thought" that  120 Hz AC WAS audio) was what was 
causing the relays to pull in.   I  found out what was going on pretty 
quickly.........I checked all the power  supply lines with an AC-coupled scope.   
One (that ultimately powered  the mike amplifier and VOX stages) had a 
significant amount of AC ripple, and  changing out the bad 'lytic in that circuit 
was all it took to fix the  problem.   Seems like I had a similar situation 
occur in a Drake TR-4  as well.   Good luck.
 
73, Herman, N4CH.
 

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