SB-102, the saga continues

[Kenneth G. Gordon]

On 23 Sep 2007 at 14:16, Emil Dular wrote:

Emil:

As others have pointed out, there are several things you 
must check for:

1) Make certain all the small screws and nuts which hold 
each circuit board to the chassis are tight. Make certain 
there is no corrosion between any board and the chassis. 
Use a little DeOxit between them if you suspect a poor 
ground. I had an HW-101 which oscillated in the finals at 
odd times which was due to bad grounds in the circuit 
boards. When all the screws were tightened (and there 
were a LOT of loose ones!) the oscillations stopped.

2) Make certain you have a shield over both the driver 
(6CL6) and the tube immediately to its front. Make certain 
those shields are solidly grounded to the small "pin" that 
sticks up near it. Some people solder the two together. Be 
careful not to push down on the shields so hard that they 
short out some of the socket.

3) Make sure that the neutralizing "wire" for the driver does 
not stick down into the hole provided for it more than about 
1/4"

4) Make certain you are NOT using any of the later model 
6146 variants as Glenn Zook pointed out. In MOST SB/HW 
transceivers these WILL oscillate and it will be impossible to 
neutralize them, or the neutralization will not "hold", or will 
not be effective on specific bands.

Remember, that if you cannot find any 6146s, use 6883s 
instead. Since these are 12 volt versions, you will have to 
wire the two final filaments in parallel instead of series as 
they are now. Or, if you can find any 6293s, use them and 
leave the filament wiring alone.

5) You might also want to check the RF choke which 
connects your bias supply to the final grids. If that is not 
soldered in place properly, or is open, or intermittantly open, 
or its associated by-pass capacitor is shorting, your bias 
could be "going away".

Since your finals are drawing so much current I suspect you 
are having multiple problems. Start with the easiest first and 
work up. I tried to organize my list in order of difficulty. :-)

Ken Gordon W7EKB

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