SB400 - Low Grid Drive on 40 Meters

[Kenneth G. Gordon]

Mark:

I have been reading the mail here for some time concerning this 
problem.

I have a question that I hope you don't mind answering:

1) In the TUNE condition, how much grid drive does your meter 
show on the other bands?

(I am suspecting that any metering resistors included in the grid-
current part of the circuit have gone bad.

2) I had a somewhat similar problem with my never-used SB-401 
some time ago. However, in MY case, I had no grid drive on 40 
meters at all. 

The problem was eventually traced to an internal short in one of the 
two 6EW6 mixer tubes, specifically, V-4. The symptoms were 
VERY odd, in that grid-drive and output on all other bands was 
absolutely correct, the HET oscillator output for 40 meters was the 
correct level and frequency, and everything else appeared to be 
completely correct. I simply had no 40 meter output. 

However, I DID have output at about 8.6 Mhz.

At one point,I even removed the 8.6 Mhz trap and still had the 
problem.

In your case, having approximately correct 40 meter output, I am 
suspecting the grid metering circuit is at fault.

BTW, those 6EW6s are somewhat special in that they exhibit an 
unusually high transconductance. I understand from other sources 
that they are also somewhat "delicate".

Ken Gordon W7EKB

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