SB-200 issues...

[Kenneth G. Gordon]

The other day, I noticed that my SB-200 with supposedly 
new finals, was showing much reduced output, which, on 
80 meters, is usually about 700 watts.

Checking the plate idle current, I found that it was just 
barely moving the meter.

I have recently installed one of those Harbach SK-201, 
soft-key kits in order to use it with my TS-940SAT.

Thinking that perhaps C-19 (?) the 2 mfd 150 VDC cap was 
shorted, I temporarily removed it from the circuit. There was 
no change.

Jumpering the proper end of R-16 33 ohms 1 watt to 
ground, thereby bypassing the SK-201, I find close to 90 
mA of idle current (at least within the accuracy of my plate 
(cathode) current metering resistors), which according to 
the manual is about right.

I have yet to temporarily install a 0-100 mA meter in the 
centertap of the filament transformer to be more certain of 
the idle current, but in the meantime:

since the SB-200 apprently does not use any fixed grid 
bias, am I correct in assuming that the two resistors, R-16 
33 ohms, 1 watt, R-15, the 1.5 ohm metering resistor, and 
the two 33 ohm 1 watt resistors, R-21 and R-22 are the 
sole sources of grid bias?

I have the schematic for the Harbach SK-201. The "switch" 
in that unit is a MOSFET. At this point, it is looking to me as 
though that MOSFET is bad, exhibiting high resistance 
when "closed", but if it is bad,1) what could cause it to go 
"bad", and 2) why does the system still appear to properly 
operate the relay?

I am confused.

Ken Gordon W7EKB

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