[Heath] SB-200 bias shift - solution found

[Kenneth G. Gordon]

Well, this has been driving me nuts: when I use the Harbach soft-key unit with my SB-200, 
grid bias is -12VDC when the rig is keyed on. It should be ~2.0 VDC.

Keying the amp withOUT the Harbach unit provides ~2.0 VDC bias.

The IRF-9610, the MOSFET keying transistor, is supposed to have an ON resistance of 3 
ohms or less. MIne was exhibiting an on resistance of about 180 ohms or so.

So, I bought a new IRF-9610 from Mouser and installed it today.

Result: no change.

Hmmm, says I, what is going on here?

After spending an hour or two checking and rechecking everything "checkable", and finding 
nothing wrong, I thought, just for something to do, I would check to see how the bias supply 
voltage would change from no load to full load. Voltage drop was significant, around 60 VDC.

Ah, says I, perhaps bad components in the 1/2 wave -130VDC bias voltage circuit? 

So I shunted C3, the 33 mfd electrolytic with a 500 mfd 200 VDC cap. Voltage drop was the 
same, both with and without the Harbach unit. But, since there is a 465 ohm resistor, R1, in 
series with the rectifier diode, that was to be expected.

HOWEVER, when I reconnected the Harbach keyer, there was also 6 VAC at the gate of the 
IRF-9610 withOUT the added filter cap. 

WITH the added filter cap, there was 0.05 VAC at the gate of the IRF-9610...

...and lo and behold, the bias voltage at the grid was now ~2.0 VDC!

Apparently, the AC voltage at the gate of the IRF-9610 was sufficient to prevent it "closing" all 
the way, so it exhibited ~180 ohms instead of ~3 ohms, which "raised" the grid bias voltage to 
-12 VDC.

I was very surprised at this result.

Ken W7EKB