SB220\221 Bias mod

[357]

I guess I remove the red jumper and run the red wire direct to the coil and move the resistor 1 hole down.
Anyone done this before, will it cause me any trouble?
Will it mess with the Harbach Softkey?

thanks

 

[357]

OK I went ahead and did the mod. So far nothing out of the ordinary, no grid current in RX or anything silly.

I also added a 50PF mica cap on the jumper bar of the relay and got the SWR down when the amp is inline on 10M.
Something like 47-48pf would be perfect but I only had 50pf kicking around

 

[BlowinSmoke]

The best capacitor to use to completely null the SWR is 33pf. It nulls any rectance and drops swr flat. Also note that on the SB-220 vs the SB-221, the input switch pin numbers were also reverse. That means that comparing the SB-220 to the 221 the common pins of the input switch were reversed. That means that the coax on the top pin on the 220 was then connected to the bottom pin on the 221. And vice versa. This is important to know is you ever do any work on the input circuits. The SB-221 did not come with the parts to enable 10 meters in the original kit. If a 10 meter kit was not purchased at time of the kit build then the easiest way to add 10 meters is to obtain the a completed network from an SB-220 and install it in place of the old 221 network. Remember about the input pin reversal. If you fail to observe that the input and output of the input will be reversed and the input SWR will be sky high.

 

[357]

I tried a variable 404 arco on pin 7 of the relay, input coax and got the pass through swr down good but the input swr with the amp running went up and output fell off 300w.
I have the best luck with 48ish pf from the 1-2 jumper bar to ground for pass though and then it disengages and uses the input coils for a 1.6 and the transmitter seems to not fold back too bad, still get 1150 key down.

I'm just gonna leave it my eyes are very unhappy and its a pin the tail situation.
I dont like that

 

[Shadetree Mechanic]

Any updates?

 

[357]

Any updates?

She's getting on down....just need some skip.
The bias mod she runs nice and cool