[Heathkit] SB1000 problem

[Bruce Hudler WK3A]

Hello Group,
I'm working on a SB1000.
Originally it had a homebrew Soft Start board (it is wired for 120V) in it, mounted on top of the power transformer. That had to come off and changed back to original configuration so that I could install the SBA2001-1 QSK board (used one), which lives on top of the power transformer now. 
I've got everything working now after replacing one of the open PIN diode D101 on the QSK Board. 
Tunes up OK and on the air tests seem to be fully functional 160m - 10m
Now the real question - most often the damn thing trips the mains breaker instantly (15A 120V) in the house when powered up. After quick reset, the amp works normally and I'm puzzled by this. I powered off everything on the circuit, and tried a different breaker circuit.  No difference.
I don't know if this is why the original HB SS was installed or something to do with the QSK board. 
The amp is new to me so I don't have any past history. 
 Thanks
Bruce WK3A