[Amps] SB-220 troubles

[n1miw at cox.net]

I recently purchased an SB-220, and I went through it to see what might have been changed/modified over the years before I powered it up for the first time - in particular, the wiring change to the relay to protect the filament transformer (which was done). When I felt everything looked good enough for the first power-up, I plugged it in and turned it on - without the perforated shield in place to open the interlock. I ended up losing one of the diodes on the PS. It didn't seem like too big a deal - I just replaced the diode.

   Upon firing it up again (interlock bypassed permanently now), I had about 2600V in the tune position. I shut it off, put the covers on, and connected it up to the rig to see what condition the tubes were in. I was mostly concerned with 10 meters, but the output is right around 1000 watts, so I believe the tubes are fine (weak tubes have lower output on 10, right?). I shut the 220 off again to reposition the amp better on the shelf, turned it on again, and heard 3 loud snaps, and then 1 loud BANG. Little tendrils of smoke came out of the case around the PS board area, so I took it apart to inspect the damage.

   I saw the 3 resistors on the upper right side of the original board were literally blown in half, chunks missing, etc. I now have a new PS board on order.

   I'm curious why this happened AFTER I was able to see the amp working, putting out strong RF on all bands? I want to believe that when I first shorted out the PS via the interlock, I damaged that 1 diode, but weakened other components on the board. With that last startup, the inrush was enough to blow the PS completely. Inrush protection on order as well. Looking through the archives, I didn't see anything relating to an interlock "mishap", so I'm hoping you guys here can have an answer.

   This is what I get for being "slightly careless" with this amp, and not thinking everything through before throwing the switch.   ...Larry N1MIW