[Amps] SB-220 troubles

[Scott McGrath]

Bad caps - blown diode was your first clue HVPS is designed to take the 
interlock crowbar on the plate supply without failure.    You could have 
a fresh internal short on the tubes as well.    To test once you get new 
PS remove the tubes and bring up HV normally should be ok.    Check the 
tubes with a ohmmeter to ensure you have no resistive paths anywhere but 
the filament if ok then reinstall in amp AFTER CHECKING HV WITH 
VOLTMETER TO MAKE SURE ITS REALLY NOT THERE!!!   We don't need any 
accidental SK's time takes enough of us as it is.   Then bring up HV but 
be ready to shut down QUICK

Scott N1JIN

n1miw at cox.net wrote:
> 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
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