On 10m...................... Parasitic maybe???? Any other damages, like plate
meter and meter shunts?? Is filament winding of trannie still intact?? Any
small pieces of "stuff" rattling around in either of the tubes?? Band switch
look o.k.?
Just thinking out loud.
73
Chuck
W4MIL
-----Original Message-----
From: n1miw@cox.net
To: amps@contesting.com
Sent: Thu, 12 Feb 2009 11:10 am
Subject: [Amps] SB-220 troubles
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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