[TenTec] 255 Power supply

Al Gulseth wb5jnc at centurytel.net
Sat Mar 12 16:33:14 PST 2011

I'd guess that one of the pass transistors bit the dust (shorted). When that 
happens the crowbar circuit fires and shorts the output to ground to protect 
the rig (as you alluded to with your comment "it seems to be trying to power 
into a dead short"), resulting in the blown fuse (although according to the 
manual the internal 20A is the one that should blow in this scenario.) There 
are other possibilities but I think this is the most likely.

Regardless, since it almost immediately blows the fuse it's a "hard" failure 
and should be easy to track down. It's those intermittent failures that have 
helped make the top of my head resemble a cue ball HI HI!!

If it is a bad pass transistor I'd suggest replacing both of them while you 
have the PS apart. Last I checked Mouser etc. carried the 2N5301s at a 
reasonable price.

73, Al

On Sat March 12 2011 5:12:25 pm Jim Anderson wrote:
> I am having a bit of a problem with my 255 power supply. That is the one
> with the speaker that matches the Omni D series.
> This started with a shut down due to high SWR. Normally when this happens
> the power supply and radio shut down and then recycling the power brings
> things back to life. Now, when I turn it back on the fuse on the back that
> protects the AC side blows. If I am quick I can watch the amp meter on the
> face of the supply go full scale to the right just before the fuse goes so
> it seems to be trying to power into a dead short. It is not the radio as I
> have hooked my other TenTec supply up and everything works as it should.
> Also, it acts the same just sitting on the bench not hooked to anything but
> a jumper on the white wires.
> Any help would be greatly appreciated.
> Jim
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