On Sat, 2008-11-01 at 22:44 -0500, Tom Macon wrote:
> Thank you to all those who have responded - I certainly appreciate it.
> Here's some general comments in response to suggestions that I've received.
>
> I normally run the Omni on a 961 supply that I picked up a couple years ago.
> When I started having Herc problems, I tried putting the Omni back on the
> 9420 and found that it no longer worked, as I described in the original post.
>
> Here's a little more detail on the voltage trip problem. When the Herc is
> switched on, the Overload light comes on immediately and the relays for the 4
> supplies do not stay closed. The overload light and VCC fade away after 4-5
> seconds as the caps discharge. The over-voltage SCR on the Herc's
> distribution board remains tripped (since it's powered by the always-on
> supply) but since the Overload light is powered by the switched voltage, it
> does not stay on.
>
> The voltage trip problem started with occasional trips while I was talking.
> They became more and more frequent, and now it does it almost every time I
> switch on the Herc. I'm guessing one of the supplies is going unstable, but
> so far haven't been able to tell which one.
>
> With a digital scope, I have succeeded in capturing a momentary spike of
> about 20v on the switched voltage line that occurs at switch-on. It seems to
> decay toward the normal supply voltage but, since the whole thing shuts down,
> it's hard to tell. I wish I could scope the voltage of each p.s. while the
> Herc is running but I can't if it won't stay on. I think testing the
> individual supplies is the next step - I just gotta figure out a suitable way
> to do it.
>
> TU again for your help!
>
> - Tom, K9BTQ
Sounds to me like you have a melted pass transistor. Standard supply
failure mode and why there is a crowbar. Check the resistances from
collector to emitter. One will be very low.
Can't say it couldn't be a driver transistor or a control IC, but the
pass transistors are the most prone to failure because they run hottest.
73, Jerry, K0CQ
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