[TenTec] Centurion 422 Fuses blowing

[W. Keith Hibbert]

Hi Brian, Keith here in Amherst, NY

It's been 40 years since I had a big rig/amp, but losing the primary 
fuse tended to be a power supply problem in my gear back in 1968 - 1971. 
  Problem there, I ran 866's for the rectifiers and they have gone the 
way of the dodo around here anyway...

I had high leakage in the filament transformer for the rectifiers, when 
the full HV came up, the leakage caused a current surge that popped my 
fuses.  Now, to translate that kind of a problem to your (more) current 
tech, what may be happening is a filament to plate short or leakage path 
in one of the 3-500Z's.  Another possibility would be a high-resistance 
leakage path in the transformer, in this, I am  assuming the filament 
windings are on the same core as the HV.  If not, check the filament 
transformer.

Now, in that Dark Age of QRO AM, I had a Megger and was able to check 
for high resistance faults.  I haven't had one for a long time, possibly 
sometime during an earlier geological epoch, I can't remember,  I would 
have to rig a test fixture to test for something like that now, probably 
a low-current HV supply, some 10 Megohm or higher loads and an isolated 
meter to look for leakage current.  Bringing back test bench memories, wow!

Gut feel keeps sending me to the rectifier stack and/or power 
transformer.  Is there an MOV across the primary?  Maybe it's firing 
under load?  How about acrid, loose electron smells/  Any ozone???

I would almost have to have it in front of me, but I hope my rambling 
helps somewhat.

73, Keith, WB2VUO
Amherst, NY: Grid FN02ox


On 01/30/2014 08:35, Bry Carling wrote:
> I havea nice Tentec Centurion 422 and the main 20A fuses blowing.
>
> It started spontaneously.
>
> I can turn on the main power OK and the filaments light up on the 3-500Zs.
> The 12 volt power supply is OK because the pilot lights come on.
> However when I flip the standby - operate switch to the Operate positionm the fuse or fuses
> will blow.
> I replaced them with good 250V 20A cuses.
>
> I checked the plate transformer windings and everything seems normal for all three windings.
> Nothing is shorted. The safety ground for the cover-ofrf condition is not across the HV plate
> supply. I checked that nothing else is shorting out the plate supply either.
>
> Does anyone have any idea what might be going on here?
>
> Many thanks - Brian Carling, AF4K