[TenTec] anyone lose a centurion hv xfmr?

[Dr. Gerald N. Johnson]

On Wed, 2006-04-12 at 01:32 -0500, John (KE5C) wrote:
> the short question is, does anyone know the approximate secondary resistance of 
> a centurion hv xfmr?  the reason i want to know is that i am trying to determine 
> if I've lost my centurion hv xfmr without actually measuring the secondary 
> voltage.

I'd say your secondary winding is open. The DC resistance of a typical
E-I core winding is typically under 4% of the output load. The DC
resistance of a tape wound core winding is often under 2%. So your
output load is 1500 volts, at half an amp? 3000 ohms. 4% is 120 ohms. 2%
is 60 ohms.
> 
> the full story is that i had the amp on but not in use.  i thought something 
> fell down in the shack due to an unusual noise.  everything looked in order, but 
> then i noticed the amp was off and one of the line fuses had blown.  i took the 
> cover off and saw nothing unusual and decided to replace the blown fuse and put 
> the cover back on.  this seemed to bring the amp back to life, so i put it in 
> line and turned the orion down to about one watt and keyed.  all seemed to work 
> (i am using a qsk loop), so i advanced the drive to 20 watts and fortunately i 
> was monitoring grid current because it went sky high in a hurry.  i switched the 
> meter to hv, and there was none.  after some *extremely* careful 
> troubleshooting, i determined the hv primary is receiving 230 vac, but i have no 
> way (and little courage) to try to measure the transformer secondary voltage. 
> however, the secondary resistance (with the rectifier board connected) is 
> initially more than 1k ohm.  the apparent resistance falls with time due to the 
> capacitors charging via the vom, and reversing the test polarity restores the 
> resistance to more than 1k ohm until the other bank of capacitors start to 
> charge.  this also suggests to me that both sides of the full wave bridge are 
> intact.  hence, i am thinking i may have an open hv xfmr secondary, although why 
> would that blow a fuse???.  i don't know what the xfmr resistance should be, but 
> certainly less than 1k, right?

You are seeing only the rectifier, filter capacitors and bleeder
resistor. The winding is open.
> 
> thanks for any comments, john
> 
Certain Heathkit amplifiers (single or dual 3-500Z) lost transformers
spontaneously a few years ago. I think they finally found the cause was
a LF parasitic in the PA sitting idle that excited a resonance in the
transformer winding damaging insulation with heat at only some interior
spot. With time that insulation failure lead to shorted turns. Shorted
turns heat a great deal but don't change the primary current much.
Eventually the heat from those shorted turns wipes out more insulation
until the number of shorted turns gets large enough to draw lots of
primary current, then the fuse blows and about that time the heat opens
the winding with the shorted turns. What it took to prevent the
transformer failure was to increase the B+ bypassing to keep the LF
parasitic from using the transformer winding. Neither the ordinary plate
feed choke nor the ordinary HF bypass capacitor had much effect at VLF.

The suggestion of feeding the primary with 6.3 (multiply the meter
reading by 20) or 12 volts AC (multiply the meter reading by 10) is a
good safe way to check winding voltages on a high voltage transformer
especially when your VOM doesn't have a high enough voltage range.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer