[TenTec] Centurion Amp Woes

[Dr. Gerald N. Johnson]

On Sun, 2006-10-08 at 10:41 -0500, Charles Delaune wrote:
> I'm having an on going problem with my NEW Centurion. Several weeks ago the
> line fuses blew for no apparent reason. The amp had been on for several
> hours, but in Standby mode. All of a sudden the amp bit the dust as both
> line fuses blew. The line voltage is 240v and checked O.K. The tubes
> checked-out O.K., as well. That is to say, no shorts between elements
> (checked with ohmmeter) or plate arcing (removed tubes and replaced line
> fuses then turned amp back on - fuses blew, again). After I made these
> initial checks I sent the amp back for repair. It was repaired then returned
> with the following list of items which were replaced:
> 
> Relay and cover (relay contacts were welded)
> Q1 (transistor from HV-AC board)
> Rectifiers (looks like all of them) in HV rectifier board 
> Power Transformer (Hi impedance short)
> 
> I've never heard of a Hi impedance short, by the way! In electronics school
> I was taught that a short, if anything, is zero or near zero resistance.
> Anyway, it looks as if the power transformer was defective and took all
> those other components with it when it bit the dust!
> 
> Since the amp was repaired (about two weeks ago), it has been operating
> flawlessly. Then, last night, after making a few contacts and being on for
> approximately six hours the line fuses blew AGAIN!!! The fuses blew about
> two hours after I had made my last QSO on 40 meters with the amp. Like
> before, the amp was on but not being used. Then, out of nowhere the line
> fuses blew. I'm going to assume that, once again, I have a defective power
> transformer! Looks like, Monday morning, I'll be on the phone, once again,
> with Ten Tec! It's a shame because when the damn thing is working it's one
> of the finest amplifiers I've ever operated, but I'm getting close to the
> point where I'm ready to tell them to either replace the amp or refund my
> $$$! 
> 
> My question is...have any of you other Centurion owners experienced this
> problem? Is there a power transformer issue with the Centurion. In other
> words...is it possible that there was a run of defective power transformers
> that "fell through the cracks", sort of speak? I understand Ten Tec builds
> their own power transformers for the Centurion. If there is an "issue" with
> those transformers I feel the problem will only happen again if the
> transformer is replaced or I opt for a new Centurion. 
> 
> Charlie - N5BLY
> 
>  
> _
There was a previous amplifier sold as a Heathkit and a commercial
maker. It had a similar failure mode, wreck the middle of the high
voltage winding or the plate transformer while sitting at idle. The
problem turned out to be that the RF bypassing on the PA plate feed, the
low value PA plate feed choke, and the LF characteristics of the plate
transformer made a parasitic circuit for LF (50 to 100 KHz likely)
oscillation that put excess voltage to the middle of the plate
transformer winding. The amplifier is still on the market so I presume
the parasitic has been cured, probably by increasing the value of the RF
bypassing capacitor, perhaps adding a substantial RF choke on the DC
side of the plate feed bypass. At least those are the places I'd start
with circuit modification to swamp the LF parasitic. That was a single
3-500Z as I recall.


-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer