I've never seen a Centurion, but if they are anything like ALPHAs, they
may have a resistor in a step start configuration -- if this resistor is
popped (usually a big 10W+ ceramic job) it can and has caused Alpha 78s
to exhibit precisely this symptom. Just a thought.
Good luck
Dave N7UE
On Wed, 27 Aug 1997, Rich Measures wrote:
> .......
> > My 422 TenTec Centurion has worked fine for several years. I haven't
> > used it all summer, but decided to turn it on last night. As soon
> > as I turned the power switch on, both primary line fuses blew along
> > with the circuit breaker. No smoke, and no visible damage after
> > inspecting it inside and out. I took a reading across the 220V line
> > cord (unpluged of course) with the fuses replace and read 6.6 ohms. Is
> > this normal? Any ideas? The only thing I did before applying power
> > was switching the standby switch from operate to standby.
> > TNX, Denis, W4DC
>
> You are not the only Centurion owner who has experienced this problem,
> Denis.
>
> I would troubleshoot this amplifier thusly:
> Replace fuses; turn on. If the HV is normal, turn off and check Q1 and
> D1 on the FILAMENT AC BOARD for a short. (Q1 and D1 function like a
> zener bias diode). Also check D21 on the HV RECTIFIER BOARD for a short.
> Replace if necessary. These component shorts can either be caused by a
> HV arc to the chassis, or it can be caused by an intermittent VHF
> parasite, which typically causes a brief HV arc from the anode circuit to
> the chassis.
> - Check the resistance of R1 and R2 [100 ohms, 2w]--the parasitic
> suppressor resistors-- on the PLATE CONNECTOR BOARD. (note - This
> operation requires unsoldering one end of each resistor to make the
> test.) If the values of resistance are above the tolerance limit, IMO,
> an intermittent VHF parasitic oscillation was probably the reason for the
> problem. If the measured resistance is below the lower tolerance, such a
> change is typical during 10m operation, and, IMO, no VHF parasitic is
> indicated.
>
> Shortly after the first article on parasitic suppression was published in
> the October 1988 issue of *QST*, Ten Tec reportedly tried using low
> VHF-Q/low VHF-Rp parasitic suppressors. However, during 28MHz operation,
> the Ten Tec engineers observed signs of heating in the VHF suppressors,
> so they went back to using their conventional suppressor design, which
> ran cooler at 28MHz. IMO, it is not possible to build a low Rp VHF
> parasitic suppressor that does not dissipate 1 - 2 % of the available
> power at 28MHz.
>
> Rich---
>
> R. L. Measures, 805-386-3734, AG6K
>
>
> --
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>
David B. Ritchie, Esq.
D'Alessandro & Ritchie, P.C., Intellectual Property Lawyers
1731 Technology Drive, Suite 710, San Jose, CA 95110-1312
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