.......
> 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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