Frank - I'm a Centurion owner who has done quite a bit of work on the
amp and will offer some info that may be of help. You might consider
posting a question on the eHam Amplifiers forum which is frequented by a
couple of fellows who have been repairing amps for many years.
Since I have no way of knowing exactly which Centurion level you have
nor your overall level of knowledge, forgive me if I cover material you
already know.
First, there are multiple versions of some of the Centurion boards and
the Filament/AC board which contains the biasing components is one of
those. Apparently the earliest Centurions didn't cut off the tubes when
not transmitting and you'd see 150-200 ma of plate current with the
Standby/Operate switch in Standby. It's always worth comparing the
latest Centurion schematic for that board with the actual component
layout. Some Centurion manuals show the board level on the diagram so
you can compare what you have to the diagrams. My late manual (for a
black-face Centurion) no longer shows the levels. Assuming the latest
board level, common failures are Q1 and Q2 and the component failures
can be brought about by tube shorts or discharging the filter caps via
the interlock switch or an insulated screwdriver or chicken-stick. The
failure symptom is always plate current with the Standby/Operate switch
in Standby.
I've never measured the voltage at the K terminal (center tap of
filament transformer), but in transmit I'd expect to see the voltage
developed by the zener, which is 8.7V I believe. If the bias circuit is
working properly, with no signal and PTT not active, resistor R2 on the
Filament/AC board is in series with the K terminal to ground, thus
cutting off the tube. Not sure what the voltage developed is, but I'd
guess somewhere above 170V or so.
Regarding your problem of no output - I'd first suspect the Input low
pass filter board. You didn't mention whether this problem happens on
all bands or just one. Could be a failure of one of the relays on that
board, or failure of all of them due to the 12V supply to the board.
I'd first check to see that the 12V is getting to the board via the pin
on connector #19. That voltage then goes to the LP filter selector
connector on the front of the bandswitch - make sure that connector is
in place. Then check connector 19 on the low pass filter board again to
make sure the voltage is showing up on the pin for the band you've
selected. Failure to select an LP filter relay will result in an open
circuit on the amp input, resulting in all or most of the symptoms
you're observing.
73, Floyd - K8AC
On 4/26/2018 10:23 PM, Frank Sweeney wrote:
Ten Tec Centurion
Initial problem Description: occasionally on initial Turn, amp will not
drive No Grid or Plate current, high swr back to xcvr. Gentle tap near
relays fixed the problem.
Secondary problem description:
Within 2 weeks of daily use with initial problem the amp now exhibits about
10 mA grid current with no excitation, finals glowing and getting hot. This
condition went un-noticed for approximately 10 minutes.
Trouble shooting: 1. DMM shows short of Q1 Bias in circuit, upon removing
Q1 (PITA) found Q1 and all other active parts to be ok in the bias circuit
Short still present and pulled tubes and found F-G short in one tube
Installed new 3-500
3rd problem description: the amp will not drive, No Plate current, No Grid
current High SWR back to the xcvr
Trouble shooting: using external supply to power the QSK board, check
switching, Relays are switching, Output relay when closed reduces
received signal. Input relay is moving a signal from my signal generator
through the input filters to the cathodes as measured on my Oscilloscope.
The tubes are acting like they are biased into cutoff. Retest Bias circuit
components al test OK
Replaced ICs in QSK board even though the QSK board appeared to be working
correctly, added IC sockets and 15V transorbs while I had it apart. Did
not fix problem
Bias resistors test good
Help what else can it be?
How do I measure the bias voltage? Is it between ground and terminal K or
is it between connector 22 and Terminal K?
What voltage should I read at terminal K of the filament board during
1. Stand-by
2. Transmit
Thanks in advance for any help
73,
Frank K9IC
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