[TenTec] Centurion Trubleshooting help

[Floyd Sense]

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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