[TenTec] Centurion Trubleshooting help

[MadScientist1948]

I am sure you already knew this but, sad personal experience told me that you must not run 3-500 tubes in any position but vertical. If you do, you will get the F-F short you got. Just a reminder in case you did not know.

The G-F short may have damaged one parts on the Filament-AC hard.

Try removing connector 22 from that board and applying ~5 VDC positive there to see if you get plate current as this will pull the cathode to ground plus the bias voltage.

You should get normal idle plate current.

This is the point at which keying voltage from the QSK board gets to the 3-500’s to set the cathodes to ground offset by the bias voltage (it uses the B- from the HV Rectifier board).

Anything to functioning on that board would give you a no plate current condition.

Gary
> On Apr 26, 2018, at 9:23 PM, Frank Sweeney <k9ic.ars at gmail.com> 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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