[TenTec] Centurion Problem

[Floyd Sense]

Mike - I've had a bit of experience working on two Centurions and have 
seen the same problem a couple of times.  In my late Centurion 
operator's manual, it shows a 2N3055 for Q1 on the Filament-AC board, 
and that's what I used in the last repair.  You should also check Q2 on 
the Filament-AC board - on my first Centurion that was also blown.  The 
diagram shows Q1 as a MPS U01 transistor, but those are rather expensive 
when you can find them.  It's just a bipolar NPN.  When I last replaced 
one, I used a MPSW42, a high voltage NPN good for 300V.

/Any kind /of arcing event is likely to blow Q1, including a shorted 
tube or even removing the amp cabinet top before the power supply is 
completely discharged.  If you do that, you get an arc to ground when 
the safety interlock switch activates.  At the advice of someone on the 
Amplifiers forum on eHam, I installed a 60 volt transorb across resistor 
R2 on the Filament-AC board to hopefully prevent loss of Q1 in the future.

73, Floyd - K8AC


> Message: 1
> Date: Mon, 20 Nov 2017 11:47:08 -0600
> From: Michael Marx <sndtubes at vacuumtubes.com>
> To: TenTec at contesting.com
> Subject: [TenTec] Centurion Problem
> Message-ID: <D6A233D0-59DF-4229-A537-0CADE2424698 at vacuumtubes.com>
> Content-Type: text/plain;	charset=us-ascii
>
> Hi Guys,
>
> I have a Centurion that experienced some kind of arcing event which resulted in one of the traces on the Filter/Rectifier board that connects to the secondary of the transformer to actually vaporize.   I bridged the trace, replaced the capacitors and checked the diodes.  After repairing / rebuilding the board, I reinstalled it and now have normal B plus.  HOWEVER, I have 200 mA of standby plate current.  This suggests the bias transistor is blown and I have had this problem in the past.  Unfortunately, I replaced the bias transistor and still have 200 mA of standby plate current.  My question is:  Do any of you have an any idea of what to check besides the bias transistor.  Or, is there some common cause for the transistor to fail?  I bought 5 of them from Mouser and hesitate to keep replacing them until I know what is causing the failure.  The transistors I bought are IDENTICAL to the transistor I bought from Ten Tec the last time it failed.
>
> thanks
> 73
> Mike WB0SND
>