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@vacuumtubes.com>
To: TenTec@contesting.com
Subject: [TenTec] Centurion Problem
Message-ID: <D6A233D0-59DF-4229-A537-0CADE2424698@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
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