[TenTec] Centurion Squeal and Oscillation

[Henry Pfizenmayer]

>I don't recall the details of the original post but I seem to remember that
> the problem was on 40M and did not appear on other bands.  This raises the
> question of how does the fan being loose relate to 40M only?

 Squeal was a function of time on and heating , not the band of operation.

The oscillation just started at about the same time . (In fact in 
retrospect - I think it was oscillating when new and I did not catch 
it -being a new amp ,and not familiar with TT qsk approaches ,  I was 
suspecting the qsk circuitry acting up and keeping bias on with key up - I 
use an external wattmeter and seldom look at the internal power meters- so 
missed the fact that it was showing 250 watts forward and reflected 
internally because there was nothing coming out on my wattmeter -shame on 
me)   It blew the bias transistor after the first 7 qsos' with it brand new 
and TT wanted it back to look at it and the tubes so I sent it back.- thats 
a looonnngg story having to do with metering screwed up when it came back - 
but after a month or so finally got it working )
>
> As to input cable length, I'd expect that certain lengths, depending on
> source Z  loading, Orion output vs. Omni VI output, might have some effect
> relating to a feedback path.  On the other hand, in receive the Centurion
> input should be disconnected via the T/R relay.  Haven't looked at the
> schematics.

 In my mind they do some pretty screwy things with the input networks - 
looks like it was done about as hard as possible but I don't know what their 
approach was . While working on this - I also noticed there is no rf 
decoupling on the input
tr reed relay coil  that switches the exciter either to "through" or to the 
amp filters.
That relay  sits on the same board as the output tr relay and it has a long 
set of wires running to the qsk   board over by the input filter board - so 
any capacitance between the relay coil and the reed will couple rf over the 
wires which run by the amp output . I am not sure but I even think the 
ground for the input tr reed does not get picked up until it gets to the qsk 
board, the schematic shows it as floating , but I did not tear out the relay 
board to see if the coil was grounded on one side on that board.

 I almost went ahead and added a .01 or .1 and a VK200 ferrite bead  but got 
to thinking , the qsk board output that drives that relay is an emitter 
follower and they can have a propensity to oscillate with a capacitive load 
, and that may be why they did not bypass that line (or timing issues) .

Plus I have a sort of self imposed  rule when trouble shooting - don't start 
re-designing !!! (Especially since its still in warranty )


Thanks for comments _ Rgds-- Hank