Re: Introduction and 465 Help Request


Bill Wollam
 

--- In TekScopes@y..., dhuster@p... wrote:
Bill,

Don't play with the gain. Both channels are affected the same way,
and that pretty much says that the problem is in the vertical
output amplifier. First thing to check is the collector load
resistors
that
go from the board up to the terminal post. If they're the beige,
rectangular, silicone-covered resistors, they may be your problem.
They're double-counterwound, inductance-cancelling resistors and a
common problem is that one of the two internal parallel windings
opens up, increasing the resistor value by a factor of two. Those
were Tek-made parts and Tek quit making them (they were very
troubleshome) and started using outsourced parts. If one or both
of
those resistors have an open winding, they'll be inductive, the
gain
will be 'way low and the pulse response will be a HUGE ringing.

Dean
Dean, thanks for your quick response. I m not sure which collector
load resistors you are speaking of but you mention them going from
the video output amp board to the terminal assy. so I assume you mean
the two 18 ohm resistors which couple the next to last output stages
(Q4478 & Q4468) to the final output stage drivers. These appear to be
OEM and measure 18 ohms as they should.

If you are referring to the collector resistors of the final stage,
they are 430 ohm wire wounds at 7 watts. The final coupling resistors
to the vertical plates are indeed TEK home grown which consist of a
200 ohm resister paralleled with a .3uh choke winding. These have
almost no DC resistance for obvious reasons. BTW, there are two
different versions of the TEK video output amplifier and I have the
discreet component version as you can probably tell by now.

As you suggested, I am looking at component values but haven't found
anything suspicious yet. Your help is appreciated.

Bill

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