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Introduction and 465 Help Request


Bill Wollam
 

Hello, I just discovered this group and look forward to
participating.
Right now I can use some technical help, though.

I am wrestling with a TEK 465 in the 32xxx serial # range that seems
to work fine in all areas except vertical position display. Both
channels have the same symptom, that is they have what seems to be an
accurate display when the vertical position is centered, but when the
position control for either channel is moved to either the top or
bottom of the screen, the trace only goes as far as the next to last
graticule and the signal gets compressed in amplitude until it
disappears altogether. I have been spending most of my troubeshooting
time in the vertical amplifier module because it appears that at
least one stage may be driving into cutoff or just the oposite (too
low gain) because the voltages on the vertical deflection plates do
not show the movement that the manual indicates. I have not yet tried
adjusting the gain pot because I need to find a signal source
compatible with the manual procedure on this. The manual gives you
the process but not much theory to allow for deviation.

If anyone has seen this before or has any suggestions, I would
appreciate hearing it. BTW, I am using a 453 to troubleshoot which
has an intermittent high voltage supply. That will be the next
project. Thanks in advance, good to have found this group.

Bill Wollam


dhuster@...
 

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


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