[GreenKeys] OT: Need help to T-Shoot this Tek 485 scope

[Paul Pennington]

I had a similar problem with my Tektronix 2235 scope.  It had a fuzzy trace
and the focus control didn't change it.  Tek likes to use 6 or 8 resistors
in series for high voltage circuits, and I found one of them was open.
Look on your schematic for something similar in the brightness control
circuit.

Paul in Augusta

On Tue, May 9, 2017 at 1:57 AM, Gil Smith <gil at baudot.net> wrote:

> I found a wealth of info on tek scope repair on a yahoo group:
> https://groups.yahoo.com/neo/groups/TekScopes/info
>
> Yahoo groups is a pain, but that is where they are.  I found lots of folks
> to help when I wanted to re-cap my trusty 300 MHz 2465 and 150 Mhz 2445.  I
> started with the 2445.  I replaced all of the caps with new/better spec
> units.  When I got it back together, the trace had some jittering that was
> not there originally.  F-me.  Also, the power switch was now stuck in the
> on position, so I need to plug the power cord in to turn it on.  Very
> disappointing outcome for a couple of days of work, plus parts cost.  I am
> really pretty careful/anal and not generally a hack, but I guess I missed
> something.  I bought a replacement power switch for the 2445, but I really
> have no interest in spending hours to do the R&R.
>
> As for my prized 300 MHz 2465, I now have no interest in screwing with it,
> as it still works perfectly.  It has great resolution for seeing things
> like undershoot, which my DSOs cannot resolve.  If it craps out some day, I
> will re-visit the re-cap (I have the bag of parts), but my original plan to
> re-cap is on hold unless needed.
>
> fyi, gil
>
>
> gil smith, AF7EZ
> greenkeys moderator
> gil at baudot.net
>
>
> -------- Original Message --------
> Subject: [GreenKeys] OT: Need help to T-Shoot this Tek 485 scope
> From: "drlegendre ." <drlegendre at gmail.com>
> Date: Mon, May 08, 2017 8:41 pm
> To: Green Keys <GreenKeys at mailman.qth.net>
>
> Hope this is OK..
>
> I'm trying to fix a Tek 485 scope that has lost the ability to control the
> trace brightness aka beam current. When it arrived, it had a power supply
> defect (shorted cap in +15) which I traced & repaired. But after running
> for 3-4hrs in shakedown, the traces lit up like the sun and cannot be
> controlled.
>
> I have replaced the diodes in the grid DC restorer circuit with 1N4007
> (equiv). No joy.
>
> Can anyone offer assistance? Right now, what I really +need to know+ is
> what voltage difference should I see between g1 and cathode? I read about
> -9V, but I'd think that's awfully low for a tube with 18K of acceleration.
> Would expect more like -40 to -50V.
>
> Replies off-list if that's appropriate. Thanks!
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