Usually aged resistors in the focus voltage divider are responsible for this
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symptom. They may be potted in a hybrid, in which case you'll have to break
some traces and rebuild the divider on an outboard basis.
I am slowly learning to view all carbon-composition resistors above 100K
ohms with the same suspicion that people reserve for old paper capacitors.
-- john KE5FX
From: Bruce Lane [mailto:firstname.lastname@example.org]
Sent: Thursday, January 15, 2004 3:47 PM
Subject: [TekScopes] 7104: Self-blurring focus
Well... I think I may have a couple of leaky caps or something.
Having cycled the pots a whole bunch on my newly-acquired
7104, I find that they're doing well enough. However, it looks
like there may be an issue in the focus section.
What will happen is that the trace will be razor-sharp and
hair-fine one moment, and then it'll start blurring. This affects
the entire screen (readout included). The interesting thing is
that the focus will blur up to its (apparent) maximum degree of
'defocused,' and then it'll snap back to razor-sharp and clear.
It'll do this three or four times, and then stay sharp or blurred
at any given moment. After the 'scope's been on for an hour or
so, this behavior disappears.
Has anyone seen this symptom before? I suspect
thermal-intermittent caps in the high-voltage area, but I don't
yet have a service manual for the unit. Pointers appreciated.
Bruce Lane, Owner & Head Hardware Heavy,
Blue Feather Technologies -- http://www.bluefeathertech.com
kyrrin (at) bluefeathertech do/t c=o=m
"If Salvador Dali had owned a computer, would it have been
equipped with surreal ports?"
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