7104: Self-blurring focus


Bruce Lane
 

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.

Thanks much.


-=-=-=-=-=-=-=-=-=-=-=-
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?"


John Miles <jmiles@...>
 

Usually aged resistors in the focus voltage divider are responsible for this
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

-----Original Message-----
From: Bruce Lane [mailto:kyrrin@bluefeathertech.com]
Sent: Thursday, January 15, 2004 3:47 PM
To: tekscopes@yahoogroups.com
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.

Thanks much.


-=-=-=-=-=-=-=-=-=-=-=-
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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Rex W. Athey <mister_twister@...>
 

I've been starting to notice a problem with most carbon resistors in general that they are way off in value. I discovered this when a couple years ago I went through some of my parts inventory and found values to be way out of tolerance. Best example I remember was 47K, probably because I had some many of them. Now when ever I get replacement resistors, I avoid carbon resistors as much as I can for this reason. Have not had any problems replacing resistors in any piece of equipment since. There are some exceptions, but I now try to avoid the aggrevation of carbons all together. I'm mostly talking old carbon resistors in inventory.

Now I'll probably start a bad resistor thread to go with the bad electrolytics / paper caps threads. Been weeding out some of them too.

Rex


regman10
 

I have found the same thing. I always verify the value of caps and
resistors in my spare parts box before using them.


I've been starting to notice a problem with most carbon resistors in general
that they are way off in value.