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"Juan Jimenez" <flybd5@xxxxxxxx> wrote in message
news:451bec7e$0$19619$88260bb3@xxxxxxxxxxxxxxxxxxxx
> Hi folks,
>
> Does anyone have a calibration/maint manual for a JDR Instruments model
> 3500 o'scope? I bought it cheap for my son ($100), he is building his own
> analog synth, and it works great, but the focus control no longer brings
> the trace into focus when I turn it all the way to the right. I need to
> adjust this but I don't have a manual.
>
> Detailed instructions will do as well.
>
> bd5 at
> bd5 dot com
>
> Thanks!
>
>
If it's not a problem with the tube itself, then likely one of the high
value resistors in the chain which divides down the PDA voltage to the right
level for the focus grid ( via the focus control ) will have gone high in
value. In the absence of a set of schematics for it, I would follow the
connections from either end of the focus control ( especially the clockwise
end which is probably the higher voltage end ) back to wherever they go, and
look for some big-ish high value resistors right there, and carefully check
their values. If you find one that has changed substantially in value, make
sure that you use a resistor with a high voltage specification to replace it
with ( many readily available resistors are only rated for operation up to a
couple of hundred volts )
Arfa
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