From: byterock@...
Date: Tue, 26 Oct 2021 10:56:58 PDT
Just a general general questions on replacing the paper caps.
Does it matter if the replacement caps are placed with their
foil side orientated in the same direction as the old caps?
?
I know in audio circuits it cam make a difference but does that
hold true on scopes as well?
Hi.
This is a job I need to do as well, also on a HP-130 'scope.??
In my case, a HP 130 B sn: 0001-04304? That was used as part of
the ground support for the UK's "V Bomber" fleet back in the
cold war days.
(I bought it from one of the XH558 support charity auctions.?
XH558 is/was the last flying Vulcan bomber.? Also the last truly
100% British designed and built 4 engine Jet aircraft.???? ?
? Sound on, and volume UP!)
All I've done so far, is find that the Power transformer is good,
as are all the main tube filaments, having disconnected the HT
windings to prevent electrolytic cap disasters.?? Even the front
panel power indicator, and display graticule lights still work.
In general, if HP saw fit to put foil cap's in a particular way
around as per the outer foil marking, it would probably be best
to fit the replacements the same way.
The "Fun" bit, these days, is finding replacement caps (likely
not paper dielectric) that even have the "outer foil" marked!
If you have another 'scope elsewhere, and a MF signal source
that can feed a capacitive load, then you can "test" unmarked
parts and find out which lead is the "Outer" foil.
Feed the cap from the signal source, via a screened lead.?? Use
the 'scope as a crude 'E' field probe, and place it on the side
of the cap' mid way between the leads, with the probe ground
lead going to the "earthy" side of the cap' as driven by the
source.
Compare the signal level with the cap' fed each way around.??
The largest signal, will indicate that the "Outer" foil is being
driven, not grounded.
I have done that many times with the 1uF 2kV "Wrapped Mica"
types we used in the tube based high power RF amps we support.?
(RF Output and Plate-line term' Decoupling.)
I was always taught, that the "Outer" foil of such cap's should
usually be connected to the lowest impedance path to "Ground",
or in the case of ramp generators, the "hot" end.? (I.e.? The
output side of an integrator circuit.)?? No doubt there are edge
case exceptions.
One thing I could use, is a good set of schematics for the HP,
all I have at present are somewhat "noisy" PDF scans.
This is one of the (several) retirement projects!? I'd like to
get it going, but keep the "grubbiness" it has acquired while in
decades of dry storage.?? Internally it is clean, but some of
the rubber shock mountings for the input stages are "a little
perished"...
Cheers.
Dave G8KBV.
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