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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?

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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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