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From: "2465bct@... [TekScopes]" <TekScopes@...>
To: "Barry n4buq@... [TekScopes]" <TekScopes@...>
Sent: Sunday, April 3, 2016 1:33:51 PM
Subject: Re: [TekScopes] Re: 2465B U400 Availability?

Barry,

I found the same sort of thing on my 2465BCT. Everything was within
specification, but there was a lot of noise that was not power line related.
There was / is one which does have some power line related ripple, but well
within specification (I don't remember which, off-hand). It was also laced
with noise in addition to the power line ripple.

When I examined the schematic, I found that the +10v supply does NOT have an
output filter. It then feeds in as a reference to most of the remaining
supply
voltages. What happens is that every one of those attempts to duplicate the
noise on the +10v supply.

I wasn't happy with that so I modified my A2A1 board slightly and my noise is
much, much better. What I did was, first and most important, to put a 330uF
capacitor across VR1293 to clean up the +10v supply. That helps a lot. Then I
also increased the filtering capacitors across the board. The result is much
cleaner supply voltages. The supply voltage with power line ripple still has
it, but the ripple is now clean and not overlaid with a lot of noise. Here
are
the changes and parts that I used. They should be the same for your board.

A2A1 C1220 290-0939-00 10uF +100-10% 100V UHE2A121MHD6
120uF 100V 20% 105c 0.64
A2A1 C1240 290-0939-00 10uF +100-10% 100V UHE2A121MHD6
120uF 100V 20% 105c 0.64
A2A1 C1260 290-0942-00 100uF +100-10% 25V UHE1E331MPD
330uF 25V 20% 105c 0.46
A2A1 C1280 290-0942-00 100uF +100-10% 25V UHE1E331MPD
330uF 25V 20% 105c 0.46
A2A1 C1300 290-0942-00 100uF +100-10% 25V UHE1E331MPD
330uF 25V 20% 105c 0.46
A2A1 C1330 290-0942-00 100uF +100-10% 25V UHE1E331MPD
330uF 25V 20% 105c 0.46
A2A1 C1350 290-0942-00 100uF +100-10% 25V UHE1E331MPD
330uF 25V 20% 105c 0.46
A2A1 C1400 290-0943-02 47uF 20% 25V UHE1E331MPD
330uF 25V 20% 105c 0.46
A2A1 C1402 290-0943-02 47uF 20% 25V UHE1E331MPD
330uF 25V 20% 105c 0.46
A2A1 VR1293 Add capacitor across to filter 10v output. UHE1E331MPD
330uF 25V 20% 105c 0.46

Note that the increase in capacitance for these filters is not a problem. It
does NOT affect the scope startup or operation. The reason why is because 1)
the scope limits the start up current flow using the thermal resistors and 2)
the scope sequences the start up based on the rise in voltages. Between those
two circuit features, the scope is protected against an increased surge
current and the start up sequencing still works. Since I have made these
changes, I have never seen a startup issue from the scope nor have I blown a
fuse.

To remove the power line ripple from the one remaining voltage would probably
require substantially increasing the main capacitors on A3 - C1021 and C1022.
Those are originally 290uF and most people replace them with 330uF. The
problem there is size. Because they are high voltage (200v) they are fairly
large and there isn't much room for larger capacitors on A3. Possibly an
inductor could be added between the output voltage and ground, but that might
also be difficult due to space.


Mike

Regarding ripple measurement: When measuring (using the 2445), I don't
exactly see what appears to be PS ripple, but lots of low-level noise (when
viewed with LINE triggering). I assume that's CPU and/or other digital
noise. It presents itself as a band of about 5mV. Is this normal? I have
a 10x probe inserted into J119 and the ground clip attached to one of the
heat-sink fins on one of the Uxxx modules. I'd think this "noise"
shouldn't
be getting back into the rails but perhaps it's normal (or, perhaps,
something wrong with the way I'm measuring it)?

One of the rails (I don't remember exactly which one and will have to
check again to find that out) exhibited a small "double blip" that appeared
to me like it's coming from a typical unfiltered full-wave bridge but it
was
only a few mV. Not sure if that's normal but I think it was within
tolerance.