Re: 5440 no HV or beam

Fabio Trevisan

Hello Rick,
I've been following from a distance. I don't have any experience with the
But I do have some experience with HV probes and there's one tricky thing
with them that it took me sometime to figure out and you better be aware of.
If your meter is Auto-range, and especially, if it auto-ranges all the way
down to its lowest range (e.g. 200mV for a 2000 count meter, or 400mV for a
4000 count etc...) Then it's almost 100% sure that in its lowest range it
doesn't have 10MOhms impedance and it may have as much as 1Gohm depending
only on the input stage of the AD.
Even if the lowest range is a separate scale manually selected... Then this
scale (the lower one) may have 10Mohms, but the other ones that are
auto-range don't.
On the 1:10 scale (2V on a 2000 count meter), impedance is tipically
11.111111Mohms, on the 1:100 scale, it's 10.101010Mohms, on the 1:1000
scale it's 10.010010Mohm and on the 1:10000 scale, it's 10.001000.
Therefore, if you leave the DMM in auto-range and read lower voltages, the
input impedance may become 11.111M and your readings can be up by about
11%... And if the DMM is one that autoranges to the most sensitive scale,
the impedance will be very high (1Gohm thereabouts) reading will overflow
and range will change. But as soon as next range kicks in, the impedance
falls, the reading falls and it will switch again to the most sensitive
range, over and over again and the reading won't stabilize.
All this is due to the fact that most auto-range meters need to put the
10Mohm resistor in series with its input (and not in parallel) so that,
even if it's auto selected to its most sensitive scale (1:1} and you try to
probe a much greater voltage, the 10M ohm series resistor and the
protection diodes will protect the input... For the other attenuation
factors, the meter's auto-range circuitry can use analog switches to
insert, from the input (which has the 10Mohm series resistor) to ground,
resistors which are fractions of 10Mohm, e.g. 1:9th ; 1:99th ; 1:999th and

So, watch out for auto-range DMMs and lower ranges while using an HV probe
as tbe readings can become quite off.


Ranges from 20V

On Oct 21, 2016 10:21 PM, "rpbale@... [TekScopes]" <
TekScopes@...> wrote:

Thanks for chiming in so consistently. Very much appreciated.

Magnus, I've checked all the voltage on the horiz. board and it checks out
fine. Here's the schematic with my measurements in yellow.

David, I'm checking the HV board next especially the -3k point. I'm using
a HV probe which I'd never used before. If it really is 150v then I could
use my regular meter.


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