[volt-nuts] New HP3458A

[Todd Micallef]

Since it was brought up about the possible leakage between Guard to Low, I
decided to check three 3458A. All three measured "open" resistance between
the two connectors in the open position as measured with an 8846A.

On two, I measured ~0.14-0.17 ohms with the switch closed. The first two
look fine and they are steady regardless of how I play with the switches.

The third meter measured around 100ohms and then drifted down below 90ohms
over a period of several seconds. Wiggling the button on the front of the
meter will make it jump around. This is the same indication that I had with
the bad front / rear switches on two of these meters.

The CLIP shows nothing else in between ground and guard, so it appears the
switch will need to be replaced. The switch should be a lot easier to
change compared to the bigger schadow switches. Fortunately, these are
available from Keysight for < $7 plus shipping.


Todd


On Fri, Aug 15, 2014 at 11:06 AM, Jason Watson <watson.jason at gmail.com>
wrote:

> I've also seen excessive Guard to Low leakage when varistor RV501 has gone
> bad (it's located on the front/rear switch pcb and it's possible to replace
> it while leaving the circuit board in place if you are careful).
>  HP/Agilent/Keysight Part number is 0837-0196, cross referenced to a Harris
> Corp. V430MA3A.
>
>
> On Fri, Aug 15, 2014 at 10:46 AM, Stephen Grady <grady.steve at gmail.com>
> wrote:
>
> > Randy,
> >
> > I have come across a few 3458A's that had leakage between Guard and Low
> > when
> > te guard is in external guard position. This was due to a leaky external
> > guard switch and/or leaky front rear switch. This can be quickly
> determined
> > by measuring resistance between guard and low with guard external. This
> > normally in not an issue except when you are using 3458A to measure
> voltage
> > with low above earth potential say in a bridge the guard low leakage will
> > be
> > loading other arm of the bridge.
> >
> > Kind Regards,
> >
> > Steve Grady
> > Sydney, Australia
> >
> > -----Original Message-----
> > From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
> > Behalf Of Randy Evans
> > Sent: Friday, 15 August 2014 1:22 PM
> > To: volt-nuts at febo.com
> > Subject: [volt-nuts] New HP3458A
> >
> > I picked up a used HP3458A today, which I needed for some precision DC
> > measurements i need to make. It passes all the self tests and the Auto
> Cal
> > but is there any thing else I can check ( I have a 14 day RR).  It reads
> a
> > 10V standard I made within a few tens of ppm, but it's not a 732A but
> that
> > is at least comforting.  It also reads an ESI 10Kohm standard resistor
> dead
> > on.
> >
> > The only problem is that the display has some faint pixels in some
> > locations, with three in the second row for every digit location dead.
> >  Likely a pixel driver I would think.
> >
> >  I am not too familiar with it yet but I noticed when I push the auto
> zero
> > button, the display has a blinking square until I hit a measurement mode
> > button; e.g., DCV, ACV, OHM, etc.  Is this normal?
> >
> > Regards,
> >
> > Randy Evans
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