[volt-nuts] 845ab Led Mod Update
wpgold3637 at att.net
Thu Sep 25 11:14:33 EDT 2014
Sorry I should have said "uv" where ever I said "mv". Too early in the
morning where I live in the world.
----- Original Message -----
From: "Bill Gold" <wpgold3637 at att.net>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Thursday, September 25, 2014 8:10 AM
Subject: Re: [volt-nuts] 845ab Led Mod Update
> I check my 845ab yesterday, S/N 4470002. It still has the neon
> bulbs/photo resistors chopper.
> From what I can observe the Peak to Peak noise on the 1 mv range is
> about +/- 0.1 mv, or a total of around 0.2 mv. As far as temperature
> I don't know. Never tried to measure that parameter.
> Sure would love to know if the FLUKE modification to the 845ab made in
> 1993, when the new circuit was put into production, makes the noise lower.
> Or better, if your efforts can make the noise lower on the 1 mv range.
> ----- Original Message -----
> From: "Dallas Smith" <dosmith at outlook.com>
> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Sent: Wednesday, September 24, 2014 12:06 PM
> Subject: [volt-nuts] 845ab Led Mod Update
> > Well I finally got the 615nm Led's installed and the results seem so so.
> > The zero drifted was significantly on the 1uV range. So I used two
> > LT1634 1.25 Volt references with the current set by two 43k resistors
> > from the ±15 volt supplies. The previous reference used came from the
> > -0.4 volts at TP1 and +0.6 volts at TP8. Removed the 150k resistor at
> > the positive end of the zero control and lifted the negative end from
> > the circuit board the connected the ±1.25 ref to the zero pot. Much
> > better stability, drifts by noise about ±.1 to ±.2uv now on 1uv range.
> > Is this normal now? Never used a new Fluke 845ab before. The white led's
> > seemed to filter the noise better with its persistent, may go back. The
> > zero seems stable with ±2 F change in room temperature now.
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