[volt-nuts] 845ab Led Mod Update

Bill Gold wpgold3637 at att.net
Thu Sep 25 11:10:14 EDT 2014


    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 drift
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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