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Newer Fluke 845ab manual with chopper mod?

DS
Dallas Smith
Sat, May 3, 2014 4:18 PM

Does anyone have the schematic for the Fluke 845 chopper modification as
stated in front of manual to be released approximately Dec 1993?

Manual statement:
'The new circuit now employs optically-isolated Bilateral Analog FETs,
and is fabricated on an additional circuit board.'

Frank, from Volt-Nuts 21 Mar 2010, suggested that: The 335A_D manual
contains the coil chopper for the calibrator circuitry, (page 119/120)
and the opto chopper version for the included Null volt meter, same
circuitry as of the 845A/B. Compare this with the 335_D manual,(page
169) where you can find the MOS FET version for the calibrator amplifier.

Has anyone tried this circuit for the Fluke 845ab?

Does anyone have the schematic for the Fluke 845 chopper modification as stated in front of manual to be released approximately Dec 1993? Manual statement: 'The new circuit now employs optically-isolated Bilateral Analog FETs, and is fabricated on an additional circuit board.' Frank, from Volt-Nuts 21 Mar 2010, suggested that: The 335A_D manual contains the coil chopper for the calibrator circuitry, (page 119/120) and the opto chopper version for the included Null volt meter, same circuitry as of the 845A/B. Compare this with the 335_D manual,(page 169) where you can find the MOS FET version for the calibrator amplifier. Has anyone tried this circuit for the Fluke 845ab?
DS
Dallas Smith
Tue, May 6, 2014 5:15 PM

Did Fluke ever make the 845ab Null Meter with the optically-isolated
Bilateral Analog FET chopper? Is so where's the schematic? The 335D is
not optically isolated. Will not having this isolation effect the
leakage? What is the part number of the optical-isolated Bi-FET?

My Fluke null meter new neon's are now starting to flicker with unstable
zero again.
I modified the meter to use curve tracer selected INS-1 Russian neon
dots. Selected for equal break down voltage and brightness. Worked well
for about two years. The NE2? with the isotope in it is impossible to find.

Has anyone used some type of circuit modification for this meter?

Dallas

On 5/3/2014 12:18 PM, Dallas Smith wrote:

Does anyone have the schematic for the Fluke 845 chopper modification
as stated in front of manual to be released approximately Dec 1993?

Manual statement:
'The new circuit now employs optically-isolated Bilateral Analog FETs,
and is fabricated on an additional circuit board.'

Frank, from Volt-Nuts 21 Mar 2010, suggested that: The 335A_D manual
contains the coil chopper for the calibrator circuitry, (page 119/120)
and the opto chopper version for the included Null volt meter, same
circuitry as of the 845A/B. Compare this with the 335_D manual,(page
169) where you can find the MOS FET version for the calibrator amplifier.

Has anyone tried this circuit for the Fluke 845ab?


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Did Fluke ever make the 845ab Null Meter with the optically-isolated Bilateral Analog FET chopper? Is so where's the schematic? The 335D is not optically isolated. Will not having this isolation effect the leakage? What is the part number of the optical-isolated Bi-FET? My Fluke null meter new neon's are now starting to flicker with unstable zero again. I modified the meter to use curve tracer selected INS-1 Russian neon dots. Selected for equal break down voltage and brightness. Worked well for about two years. The NE2? with the isotope in it is impossible to find. Has anyone used some type of circuit modification for this meter? Dallas On 5/3/2014 12:18 PM, Dallas Smith wrote: > Does anyone have the schematic for the Fluke 845 chopper modification > as stated in front of manual to be released approximately Dec 1993? > > Manual statement: > 'The new circuit now employs optically-isolated Bilateral Analog FETs, > and is fabricated on an additional circuit board.' > > Frank, from Volt-Nuts 21 Mar 2010, suggested that: The 335A_D manual > contains the coil chopper for the calibrator circuitry, (page 119/120) > and the opto chopper version for the included Null volt meter, same > circuitry as of the 845A/B. Compare this with the 335_D manual,(page > 169) where you can find the MOS FET version for the calibrator amplifier. > > Has anyone tried this circuit for the Fluke 845ab? > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > >
CS
Charles Steinmetz
Tue, May 6, 2014 6:06 PM

Dallas wrote:

Did Fluke ever make the 845ab Null Meter with the optically-isolated
Bilateral Analog FET chopper?

I have never seen an 845 with FET chopper, despite the engineering
change specification most 845 owners have seen.  I've heard of one or
two 845s that may have been repaired (by Fluke) with a FET chopper
assembly (perhaps using 335D parts ??), but I'm not sure any 845s
left the factory that way.

These days, there are integrated chopper-stabilized ("zero offset")
op amps that can do better than chopper amplifiers (e.g., LTC1050,
LTC1052, LTC2054, LTC2057, LTC7652, ICL7650, MAX4238/4239).  Randy
Evans started two threads here back in December 2013, one on
modifying the Fluke 887 and one on modifying the 845.  He made
available his notes, which were quite thorough.  Check the archives
starting on Dec. 22, 2013.

Best regards,

Charles

Dallas wrote: >Did Fluke ever make the 845ab Null Meter with the optically-isolated >Bilateral Analog FET chopper? I have never seen an 845 with FET chopper, despite the engineering change specification most 845 owners have seen. I've heard of one or two 845s that may have been repaired (by Fluke) with a FET chopper assembly (perhaps using 335D parts ??), but I'm not sure any 845s left the factory that way. These days, there are integrated chopper-stabilized ("zero offset") op amps that can do better than chopper amplifiers (e.g., LTC1050, LTC1052, LTC2054, LTC2057, LTC7652, ICL7650, MAX4238/4239). Randy Evans started two threads here back in December 2013, one on modifying the Fluke 887 and one on modifying the 845. He made available his notes, which were quite thorough. Check the archives starting on Dec. 22, 2013. Best regards, Charles
DS
Dallas Smith
Tue, May 6, 2014 6:29 PM

Thank you Charles,

The Dec thread looks interesting, maybe a good solution. But was not
looking forward in replacing the amp circuit. I think I'll try a optical
isolated version of 335D circuit. This MOSFet is easy to apply to the
null meter circuit without a major overhaul. If successful I'll share my
simple mod. If the performance fails, I'll try the Dec 2013 method.

Dallas

On 5/6/2014 2:06 PM, Charles Steinmetz wrote:

Dallas wrote:

Did Fluke ever make the 845ab Null Meter with the optically-isolated
Bilateral Analog FET chopper?

I have never seen an 845 with FET chopper, despite the engineering
change specification most 845 owners have seen.  I've heard of one or
two 845s that may have been repaired (by Fluke) with a FET chopper
assembly (perhaps using 335D parts ??), but I'm not sure any 845s left
the factory that way.

These days, there are integrated chopper-stabilized ("zero offset") op
amps that can do better than chopper amplifiers (e.g., LTC1050,
LTC1052, LTC2054, LTC2057, LTC7652, ICL7650, MAX4238/4239).  Randy
Evans started two threads here back in December 2013, one on modifying
the Fluke 887 and one on modifying the 845.  He made available his
notes, which were quite thorough. Check the archives starting on Dec.
22, 2013.

Best regards,

Charles


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Thank you Charles, The Dec thread looks interesting, maybe a good solution. But was not looking forward in replacing the amp circuit. I think I'll try a optical isolated version of 335D circuit. This MOSFet is easy to apply to the null meter circuit without a major overhaul. If successful I'll share my simple mod. If the performance fails, I'll try the Dec 2013 method. Dallas On 5/6/2014 2:06 PM, Charles Steinmetz wrote: > Dallas wrote: > >> Did Fluke ever make the 845ab Null Meter with the optically-isolated >> Bilateral Analog FET chopper? > > I have never seen an 845 with FET chopper, despite the engineering > change specification most 845 owners have seen. I've heard of one or > two 845s that may have been repaired (by Fluke) with a FET chopper > assembly (perhaps using 335D parts ??), but I'm not sure any 845s left > the factory that way. > > These days, there are integrated chopper-stabilized ("zero offset") op > amps that can do better than chopper amplifiers (e.g., LTC1050, > LTC1052, LTC2054, LTC2057, LTC7652, ICL7650, MAX4238/4239). Randy > Evans started two threads here back in December 2013, one on modifying > the Fluke 887 and one on modifying the 845. He made available his > notes, which were quite thorough. Check the archives starting on Dec. > 22, 2013. > > Best regards, > > Charles > > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > >