After making a mess in my garage while searching for it, it turns out that
that my Fluke is a 731B.  It appears to use the same Voltage Reference IC as
the 731A, so no big deal there.
I found a previous discussion about the 731/732 reference ICs.  Seems that
the ICs are either from Motorola  (MCA1914/MCA1924/MCA1934) or GE (RA3).
Doubtful that GE continued production of that IC very long, so my guess is
that these are Motorola parts.  It's all moot, since none of the parts are
still in production.  I'm afraid to contact Fluke for a quote on the
IC/Resistor set.  Don't think my heart could stand the shock.

I'd really like to get this unit back in operation, but I don't want to
break my bank account to do it.  I have a few LM399s on the shelf, so I
guess that will be my best approach to a repair.

Thanks for the discussion,
Dave M

M K wrote:

Dave,

What is the problem with the original reference? I have not heard of too
many failing. There was also a reply from Hank on another message. Perhaps
he can provide the needed parts from a 732A?

Todd

On Mon, Sep 14, 2015 at 6:45 PM, Dave M dgminala@mediacombb.net wrote:

The original reference has a shorted Zener.  I verified that with a curve
tracer (that was a couple years ago, but I remember it well.

I saw the post from Hank, and have emailed him about the 732A stuff that he
has.

I think my easiest and least expensive approach to getting this instrument
back on the bench is to build a circuit with an LM399A and fix it onto the
731B reference board.

Cheers,
Dave M

Todd Micallef wrote:

When the people fear the government, there is tyranny. When the
government fears the people, there is liberty -- Thomas Jefferson

Dave M

今晩は,スタインメツさん、

On Sun, 13 Sep 2015 08:22:29 -0400
Charles Steinmetz csteinmetz@yandex.com wrote:

Why would this be a mistake?

		Attila Kinali

--
Reading can seriously damage your ignorance.
-- unknown

I wrote (regarding replacing the op-amp in a Fluke 731A or B voltage standard):

Attila wrote:

Let me count the ways....

First, the ground rules:  My suggestion was to modify the 731 (we now
know the OP has a 731B) with as little invasion as possible.  I
suggested (i) replacing the reference with an LM399 and changing the
gain-setting voltage divider (R2 or R3) to obtain 10v with the new
reference (only a small change would be necessary), (ii) if desired,
replacing the op-amp, and (iii) replacing all of the divider and
zener bias resistors with lower-tempco units (moot, now that we know
the unit is a 731B, which has low-tempco WW resistors).  I do not
recall how much current the selected resistors, R5 and R6, deliver to
the existing reference; the LM399 seems to be most stable over the
long term when operated at 1.5 to 1.8mA, so R5/R6 should arguably be
changed (if necessary) to get the current into that range (about
2k).  Any replacement resistors should have tempcos at least as good
as the original WW resistors, but anything better would be wasted on
a circuit that sits out in the open like the innards of the 731B.

My notion was to use an op-amp that will drop directly into the LM308
position, with only a few component changes on the circuit board.  If
someone wanted to do something more elaborate, there may be other
possibilities, but even then I do not believe an AZ op-amp would be a
good choice for the reasons given below.  [since Fluke used an LM308
in the TO-5 package, which is not a current package for either of the
op-amps I recommend, in reality a small adapter board would need to
be made (or the op-amp would need to be mounted on stilts -- ugh, PC
meets dead-bug)].

I have now recommended two candidate op-amps -- the OPA277 and the
LT1012.  I have not run the numbers to see which is better in this
application, but I suspect the LT1012 is.  The LT1012 was once made
in the TO-5 package -- if one can be found today, that would decide
the matter for me, for two reasons: (1) it fits with no surgery, and
(2) the hermetic package resists drift due to atmospheric humidity
and pressure.  The LT1012 also has an overcompensation pin, like the
LM308, which is handy for making the particular circuit used in the
731B stable.  So, I'm recommending the LT1012 in preference to the
OPA277, even if it is not in the hermetic package.

Finally, note that the LM308 is a respectable performer even today,
and with everything out in the open on the PC card it is quite
possible that other errors may completely swamp any improvement the
LT1012 might promise.

So let's start counting:

1. Maximum power supply voltage.  The 731B uses an arrangement in
   which it is powered by the regulated 10v supply.  Precious few AZ
   op-amps can handle a power supply of more than 5 or 6 volts, and the
   ones that will are not the best choices for other reasons.  Even the
   reference output (not quite 7v, typically), which is connected to the
   op-amp's noninverting input, is higher than the vast majority of AZ
   op-amps' maximum supply voltages.  So, even changing the topology of
   the amplifier would not help.

2. Switching noise.  All AZ amps suffer to a greater or
   not-so-lesser degree from switching noise.  Some of them allow the
   designer do a halfway decent job of filtering the output -- but all
   of them put all sorts of hash and garbage onto their supply
   lines.  As noted above, the supply is connected directly to the
   regulated output in the 731B, so there is no good way to clean it
   up.  (Same for the AZ amp's output noise in the 731B circuit.)

3. Input noise current.  AZ amplifiers have little or no 1/f noise,
   because it is cancelled by the AZ action.  However, they have
   substantially more input bias (leakage) current than regular CMOS
   op-amps and, consequently, a higher input noise current
   density.  This is a dirty little secret of AZ amplifiers -- most
   manufacturers calculate the input noise current density as the shot
   noise of the input current (if they even list a noise current spec --
   many do not).  However, the actual measured input noise current
   density is frequently as much as 100x higher than this calculated
   value.  So, even ignoring switching noise, many AZ amplifiers are not
   even as quiet in real-world circuits as run-of-the-mill JFET
   op-amps.  Since both of the op-amp input nodes are moderately
   high-impedance, input current noise will be a factor in the noise analysis.

I'm sure I've forgotten a few, but those are some of the high points.

Best regards,

Charles