That looks like a good one from what I can see. It would be nice to check it
out in person to see if the display has any dim digits. Looking at the
picture seems to show it has some dimming compared to the manual range
indicator. The price is pretty good also. Maybe they might negotiate down
slightly.
It would run you about $600+ to send in for a Keysight calibration.
Nice work on finding the problem with your 3457A.
----- Original Message -----
From: "Jerry Hancock" <[email protected]>
To: "Discussion of precise voltage measurement" <[email protected]>
Sent: Tuesday, January 05, 2016 8:06 PM
Subject: Re: [volt-nuts] New member with 3457A problem
Hello again, Just for the record, after spending another dozen or so hours
on this problem, I found U216, “Precharge offset adj” to have a low
resistance from pins 1,5,6 to pin 4, essentially one of the 4 transistors
in the CA3096 is shorted (11.28 ohms) though not completely. I found this
by scoping the signal coming into the input amplifier at pin 10 of U101,
“Input Hybrid”, while putting the meter into an auto cal loop. This
problem was difficult to track down as unlike the other Service Manuals,
there are no waveforms available in the 3457a manual. I noticed that
during the precharge cycle the waveform was taking a large negative dive
instead of two relatively equal pulses, one that reads the signal and the
other during precharge, I assume. This negative voltage from the
precharge offset was then not being drained efficiently when the leads
were open resulting in a negative to zero open voltage displayed on the
meter compared to the typical ~+2.5V.
So for the record, if you get a “0 Hardware error” followed by a 512 or
560 (512 + 32 + 16 errors signaled) in reply to the “auxerr?” command,
take a look in the area of U216 and check the four transistors that are in
the CA3096E.
Lastly, as I commented on another site, after reading through the 3457a
theory of operations, that there are a number factors that need to be
calibrated out of a meter to ensure accuracy and repeatable measurements.
In this case, obviously the input amplifier needs to have a stable offset
voltage (zero). The precharge offset charges the approximate 10pf of
stray capacitance out of the input amplifier to keep it from impacting the
readings. In conclusion, it takes more than an accurate ADC, gain
resistors, etc to make a good meter. I followed a number of threads where
people were attempting to make their own 5.5 digit meter, I played around
with some circuits myself, but nowhere did I see stray capacitance taken
into account.
Thank you for reading. By the way, there is what looks like a nice 3458a
on craigslist in the SF Bay area. Initial price of $2,900 was dropped
last night to $1,950. I wish I had some leftover cash from Christmas!
Jerry
On Jan 3, 2016, at 6:26 PM, Jerry Hancock <[email protected]> wrote:
I’ve been reading the list for a while, never had a reason to post until
now.
I have an HP 3457a, actually two of them, both work great, accurate,
stable, etc., except one won’t autocal without an error 512 in auxerr -
ohms recharge offset. I’ve had the meters side by side for hours the
past few days and can’t seem to locate the problem. There is a
difference in the Ohms precharge offset in the failing meter. It looks
like a factor mod was performed to lower U216 below +15 by adding
resistors to legs 13 and 10. I know there were service notes sent out by
HP in 1992 related to the 512 error. the other thing of note is that
occasionally when I power on the meter, it will also get a 16 error,
which is the offset. I ran diagnostic 4 and calibrated the offset but
occasionally the meter will not pass the test no mater how the offset it
adjusted. For those of you not familiar, the diagnostic 4 puts the meter
in a loop displaying HI or LO depending on the adjustment until the
repair tech (me in this case) adjusts the offset correctly. At that
point the
meter displays ‘Passed’. When the meter fails, usually when cold, the
display oscillates between HI/LO like the ADC doesn’t have a connection
and the input is floating. I tried swapping U101, the input hybrid,
between the two units without luck.
So I am wondering if anyone on the list has run into this problem in the
past, and if so, the solution would be appreciated.
Thanks.
By the way, I have a number of voltmeters, can’t seem to get enough, and
am pretty good with 3455A repairs having four of them. My black face
3455A is one of my favorites. I also have 2 x 3456A, 2 x 3457A, 3 x
3478a, 2 x 3476, a bunch of Keithely (191, 178, 199, 195, 196, etc) and
others, mostly older HP units like the 400. I am currently eyeing a
3458a. I guess I am a volt-nut at heart.
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