[volt-nuts] New member with 3457A problem

[Pete Lancashire]

Seen a lot worse even on rentals.  Pretty much the same here, other big $
projects going on right now so can't make an offer.



On Tue, Jan 5, 2016 at 5:28 PM, Tom Miller <tmiller11147 at verizon.net> wrote:

> 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" <jerry at hanler.com>
> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> 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 <jerry at hanler.com> 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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>>
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