[volt-nuts] New member with 3457A problem

[Tom Miller]

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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