[volt-nuts] HP 3458A repair.

T. Micallef tmicallef at gmail.com
Tue Sep 17 18:55:32 EDT 2013

John Phillips <john.phillips0 at ...> writes:

> Hi,
> I have a 3458A that we sent to Agilent for calibration which it failed.
> Before we sent it we calibrated it and it looked good to us. The infor. we
> revived led us to believe that the cal memory may have caused the failure.
>  We ask that it be sent buck to us and paid half the cal charges (about
> $800) insted of the $2660.64 they wanted to repair it. We were just going
> to repalce the ram in try again.
> When we got the meter back it came with befor and afer data Like before 10
> volts read 9.9999957 and after it read 10.00009 so they did something or
> the meter drifted that much.
> The problem is  0.1 volt and 1.0 volts failed at 8 and 10 MHz but passed at
> 4 MHz.
> 4MHZ 0.1 volt reads 0.097251 Lower Limit is 0.095930 PASSED
> 8MHZ 0.1 volt reads 0.085712
> Lower Limit is 0.0959
> 2
> 0
> 10MHZ 0.1 volt reads 0.75569 Lower Limit is 0.084900 FAILED
> 4MHZ 1 volt reads 0.97272 Lower Limit is 0.95930 PASSED
> 8MHZ 1 volt reads 0.86389
> Lower Limit is 0.95920
> 10MHZ 1 volt reads 0.73514 Lower Limit is 0.84900 FAILED
> The AC after readings are the same. I do not see how AC after could be that
> identical even if they did not try to calibrate it. Did they just copy the
> before data and call it after data?
> My best guess is that if the 4 MHz is in and the higher frequencies are not
> the meter requires some kind of mechanical adjustment to get the frequency
> response   withing spec or the AC board needs to be repaid.
> Are they charging a standard repair charge to do a calibration? I do not
> see changing the memory to fix this.
> Where would you go from here if this was your meter?

Tough call. I have a 3458A that has similar issues. I have delayed its
repair since I have a second one in better shape. It will not pass SCAL at 2
and 8 MHZ nor will it pass ACAL AC.

I have scoped a test input signal at the different SCAL frequencies, and it
does appear to drop well below what the RMS converter is expecting on the
higher frequencies. If you are having similar issues, it could be part of
the compensation circuits or attenuators on the AC board. It is hard to tell
if it is a hardware issue, or the cal memory is not setting the compensation
DAC's properly based on the input frequency.

Either way, I would think if you performed ACAL AC, that it would generate
some error like Flatness DAC Convergence.

BTW, what are the dates of your NVRAM? Have you received any Checksum errors
during the selftest?

As a last resort, if you do not need the capabilities of the AC range,
you may be able to get a limited calibration performed on the DC and Ohms

The $2K+ Agilent repair should include a full calibration. The repair cost
is the difference, but still a big chunk of change.

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