HP 3458A repair & refurbishment at home - Page 1
Forum section: electronics / repair

author: Oldtestgear
author_rank: Regular Contributor
timestamp: September 30, 2017, 03:18:32 pm
content: My first post about a repair, partly because I have been encouraged to post the details by other forum members.I recently inherited a poorly 3458A that had a few issues needing repair. First was the dreaded RAM1HIGH message which was the first on the list to be fixed, I replaced both the DS1230 NVRAMs, using decent sockets for both. I switched on & it actually started self test. Only failure was down to no calibration constants. Although the DS1220 was much newer than the other chips, the cal. constants were missing.Next step was to perform a very rough & ready calibration for DCV & Ohms. I did this to see whether the fault was with the NVRAM or (I hoped) it had been replaced but not recalibrated. An hour later, I had a new set of constant fed in & it as reading values that were vaguely correct. Switching it off for 24 hours seemed like a fair test & yes the NVRAM did retain the data.Now came what I thought would be the simple task of repairing the broken front panel, green window, & the front input terminals. A few calls to Keysight suggested that I would be relieved of several £100s for the privilege but at least all the parts are readily available. At this point thing went rather awry & it took me several weeks to get the parts delivered. The staff at Keysight were very helpful & we got every detail sorted eventually. Thanks to TiN for his encouragement & sound advice which made the final outcome much simpler.Parts arrived & were then fitted. It does look very nice with a new Keysight front panel plus the input terminals are now firmly located & not flapping around. The next question is whether the 3458A is actually stable & gives repeatable results. I did not have a good enough Voltage Standard to test it & I was not willing to leave my Datron 4000A calibrator running for 14 days continuous operation. The best option I have is an HP 11107A 10K resistor. I connected this to the rear terminals & logged results twice daily for two weeks. Overall the results looked fine with the data spread around the nominal value & varying by a few ppm in either direction. Good enough.I decide to change the operating temperature of the LTZ1000 as I will never need the high temperature performance that HP originally intended. Plus the lower operating temperature should reduce drift vs. time which has to be a bonus.I then replaced the DS1220Y & performed a much more careful DC & Ohms calibration. Again, the new NVRAM retained the data.As others have stated on the forum, Keysight offered a summer promotion of half price calibration. This had to be my final step. If it passed calibration then I had successfully refurbished the DMM & it could be used for the next stage in my plan. It has just been returned from Keysight UK with a new calibration certificate. Yes it is near the limit on some AC tests but still has a full set of "passes". Amazingly, my second calibration attempt was adequate to get DCV etc. through in as received condition. I was very impressed.I was very lucky with this repair as the main requirement was hard cash more than skill but everyone deserves a dose of luck every so often & this has been mine. With a bit more luck the pictures I think I attached will actually appear.Phil

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: September 30, 2017, 03:59:06 pm
content: Bad idea to change the heater temperature of the LTZ, this will cause changes to the crystal matrix and the LTZ will not be as stable for quite some time. That LTZ was already aged by Keysight at that temperature setting, you've now go a Vref that will not be as stable for months or longer....never ever change the temperature of an aged LTZ!

author: Johnny10
author_rank: Frequent Contributor
timestamp: September 30, 2017, 04:04:09 pm
content: Why not leave Datron 4000A running 14 days? Noise?

author: Oldtestgear
author_rank: Regular Contributor
timestamp: September 30, 2017, 05:45:45 pm
content: My experience with Datron instruments is not very happy. Too many with burning tantalum capacitors and this instrument has had a few go up in smoke already. I normally replace all tants before I will leave an instrument unattended. This is based not only on my experiences but that of friends as well. Better safe than burn the house down.

author: TiN
author_rank: Super Contributor
timestamp: October 01, 2017, 09:09:26 am
content: Good job on fixing, but you missing vital photo of 3458A showing 10.000000V (joking).Changing oven temperature for 3458A LTZ will indeed make instrument unstable for few days/weeks but in long term you might get better long term stability, if that what matters for you.However testing stability for such precision is feat for a year or few, that's for sure.Also first photo confusing me, what's that about?

author: Oldtestgear
author_rank: Regular Contributor
timestamp: October 01, 2017, 09:45:27 am
content: Oops, first photo is the wrong one!Will try to sort the right pictures later today. First attempt at putting up photos & I messed it up. Must also confess that I have not had the time to switch it on since getting it back from Keysight.Phil