HP 3456A help please - Page 1
Forum section: electronics / testgear

author: GhettoTronix
author_rank: Newbie
timestamp: January 29, 2014, 10:58:58 am
content: I have a couple questions. This is a new machine for me and a big step up. I have gone thru the manual and I have done some searching without luck.I think my "B" trimpot is toast. Poor little guy has been adjusted too many times in the last 30 years. I can't figure out where it is in the schematic to determine its valueI don't want to tear it down until I am ready for the repair. I have the same scanned manual that is on the HP website. Can one of you experts recommend a suitable replacement trimpot? Perhaps point it out in the schematic? I imagine I want to spend a couple extra bucks for something good.Next up is the "MEAN" function. How do I set it up? The manual doesn't explain it well enough. It tells you what it does, but not really how to set it up for use. I tried pushing buttons to figure it out but it only throws an error at me.Thanks in advance for your expertise

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 29, 2014, 12:26:47 pm
content: Quote from: GhettoTronix on January 29, 2014, 10:58:58 amObviously, you have the wrong manual, service only?Download the operating manual, "introductory users guide", available on the agilent site.That's very well described, page 19-20 "How do I use a Math Feature?".You have to engage the STAT function, and after at least 3 readings, you can read the math registers MEAN, VAR, COUNT, UPPER, LOWER, Z.The error applies beacuse those registers have not been filled, because STAT was not started.Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 29, 2014, 12:49:33 pm
content: Quote from: GhettoTronix on January 29, 2014, 10:58:58 amDo you have the correct manual, the compilation, including service manual 03456-90006 from the agilent site?In that part of the document, pdf page 155, Section V - Adjustments, all the different pots are described, so you can look in the schematics, where to find them.Pot A is coarse adjustment of 10V DC, B is fine 10V DC. How to adjust is described in paragraph 5-14 on pdf page 156.That should be clear, in advance.. Why do you think, Pot "B" is not ok?The 10V is directly calibrated on the reference schematic, A20 / 5A or 5B, depending on which reference is built into your instrument. "A" - coarse is a rotary switch, "B" fine sits direcly beneath it, that's a trim pot.Btw.: That's pdf page 276 or 278.PS: Why do you want to trim pot "B"?Do you have something less uncertain as a calibration standard, than this box?Frank

author: GhettoTronix
author_rank: Newbie
timestamp: January 29, 2014, 10:48:02 pm
content: Ahh that makes sense on the service manual. I thought the 400 page PDF is all there was and it seemed to be lacking on information. I will look for more.The B trimpot does not act right when I tweak it. Seems like it is worn out. I bought one of those reference modules off ebay that uses the AD584LH chip (5ppm). I know this machine has not been NIST traceable for at least 5 years, probably longer, and it was 10 counts high compared to my reference that is claimed to be checked against a NIST traceable 3458A. Probably nothing to worry about but I want it to be spot on to my best known reference even if it is slightly off. I am a hard core perfectionist and the little details matter to me. Finding the sweet spot in the trimpot is a PAIN and it seems to move by a turn or two when I make an adjustment. I have it dialed in best I can for now but I don't trust it. I do plan to spend more money on a better reference down the road when I can afford it but this is what I have for now.I am keeping record of time, temp, measurements, and tweaks. I know my reference pretty well and I know exactly how much it drifts with temperature down to tenths of a degree in the range of my lab, around 21-26*C. I can measure the difference from the heat of my hands by handling it and always let it settle to room temp. I can get a couple degrees higher by using the heat radiated from the 3456A but I am not keeping record of those measurements. It is winter and cool right now so I will get a few more degrees out of it once it warms up outside. Working on the time part now to see how well it stays in tune in the coming months.When I do buy a good reference I want to replace the trimpot as well so my confidence in the machine is upped. This trimpot just doesn't seem right and it's not like I never turned one before. This is the first time I have ever suspected a problem from a trimpot. I might be wrong but it seems to be the case.

author: sync
author_rank: Frequent Contributor
timestamp: January 29, 2014, 11:07:16 pm
content: Quote from: GhettoTronix on January 29, 2014, 10:48:02 pmI would trust the HP 3456A over the Ebay reference. And IFIAK these references are calibrated against a 34401A. Which can off by 10 counts at 6.5 digits and nonetheless well within spec.btw:AD584LH 5 ppm/K (and that's only for the IC itself)HP3456A 2 ppm/K +2 counts/K0.02 counts/K (10V range)

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 30, 2014, 08:28:45 am
content: Quote from: GhettoTronix on January 29, 2014, 10:48:02 pmIf you are a perfectionist, I recommend you to first learn about the basics of metrology.Obviously, you confuse temperature coefficient of the AD584L (i.e. 5ppm/K) with the uncertainty - measured in a different unit, namely ppm - of the assembly from ebay, and the uncertainty / stability of your 3456A, which is 8ppm/24h or 23ppm/90days .If the room temperature, where you now operate your AD548L, deviates only 2°C from the calibration temperature, the output will deviate already by 10ppm, that's what you observe already.To calibrate the 3456A, you need a reference, which is uncertain to 1ppm relative to NIST, and has a T.C. of < 0.1ppm/K.I have seen descriptions of those chinese 4-channel references based on an AD584L, but the uncertainty (again: in ppm) is to my knowledge not defined at all.They show photos with a 34401A only, and that instrument - IF freshly calibrated - is uncertain to 20ppm only.Even if they would calibrate it against a 3458A, the AD548L is way too unstable (over temperature and time) to make a useful reference for calibration of 6.5 digits DMMs.A reference for the 10V range alone also is not sufficient to calibrate that instrument, for DCV only you need those 5 Cardinal Points: 1000V, 100V, 10V, 1V, 0.1V.Better trust the 3456A as is.Frank