HP 3468A / HP 3478A vs Instek GDM-8251A? - Page 1
Forum section: electronics / testgear

author: JenniferG
author_rank: Frequent Contributor
timestamp: February 25, 2017, 10:36:51 am
content: Anyone care to give the pros and cons of each? The HP's are a tad bit more money but the Instek have the desirable vaccuum flourescent displayAre the HP's more reliable & accurate, as easy to calibrate, etc.? (Btw, if I recall correctly I think I read someone had to send in their 3468A to be calibrated and that it costed $80.. whereas I know the Instek can be calibrated at home with diy dongle & 10k voltage reference).I am just trying to finalize my decision to go ahead with this GDM instead of the venerable HP meters.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2017, 11:00:42 am
content: The VFD display on the GWI is a two sided thing: if good it is very well readable, but if old the intensity can go down. The reflective LCD display is well readable if the light is good. In very bright conditions even better than VFD.I have used HP3478s. They work rather well and AFAIK have a 30 mV range for low voltages thus have a better resolution (1 µV compared to 10 µV). The meter is 300,000 counts compared to 120,000 counts for the GWI. At least for the 3478 calibration should be about as easy as for the GWI. Not sure about the older 3468 - this one could still have individual pots for the ranges. However the normal calibration is just verification of the specs and does not include adjustments - so it should not make a big difference.Form the specs the GWI seems to be 10 M input impedance even on the low DC ranges - this might be a n factor.With such an old meter one might want to have a look at the mains input RF filter and maybe electrolytic filter caps.

author: nctnico
author_rank: Super Contributor
timestamp: February 25, 2017, 11:24:27 am
content: I hate the reflective LCD displays on HP equipment. They are hard to read. The old meters also lack a lot of modern features. In the end I got a Vici 8145 4.5 digit DMM which has capacitance measurement, instant continuity and a backlit LCD with large digits.

author: guenthert
author_rank: Frequent Contributor
timestamp: February 25, 2017, 05:19:40 pm
content: Among my DMMs there's also a HP 3478a. While it does have a 30mv range and hence a 100uV resolution like its big brothers, there's quite some noise (error includes 40counts iirc). Might need logging and statistics to make better use of it. Caution: the 3478a high impedence input (>10GOhm) applies only to DCV ranges up to 3V, beyond that it's just 10MOhm (that isusuallygood enough).Regarding the display: the LCD of the 3478a is fine in a well lit place, but poor otherwise (no backlight). The HP34401a has VFD, which looks neat and is better in dim environments (might be more of a challenge in bright sunlight). I wished the manufacturers would use LEDs instead and not tiny ones like on the 3456a, but nice big and bright ones like the one on the Keithley 617.You might also want to read the specification carefully and compare it with your needs. I, for one, found the low AC current capabilities of the 3478a, err, underwhelming.

author: mtdoc
author_rank: Super Contributor
timestamp: February 25, 2017, 06:15:45 pm
content: I have both a 3478A and a GDM-8251A. They are very different meters.As pointed out, the HP has higher resolution. I've had it for several years now and it remains rock solid accurate with low drift based on my DMMcheck, an aged AD584L, and comparison to readings from my Fluke 8842A and more recently my calibrated Agilent U1252b. I find the LCD display fine but it does require reasonable lighting to read.For high resolution DC voltage, current and resistance measurements it is my go to meter. I have used it very little for AC.The 8251A does not match it's resolution or accuracy and I suspect it will drift more. Build quality is not the same. But it does have several features the 3478A does not have: Capacitance, Frequency, continuity, diode check, temperature and the dual display - which is very nice.So different beasts really. To sum up:HP 3478A - Classic high quality HP test gear. Higher resolution and better accuracy. Limited features.GDM-8251A - More modern, full featured meter. Lower build quality, lower resolution and accuracy (though still plenty for most any hobbyist use).I say get both.If you get a 3468A or 3478A you will want to change the battery that backs up the calibration data when unplugged. This needs to be done with a non-grounded iron with the unit powered on or with battery backup. There are thread on the forum which explain the details of that. It is not hard to do.

author: JenniferG
author_rank: Frequent Contributor
timestamp: February 25, 2017, 06:27:15 pm
content: I bet this Fluke 8050A (which I got for $30 shipped) ought to get me through all of the Art of Electronics sufficiently? Maybe I should just hold of on acquiring another meter until later. Money is tight anyways.

author: mtdoc
author_rank: Super Contributor
timestamp: February 25, 2017, 06:36:04 pm
content: Quote from: JenniferG on February 25, 2017, 06:27:15 pmYes, I'm sure it will. Good score BTW.I assume you have a second handheld meter though? If not you will want something so you can do current and voltage measurements at the same time. Any inexpensive meter will likely be fine.

author: JenniferG
author_rank: Frequent Contributor
timestamp: February 25, 2017, 11:55:37 pm
content: Quote from: mtdoc on February 25, 2017, 06:36:04 pmThe other meter I have is a Commercial Electric MAS830B -- purchased from Home Depot. (My man bought it about a year ago.)It is the same one this guy purchased (and shared photos of) :https://www.eevblog.com/forum/reviews/commerical-electric-(home-depot)-mas830b-multimeter/msg87206Perhaps I need a 4.5 digit instead of a 3.5 for my second meter? Maybe one that is more accurate? Are there decent 4.5's for like $30-40? Maybe I should get an old Fluke 8060A? BUt I am wondering if the 8060A has the same complicated calibration procedure the 8050A does -- heard they were just about the same except one is portable.I'd love an accurate 4.5 handheld, that was easy to calibrate, that's like $50 or less. Maybe I am asking for too much?

author: mtdoc
author_rank: Super Contributor
timestamp: February 26, 2017, 01:30:16 am
content: Quote from: JenniferG link=topic=83861.msg1145597#msg1145597There are several options for decent 6000 count meters under $50. I think Dave did a video on that a few years ago. Search and you will find ..My choice would be the Tekpower TP4000ZC as I recently mentioned inthis threadwhere you'll find a video review and a link to forum member Franky Tong's eBay store where you can get it for $39 shipped.BTW inexpensive, low count meters generally do not get calibrated.

author: Cerebus
author_rank: Super Contributor
timestamp: February 26, 2017, 02:26:45 am
content: I can't say anything helpful about the Instek, having no direct experience of it, so I'll gloss over that.As far as old HP kit goes:1) Both of those are old enough that complete service and user manuals, including schematics, are available online. Personally I hate to own anything that I can't fix, and fixing a DMM is tough without schematics. Neither uses any truly exotic or unobtainable parts (except for two HP specific hybrids which would cost more as parts than a whole second hand meter), although decoding HP part numbers into industry standard part numbers can sometimes be a bit of a black art; there are cross-reference guides available online that do the job 98% of the time.2) Old HP kit just keeps working and often significantly exceeds the nominal specifications. I have two HP bench power supplies, an HP 6002A made in 1980, and an HP 6236B made in 1984 - both still meet or exceed all their out-of-the-factory-door specifications.3) You'll often get a lot more measurement bang for your buck out of old gear, even if you miss out on some of the glossier features you might find on newer kit. The brochure price for the HP 6002A mentioned above new was $4035 USD in 1980, I got it last year for £60 GBP - the original price tells me that nobody had to skimp on the specifications or economise on parts. The 3478A has a basic DC accuracy of 0.006%+2digits (up to 1 year away from cal (0.003%+2digits within 24 hours of cal)), the Instek is 0.012%+5digits at some unspecified time after cal, so the HPs basic accuracy is at least twice as good as the Instek.

author: mmagin
author_rank: Frequent Contributor
timestamp: February 26, 2017, 04:55:20 pm
content: Aside from the non-backlit LCD, the 3478A is a great multimeter. I wouldn't pay as much for the 3468A, it has inferior accuracy specs and if you ever wanted to interface to a computer, HP-IL is kind of a dead-end today.

author: nctnico
author_rank: Super Contributor
timestamp: February 26, 2017, 05:01:51 pm
content: Quote from: Cerebus on February 26, 2017, 02:26:45 amI don't think so. On various older HP gear the wires soldered directly into the PCB boards tend to get loose. I used to own a 6002A myself and I had to redo all the wires because I could pull them out after a minor amount of wiggling. The same goes for the HP6024A PSUs I still have (which needed a lot of electrolytics to be replaced).

author: Cerebus
author_rank: Super Contributor
timestamp: February 26, 2017, 05:47:56 pm
content: Quote from: nctnico on February 26, 2017, 05:01:51 pmLet me be clear here. It was working, you opened it up, wiggled thesolid corewires until they failed and it stopped working? Now what's the failure mode here? Ah, yes - Nico. Perhaps we need to add "Nico proof" to "squaddie proof" and "copper proof". You're not adrummerare you?But seriously, the construction methods on older HP kit were pretty standard for good quality kit from their era. I agree that the wire to board and wire to front panel controls could be better - kit I was building myself back in those days tended to get Hellermann neoprene cable sleeving slipped over all those style joints - but it's still of serviceable quality and keeps working in kit that isn't heavily abused. My HP 6002A had been dropped heavily enough to dislodge one of the front panel meters and crack the meter bezel but none of the solder joints or wire cores had given up.

author: nctnico
author_rank: Super Contributor
timestamp: February 26, 2017, 05:58:23 pm
content: Quote from: Cerebus on February 26, 2017, 05:47:56 pmNow you are trying to read things which aren't there. I've never seen any solid core wire in HP gear so please put that assumption back where it came from. Also a 'minor amount of wiggling' is barely touching wire and it still falls out of the solder joint. You can talk all you want but that is an accident (fire) waiting to happen. Old gear is what it is: old and the stuff from HP isn't any different so solder joints in HP gear will also get brittle over time.

author: JenniferG
author_rank: Frequent Contributor
timestamp: February 26, 2017, 06:11:23 pm
content: Looks like I'll be opening up the HP power supply I just got (from 1979) and checking solder joints!

author: Cerebus
author_rank: Super Contributor
timestamp: February 26, 2017, 06:38:20 pm
content: Quote from: nctnico on February 26, 2017, 05:58:23 pmNot assumption, observation. I had to de-solder that meter to fix the cracked bezel. It was solid core wire, not stranded. And try not to be quite so humourless...

author: guenthert
author_rank: Frequent Contributor
timestamp: February 26, 2017, 10:52:38 pm
content: Quote from: nctnico on February 26, 2017, 05:58:23 pmWait, what? How old are we're talking about now? I have several instruments from HP and Keithley from the mid eighties, which just work (well, my Keithley 617 had a mechanically-busted GPIB connector and while repairing that I found some hints that someone else before me was in there and used a heat gun quite liberally on one of the chips, but other than that it looks inside brand-spanking new). I never opened the HPs (I'm not a big fan of fixing what isn't broken). I also have a Tektronix FG501 of the late seventies I believe, where I had to replace a couple of parts. There was also quite some rust on the frame due to improper storage over many years. I even had to replace several adjustable resistors since they just crumbled and fell apart, so I don't want gear that old anymore, but I never had to resolder a solder joint.Is this something about the climate or salt in the air due to the proximity to the Northern Sea there?