EEVblog #427 - HP 3478A Multimeter Teardown - Page 1
Forum section: electronics / general

author: uprightsquire
author_rank: Contributor
timestamp: February 21, 2013, 10:19:39 am
content: The 3478a that I've been cleaning up today is date coded '82 and doesnt have the optocouplers... There are also a few other small differences between the newer and older boards

author: BravoV
author_rank: Super Contributor
timestamp: February 21, 2013, 10:31:55 am
content: Quote from: uprightsquire on February 21, 2013, 10:19:39 amPhotos please, especially the isolation part using transformer instead optocouplers, must be interesting to see the differences.PS : Suggesting not to use external hosting for the photo like you did using tinypic at your other thread. Just use this forum's image attachment feature.

author: EEVblog
author_rank: Unknown
timestamp: February 21, 2013, 10:44:09 am
content: My photos here:http://www.flickr.com/photos/eevblog/sets/72157632806646675/Dave.

author: uprightsquire
author_rank: Contributor
timestamp: February 21, 2013, 10:51:58 am
content: By '82 I mean '84

author: BravoV
author_rank: Super Contributor
timestamp: February 21, 2013, 11:21:20 am
content: Thanks, its obvious not only the opto couplers vs toroid transformers, also the newer 3478A board has slightly different layout.Did a quick&dirty merging both photos between Dave's (upper one) vs yours (bottom) at the isolation parts.PS :Just a reminder on Dave's selling this particular HP 3478A at ebay ->http://www.ebay.com.au/itm/271158344204, proof that Dave's is newer, hopefully this will contribute higher bidding price on thisHP 3478A meterSagan's fund, hurry.... one more day left..

author: EEVblog
author_rank: Unknown
timestamp: February 21, 2013, 12:06:02 pm
content: BTW, my board is marked Rev ADave.

author: uprightsquire
author_rank: Contributor
timestamp: February 21, 2013, 12:16:44 pm
content: Quote from: EEVblog on February 21, 2013, 12:06:02 pmI have rev c.

author: Zad
author_rank: Super Contributor
timestamp: February 21, 2013, 04:11:46 pm
content: If anyone is interested, I did a tear-down of my Tektronix DMM4020 (a Fluke 8808a by any other name). No custom part numbers in this, no prizes for guessing the voltage reference!Lots of optical isolation on the serial driver section. It even had its own isolated feed from the custom main power transformer.As with the HP3478A, the PCB has segregated areas for digital and analogue systems:Just 4 Philips screws and a cal sticker to remove to get into the DMM4020, I love equipment that is clearly designed to be maintained over a long timescale. There are a few more photos here:http://s222.photobucket.com/albums/dd237/Zadpics/Tektronix%20DMM4020%20Bench%20Multimeter/

author: Jonny
author_rank: Regular Contributor
timestamp: February 21, 2013, 07:21:44 pm
content: How tall is metal shield beside the crystal? Does it meet the lid to form more of a box over the circuit?

author: EEVblog
author_rank: Unknown
timestamp: February 21, 2013, 08:19:57 pm
content: Quote from: Jonny on February 21, 2013, 07:21:44 pmNo. Maybe an inch or so tall.Dave.

author: c4757p
author_rank: Super Contributor
timestamp: February 22, 2013, 12:02:03 am
content: The 3468A has a full can over the analog section. Interesting that they thought it was not necessary for the higher-spec 3478A.

author: BravoV
author_rank: Super Contributor
timestamp: February 22, 2013, 12:42:41 am
content: Quote from: c4757p on February 22, 2013, 12:02:03 amProbably because the whole 3468A's case is all plastic instead of metal ?

author: c4757p
author_rank: Super Contributor
timestamp: February 22, 2013, 12:52:48 am
content: Yeah, that would be it.

author: BravoV
author_rank: Super Contributor
timestamp: February 22, 2013, 01:44:13 am
content: Excluding cost, physical size and ease of installation factors, technical wise whats the pro & con for using opto coupler vs inductive for the isolation like above ?Also just noticed at the toroid windings, example at the red & blue windings (are they pri & sec ?), they're not even wound in a complete single turn, looks like only half turn ?

author: obsoletemac
author_rank: Newbie
timestamp: February 22, 2013, 05:29:59 am
content: Nice video, thanks Dave.There seems to be no line voltage selector on the 3478A. Is it possible to rewire a 120V unit for 240V without replacing the transformer? i.e. are there dual primary windings?//c

author: pgross
author_rank: Contributor
timestamp: February 22, 2013, 11:56:57 pm
content: Yes, it is possible to change the mains voltage.Remove the back bezel and slide off the case. Underneath the ribbon cable to the HP-IP port there is a single pole connector and some pins for the input voltage ranging from 100-240 V.The actual line frequency (50 or 60 Hz) is selectable by the small dip switch on the back./ Peter

author: staxquad
author_rank: Frequent Contributor
timestamp: February 23, 2013, 12:15:16 am
content: Dave's unit has a ser. of 2911A65826, falling into the post 2619A37795 units with a different design to the pre, and part of the serial could be the date code, the 1st, 2nd, 3rd and 4th digit, SN : 2911A66xxx - Means : (29+60 = 89) = 1989 , Week 11 , A(merica) (info from bingo600)Also, if you don't know the serial no, but "TERMINALS" shows up instead of "TERM" for the front-rear switch front panel, it's a late unit (for those aiming to purchase a later model).

author: Wartex
author_rank: Frequent Contributor
timestamp: February 23, 2013, 12:54:22 am
content: Quote from: BravoV on February 22, 2013, 01:44:13 amhigh voltage AC will make it thru the inductor

author: robbak
author_rank: Contributor
timestamp: February 23, 2013, 08:24:28 am
content: Quote from: BravoV on February 22, 2013, 01:44:13 amAdd ease of use, and speed: You can drive the LED in an optocoupler straight off the data bus, and the signal from the photo-transistor on the other side is ready for the data bus. The toroids need driver circuits to drive them and sensor circuits to turn what comes out of them back into a useable signal on a data bus.Power, of course: those toroids would need a reasonable power level to drive them.And speed: the inductance of those toroids would limit the clock speed that you could use them on.

author: Hypernova
author_rank: Unknown
timestamp: February 23, 2013, 12:16:55 pm
content: Two down sides for opto's:1) Life time, for fast signalling if you choose to drive the LEDs near their rated current they fail after a few years, for hobby use this is generally negligible.2) Size, compared to other IC scale isolators (capacitive from TI and transformer from ADI) opto's use air gaps for isolation, for a given isolation rating they may be bulkier.

author: bingo600
author_rank: Super Contributor
timestamp: February 23, 2013, 12:27:41 pm
content: Damm i have hoped that Dave did replace the batteryI have one that soon needs a battery replacement , and was thinking of using my propane iron , and the leave it powered on during the change.Dave i think the guy buying this meter needs a "Fresh battery" installed ... Hint..Hint .. Don't you/Bingo

author: BravoV
author_rank: Super Contributor
timestamp: February 23, 2013, 12:46:23 pm
content: Dave mentioned the measured voltage was at 3.49 volt which is a big possibility it is a lithium thionyl chloride type (LTC) with typical at 3.6 volt instead of lithium sulfur dioxide type at typical 3 volt.As these old HP DMM are using static cmos ram for keeping the calibration, the absolute minimum threshold for retaining the data at the chip is 2 volt.Below the typical LTC cell discharge curve, it has a really scary vertical voltage drop when its going to die.

author: IvoS
author_rank: Frequent Contributor
timestamp: February 26, 2013, 12:35:36 am
content: I was also thinking to plug the unit to mains but disconnect the ground prong during battery replacement.

author: Smokey
author_rank: Super Contributor
timestamp: February 26, 2013, 12:46:48 am
content: Wow. The ebay auction ended and the thing sold for AU $355.00.http://www.ebay.com.au/itm/271158344204Did you have it gold plated before you put it back together or something? You at least autographed the case right?

author: EEVblog
author_rank: Unknown
timestamp: February 26, 2013, 02:02:36 am
content: Quote from: Smokey on February 26, 2013, 12:46:48 amYep, more than double what I paid for it.That's ebay Australia for you, most likely not a fan, they'd have no idea who I am.I've seen this time and time again with stuff I've sold on ebay, people will pay silly prices to buy it from someone local in Oz instead of from overseas.Almost every bit of test gear I've bought and sold on ebay, I've sold it higher than what I paid for it (inc postage).Dave.

author: triac
author_rank: Contributor
timestamp: February 26, 2013, 01:44:03 pm
content: they're not even wound in a complete single turn, looks like only half turn ?What you see is classified as one turn.

author: belba
author_rank: Contributor
timestamp: March 01, 2013, 09:54:01 am
content: Hello,athttp://matthieu.benoit.free.fr/cross/competitive/agilent/hp-part.htmyou can find a cross-reference list of many HP marked ICs. I think it could be useful.Guido

author: Buga
author_rank: Newbie
timestamp: March 02, 2013, 02:02:32 am
content: Why do these high end bench meters wander around in the millivolt range where as the fluke handhelds seem to hold a zero when turned on? Is it really that sensitive to noise

author: c4757p
author_rank: Super Contributor
timestamp: March 02, 2013, 02:09:38 am
content: This meter, like many other bench meters, has an input impedance specified to be "greater than 10 G" (yes, gigohms) on the low ranges. That's not enough to dissipate charge that it picks up. (If you manually switch it to a higher range, it will settle to zero.) Most of the handheld meters are 10M on all ranges.

author: IvoS
author_rank: Frequent Contributor
timestamp: March 04, 2013, 11:19:28 pm
content: Does anyone know where I can buy a new battery for this meter?

author: Dave
author_rank: Super Contributor
timestamp: March 07, 2013, 03:09:11 pm
content: Farnell, Digikey, Mouser, anywhere really.

author: reagle
author_rank: Unknown
timestamp: March 12, 2013, 03:36:27 am
content: Just got one for work, and did not realize that it will not take standard shrouded DMM leads. Is there a special "HP" type of leads or do I need to go down the path of chopping the shrouds off?

author: c4757p
author_rank: Super Contributor
timestamp: March 12, 2013, 03:48:14 am
content: I just chop off all the shrouds. I'm not an electrician, the times when I use any voltage above 40 or 50 or so are very rare, and I would never yank an energized plug anyway. Any time I do use dangerous voltages, I prepare an entire test setup with the power completely disconnected, power it up, and all I allow myself to do without shutting down the power is push buttons. I've got too much older stuff that won't take shrouded plugs to mess around with them.

author: reagle
author_rank: Unknown
timestamp: March 12, 2013, 04:02:54 am
content: It's just a shame to cut brand new Pomona probes, but oh wellOddly enough, my 884x flukes of the same vintage take shrouded probes just fine. Probably due to higher voltage rating on the inputs or maybe just a bit newer

author: c4757p
author_rank: Super Contributor
timestamp: March 12, 2013, 04:06:29 am
content: I think it's probably due toFluke. They had experience making handheld stuff for use by electricians, so I imagine they'd upgrade safety sooner. HP's been mostly lab equipment with few exceptions.

author: SeanB
author_rank: Super Contributor
timestamp: March 12, 2013, 05:01:05 am
content: Frankie ( Iloveelectronics) has the adaptors in his Ebay store cheap. Then you can leave the leads intact and place the adaptors in the unit permanently.

author: bingo600
author_rank: Super Contributor
timestamp: March 21, 2013, 01:51:29 pm
content: Quote from: BravoV on February 23, 2013, 12:46:23 pmIt sems to be a Panasonic 3v Lithium Cell , whatever that means. LiIon , LTC orI just changed mine , pict here.https://www.eevblog.com/forum/beginners/changing-battery-in-a-hp-3478a-can-i-use-a-3v-lithium/msg205742/#msg205742/Bingo

author: Wim13
author_rank: Frequent Contributor
timestamp: March 30, 2013, 08:08:10 pm
content: Quote from: bingo600 on March 21, 2013, 01:51:29 pmNo it is a: Panasonic Poly CarbonmonoFluoride Lithium batt. 3.0 Volts to the datasheet.I replaced the batt in mine 3478, and there was also the same panasonic 3 V, see datasheet attached..But why is the voltage 3.4 Volts.., same as above. The datasheet says 3.0 Volts..??does the battery charges itself with zero point energy ?The unit i have is from 1992, so he batt is 20 years.Just repalces it by a HQ 910a, also 1200 mAH

author: Rutger
author_rank: Regular Contributor
timestamp: April 01, 2013, 02:33:56 am
content: Quote from: IvoS on March 04, 2013, 11:19:28 pmI got mine at Battery Plus here in the US for about $ 5.00, local pickup so no shipping cost.

author: madshaman
author_rank: Frequent Contributor
timestamp: April 01, 2013, 05:33:42 am
content: Quote from: reagle on March 12, 2013, 04:02:54 amPomona makes leads that have retractable shrouds for this purpose.

author: george graves
author_rank: Super Contributor
timestamp: April 13, 2013, 10:35:59 am
content: Quote from: c4757p on March 02, 2013, 02:09:38 amThat's crazy. I had no idea. I too was taken back a bit when I first turned it on. "why the heck is it floating like that?!?!?" Anyways - I just picked one up on ebay for about $130 USD. Calibrated last in 2006 - tested it a bit today - calibration seems spot on. That's fine for my needs. Thanks Dave/EEVblog for turing me on to this meter. Seems like the best bang for the buck out there in the 5 1/5 digit meters.Two things I've been thinking of...1.) I do wish it had a back light display. I'm tempted to pull it out and see if I can't shoot some leds into the side of the glass. But at $60 for a replacement - eeek - not sure I'm brave enough to try.2.) For 4 wire resistance measurements - I know I can use any leads I want - but why are Kelvin leads better? And is there a poor-man's alternative?

author: EEVblog
author_rank: Unknown
timestamp: April 13, 2013, 12:42:49 pm
content: Quote from: george graves on April 13, 2013, 10:35:59 amNothing special about "kelvin leads", it's the "kelvin connection" that is important. i.e. those two sense wires, they have to be connected directly to the exact point you are trying to measure. How you do that depends on what you are trying to measure.Simply having 4 wires connected to normal crocodile clips, or say some sharp probe pins is a good enough poors mans version for general use.

author: Unknown
author_rank: Unknown
timestamp: April 13, 2013, 01:59:26 pm
content: Kelvin leads are nothing special, I guess the most important reason why they're expensive is the limited market. The main thing they offer is convenience: clipping four alligator clips to each resistor gets old real fast. It's also hard to get four clips on a tiny 0805 resistor.