Author Topic: EEVblog #427 - HP 3478A Multimeter Teardown  (Read 22648 times)

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Online BravoV

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EEVblog #427 - HP 3478A Multimeter Teardown
« on: February 21, 2013, 10:15:18 am »

« Last Edit: February 21, 2013, 10:18:15 am by BravoV »
 

Offline uprightsquire

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #1 on: February 21, 2013, 10:19:39 am »
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
 

Online BravoV

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #2 on: February 21, 2013, 10:31:55 am »
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

Photos 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.

Online EEVblog

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #3 on: February 21, 2013, 10:44:09 am »
 

Offline uprightsquire

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #4 on: February 21, 2013, 10:51:58 am »
By '82 I mean '84
 

Online BravoV

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #5 on: February 21, 2013, 11:21:20 am »
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 this HP 3478A meter Sagan's fund, hurry.... one more day left. ;)


.

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #6 on: February 21, 2013, 12:06:02 pm »
BTW, my board is marked Rev A  :-//

Dave.
 

Offline uprightsquire

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #7 on: February 21, 2013, 12:16:44 pm »
BTW, my board is marked Rev A  :-//
I have rev c. :)
 

Offline Zad

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #8 on: February 21, 2013, 04:11:46 pm »
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/


Offline Jonny

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #9 on: February 21, 2013, 07:21:44 pm »
How tall is metal shield beside the crystal? Does it meet the lid to form more of a box over the circuit?

Jonny
 

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #10 on: February 21, 2013, 08:19:57 pm »
How tall is metal shield beside the crystal? Does it meet the lid to form more of a box over the circuit?

No. Maybe an inch or so tall.

Dave.
 

Offline c4757p

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #11 on: February 22, 2013, 12:02:03 am »
The 3468A has a full can over the analog section. Interesting that they thought it was not necessary for the higher-spec 3478A.
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Online BravoV

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #12 on: February 22, 2013, 12:42:41 am »
The 3468A has a full can over the analog section. Interesting that they thought it was not necessary for the higher-spec 3478A.

Probably because the whole 3468A's case is all plastic instead of metal ?  :-//

Offline c4757p

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #13 on: February 22, 2013, 12:52:48 am »
 :palm: Yeah, that would be it.
No longer active here - try the IRC channel if you just can't be without me :)
 

Online BravoV

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #14 on: February 22, 2013, 01:44:13 am »
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 ?  :-//

Offline obsoletemac

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #15 on: February 22, 2013, 05:29:59 am »
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
 

Offline pgross

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #16 on: February 22, 2013, 11:56:57 pm »
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
Up & running!
 

Offline staxquad

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #17 on: February 23, 2013, 12:15:16 am »
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).
« Last Edit: March 22, 2013, 05:49:35 am by staxquad »
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Offline Wartex

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #18 on: February 23, 2013, 12:54:22 am »
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 ?  :-//

high voltage AC will make it thru the inductor
 

Offline robbak

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #19 on: February 23, 2013, 08:24:28 am »
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 ?  :-//
Add 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.
 

Offline Hypernova

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #20 on: February 23, 2013, 12:16:55 pm »
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.
 

Offline bingo600

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #21 on: February 23, 2013, 12:27:41 pm »
Damm i have hoped that Dave did replace the battery  :-[

I 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
« Last Edit: February 23, 2013, 07:17:15 pm by bingo600 »
 

Online BravoV

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #22 on: February 23, 2013, 12:46:23 pm »
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.  :o


Offline IvoS

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #23 on: February 26, 2013, 12:35:36 am »
I was also thinking to plug the unit to mains but disconnect the ground prong during battery replacement.
 

Offline Smokey

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Re: EEVblog #427 - HP 3478A Multimeter Teardown
« Reply #24 on: February 26, 2013, 12:46:48 am »
Wow.  The ebay auction ended and the thing sold for AU $355.00.

http://www.ebay.com.au/itm/271158344204

Did you have it gold plated before you put it back together or something?  You at least autographed the case right?
 


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