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

author: gilbenl
author_rank: Regular Contributor
timestamp: June 13, 2015, 08:51:10 pm
content: HP3456A offered up on eBay as "Powers up-Pulled from a working environment" with a starting price of $100. Offered the seller $50 and to my surprise, they acceptedLots of info on this great DMM out there, but not much in the way of pictures. Thought someone might find it interesting.A few impressions: I was terrified when I first turned on the meter against my 10V standard. Reading was all over the place and I don't need another wonky DMM. Don't worry like I did-It takes every bit of 1-2 hrs to really warm up to spec. In the ballpark after about 30. Initial stabs at making this thing do what I wanted were fruitless as the controls are less than intuitive. One pass through the manual, however, and it makes total sense. Very easy to use and loaded with features. Some notable inclusions:-Front and Rear Inputs-Ratio measurements-Null measurements-Variance, mean, upper, lower, (x-y)/z, dB stats-Adjustable NPLC and readings per trigger-Temperature (F and C)Some notable exclusions:-Current-Frequency-AutocalSo far, I love this meter. My sense is that the Datron 1062 is a bit quicker in terms of taking measurements, but I'm still playing with NPLC, Filter, Autozero and N Meas/Trig settings-None of which can be controlled on my 1062. The overall construction of the 3456 is remarkable. The 1062 is very well built but the engineering know-how derived from HP pedigree can't be beat.This is the best bang-for-buck meter I own. In fact, at $50+$20sh, it may be my best score yet. Highly recommended.Good history of the 3456A here:https://www.febo.com/pipermail/volt-nuts/2010-November/000626.htmlFront Panel:Wasn't sure of the age of the unit when purchased; expected an early model. Notice the rubber buttons rather than plastic. This indicates that it was among the last two revisions. Its also without a fan or filter in the back.Calibration Panel:The 3456 predates autocal/cal over the bus. Adjustments are made using these vishay pots located behind the input jack panel. Since the meter was in spec, I quickly replaced the cover as to avoid letting the accuracy out.Rear Panel:Again, notice the absent fan and filter assy. denoting a later run. 8924 according to the 5V regulator. Unit last appeared in the catalog in early '93.Size:You can't discuss the 3456 without mentioning its size. It's massive-about 6 inches deeper than a 3457. Size comparison with another notoriously massive meter-Fluke 27Top Case Removed:The top and bottom case are held in by a single screw each. Removing this reveals a black shield keeping bad influences from the analog board.Top-Power Supply and Analog:With the black cover removed, you can see the analog PCB (front) and power supply (rear) all contained within the guard.Analog Board Close-Up:Mid left of the screen is the LM299-based reference board (A25), same part used in the less accurate 3457A. Throughout, silkscreen "boundaries" indicating where each section is. Very detailed.Power Supply:Transformer resides out of the guard and somewhat inaccessible from the top. Caps, regulators, etc located on the board below. For once, a 20 yr old instrument with GOOD caps.Digital Boardmmmmm digital.Bottom:Flipping the unit over reveals a similar black inner case (not shown), beneath which resides the AC board with all of the real good stuff under yet another shield. Notice the inductors used to interconnect the boards rather than optocouplers.AC Oscillator:Reference oscillator(?) for the AC section. Funny looking thing.AC Cover Removed:3 layers deep. Let's see what all the secrecy is about...AC Hybrid-SN74LSO2N:Ceramic hybrid sitting at the heart of the AC board. Notice the hand-painted value bands on the resistor. I wonder what that guy's title was...HPIB:Opposite the AC board and out of guard is the HPIB board. Nothing particularly interesting except that insane 16,000uF cap. Hope that thing never goes.Transformer:Last and definitely least are the transformer and misc rear panel connections. This section lives under an aluminum panel separate from the black guard covers.

author: Vgkid
author_rank: Super Contributor
timestamp: June 13, 2015, 09:27:27 pm
content: If i remember correctly the analog side has its own digital section, thats where the osc comes into play.RECAP THE PSU!!!It will cause many problems, unless someone has already done it.

author: TimFox
author_rank: Super Contributor
timestamp: June 13, 2015, 10:20:46 pm
content: Those white cylindrical objects with the short color bands are actually "dogbone" ceramic capacitors (probably NP0).