Hewlett Packard 3456A Digital Voltmeter

HP Hewlett Packard 3456A Front pan­el short­ed input, still warming-up

Cleanup, restora­tion, and cal­i­bra­tion of a Hewlett Packard (HP) 6 1/2 dig­it 3456A Digital Voltmeter.

I had been look­ing for a new­er Hewlett Packard 3457A dig­i­tal volt­meter instead of the HP 3456A, to work on for sev­er­al years now, with­out much luck in find­ing one that need­ed repairs but not total­ly trashed and a rea­son­able price.
That had giv­en me some time to do addi­tion­al research on some of the fea­tures, and gen­er­al fail­ure modes of that mod­el. Even though the 3457A had plen­ty of nice fea­tures, I was start­ing to con­sid­er the HP 3456A instead. Even though the 3456A is an old­er design, it has what I con­sid­er an eas­i­er to read dis­play, no cal­i­bra­tion to loose because of a dead mem­o­ry bat­tery, decade ranges ( 1 V vs. 3V), and a low­er noise front end.
By acci­dent I came across a HP 3456A on Ebay that had some small issues and looked to be in good mechan­i­cal con­di­tion, so I put a low-bid on it.
I picked it up at a very rea­son­able cost, below my bid and with a low ship­ping charge.

The Hewlett Packard 3456A is a 6 ½ dig­it volt­meter with AC, DC, Ohms, and math func­tions. It has a res­o­lu­tion of 100 nV on the 0.1VDC range and a res­o­lu­tion of 1 mV on the 1,000.0 VDC range. AC volt range is from 1 uV to 1,000 volts, and resis­tance range is from 100 µΩ to 1 GΩ with 2 or 4 wire ohms con­nec­tions.
It uses an inte­grat­ing ana­log-to-dig­i­tal (A‑to‑D) con­vert­er using the Multi-Slope II tech­nique which was devel­oped specif­i­cal­ly for this mod­el.
This unit is a 2201A series, was built in ear­ly 1983 and sold for around $3,700 ($9,500+ in 2020 U.S. dol­lars). The 2201A series did away with the rear fan, and moved the Outguard 5 volt pow­er sup­ply reg­u­la­tor to the rear pan­el.
This is not a small volt­meter, and mea­sures 19.75″ long by 16.75″ wide by 3.5″ high (50.16 X 42.54 X 8.9 cm). 

HP 3456A Front LED display

As usu­al I gave the new unit a full inspec­tion upon arrival. Almost every­thing was in good shape, except there was quite a bit of residue and some cor­ro­sion along the back side near the pow­er inlet. It looked like the meter had sat in some water for a peri­od of time.
Luckily the dam­age was­n’t high enough to reach the boards or the pow­er inlet, and only affect­ed the frame, side pan­el , and the inter­nal rear con­nec­tor cov­er which wicked up the mois­ture and had lots of cor­ro­sion between it and the rear pan­el.
Most of the cor­ro­sion was removed with some mild clean­er and a brass brush, fol­lowed with a nylon brush and dis­tilled water rinse. One Pozidriv screw need­ed to be replaced, but it was the only casualty.

HP 3456A inside bottom covers removed
HP 3456A inside bot­tom cov­ers removed

After a thor­ough clean­ing I pow­ered up the unit with the bot­tom pan­els removed so that I could check the pow­er sup­ply. The pow­er sup­ply volt­ages are marked on the bot­tom side of the A10 board at the con­nec­tor. All volt­ages were well with­in spec­i­fi­ca­tions, with very low AC rip­ple present. To my sur­prise all the elec­trolyt­ic capac­i­tors looked to be in very good con­di­tion. The elec­trolyt­ic capac­i­tors are high qual­i­ty Nippon Chemi-Con and Sprague units, but they are still 38 years old and should be replaced. More on that later.

HP 3456A A10 Power sup­ply board removed from unit to test capacitors

After run­ning for sev­er­al hours I ran thru all the per­for­mance checks, with only a few small issues. Several of the ranges were just out of cal­i­bra­tion, and there was a small 1.2 uV off­set with the input short­ed. I also ran the front pan­el self-test which per­forms cer­tain ana­log gain, off­set, dig­i­tal checks, includ­ing cross­guard cir­cuits, A‑to‑D con­vert­er, input switch­ing, ohms mea­sure­ment func­tion, and the ac con­vert­er which all passed with no issues.

HP 3456A front pan­el cal­i­bra­tion poten­tiome­ters for DC, Ohms, and AC

The cal­i­bra­tions were easy, and are done by adjust­ing mul­ti-turn poten­tiome­ters locat­ed under the front ter­mi­nal pan­el.
The off­set adjust is a bit more dif­fi­cult if it is out by more than 0.9 uV, which it was. The pro­ce­dure requires rerout­ing the input ampli­fier’s input wire to a dif­fer­ent pin, mov­ing a jumper, and tak­ing some mea­sure­ments, before adding a select padding resis­tor between a ter­mi­nal and resis­tor junc­tion. The pro­ce­dure is well doc­u­ment­ed in sec­tion 8‑F-19 of the manual.

LM399 Voltage reference board
HP 3456A volt­meter LM399 Voltage ref­er­ence board

One mod­i­fi­ca­tion that I want­ed to make was to bet­ter insu­late the LM399 pre­ci­sion shunt ref­er­ence on the A25 ref­er­ence board.
I opt­ed for the roVa Flex Plus Aerogel Insulation sheet, for it’s high insu­la­tion qual­i­ties on the top side of the board, and some Aspen Aerogel SPACELOFT™ mat for the bot­tom side.
I had read that the LM399 can have a ppm/K fig­ure of between 0.1 to 0.2ppm/K if prop­er­ly insu­lat­ed with more than the plas­tic insu­la­tor that comes with the device.
I added cutouts per lay­er for the ref­er­ence, resis­tor lead, and capac­i­tor in the roVa Flex, and secured the top stack with cot­ton thread.
Currently I don’t have a way to test the effec­tive­ness of this extra insu­la­tion, but in the­o­ry this should make a pos­i­tive improvement.

HP 3456A Reference board additional insulation
HP 3456A Reference board addi­tion­al insulation

The next step was replac­ing ALL the elec­trolyt­ic capac­i­tors thru out the unit. I will not be replac­ing any of the ceram­ic, film, mica, or her­met­i­cal­ly sealed Tantalum capac­i­tors.
So I had thought that replac­ing the capac­i­tors on the A10 pow­er sup­ply board would be some­what easy and straight­for­ward as the board is easy to remove to work on.

HP 3456A A10 board with new Nichicon capac­i­tors installed except for C5
HP 3456A C5 capacitor board pinout
HP 3456A C5 capac­i­tor pin lay­out on back of the A10 board

Until I came to C5, the Sprague 4,000 uF at 15 volt capac­i­tor.
It is an odd 5 pin type, with 4 neg­a­tive pins around the perime­ter, and a strange­ly off­set pos­i­tive pin. I was unable to find any new capac­i­tors with a sim­i­lar pin con­fig­u­ra­tion, so I made my own C5 adapter board.

HP 3456A C5 capac­i­tor adapter board

The assem­bly is com­posed of four Nichicon 1,200 uF 25 VDC capac­i­tors for a total of 4,800 uF @ 25 volts with an imped­ance of 4.25 mOhms. The Nichicon UHW1E122MPD capac­i­tors are long life, high reli­a­bil­i­ty, low imped­ance units rat­ed at 105˚C for 10,000 hours, so they should hope­ful­ly last for anoth­er 38 years. I decid­ed to use four capac­i­tors to not only decrease the imped­ance, but to also increase the exter­nal sur­face area for bet­ter cooling.

HP 3456A C5 capac­i­tor assem­bly, 4,800 uF at 25 VDC
HP 3456A AC power inlet and Outboard 5 volt regulator
HP 3456A AC pow­er inlet and Outboard 5 volt regulator

I am also replac­ing the Corcom AC pow­er line fil­ter mod­ule with a new Kemet GL-2030F‎ unit. The rea­son for this is that the mod­ule con­tains, or should con­tain X and or Y safe­ty capac­i­tors. These will also be 38 years old and should be replaced for safe­ty rea­sons as these capac­i­tors con­nect across AC line volt­age, and/or from AC line to ground. This is an often over­looked com­po­nent as the capac­i­tors are hid­den from view.

HP 3456A A3 board
HP 3456A A3 board with large axi­al capac­i­tor for fil­ter­ing 5 volt dig­i­tal supply
HP 3456A A20 board middle section
HP 3456A A20 board mid­dle sec­tion with small elec­trolyt­ic capac­i­tor (Orange — low­er left)

The oth­er two capac­i­tors to be replaced are the big axi­al 12,000 uF unit on the A3 board, and the small 1 uF @ 150 volt (orange) elec­trolyt­ic on the A20 board.
The small red, white, and blue cir­cles on the boards are PTFE (vir­gin Teflon) insu­la­tors, which can have a vol­ume resis­tiv­i­ty above 1017 (Ω/cm). These pre­vent stray cur­rents from affect­ing mea­sure­ment cir­cuits.
Dirt, oils, and even fin­ger­prints can seri­ous­ly degrade the insu­lat­ing prop­er­ties of these con­nec­tions, so extra care is required when work­ing on these boards.

HP 3456A Function, Math, and Register buttons
HP 3456A Function, Math, and Register buttons

The Function selec­tion but­tons include: DCV, ACV, ACV+DCV, 2‑Wire Ohms, and 4‑wire Ohms. Using the Shift but­ton selects the addi­tion­al Functions: DCV/DCV Ratio, ACV/DCV Ratio, ACV+DCV/DCV Ratio, 2‑wire O.C. Ohms, and 4‑wire O.C. Ohms.
The num­bered key­board sec­tion is used for stor­ing num­bers into reg­is­ters and select­ing Math oper­a­tions. Math oper­a­tions include: % Error, Scale, Pass/Fail, dB, dBm, Null, Thermistor, and Statistics.

Here are the oth­er boards, and dif­fer­ent views inside the HP 3456A:
(Click on images for larg­er size view)

HP 3456A A20 board front
HP 3456A A20 board front
HP 3456A A20 board back with A10 board top
HP 3456A A20 board back with A10 board top
HP 3456A A20 board overall view
HP 3456A A20 board over­all view
HP 3456A A4 board Rev C
HP 3456A A4 board Rev C
HP 3456A A30 board Rev C
HP 3456A A30 board Rev C
HP 3456A A40 board Rev B
HP 3456A A40 AC board Rev B
HP 3456A A40 board Fimeline resistor module
HP 3456A A40 board Fineline resis­tor module
HP fine­line resis­tor chip mag­ni­fied view. Credit:Hewlett Packard Journal April 1981

The brown mod­ules on stand­offs are the Fineline resis­tor net­work mod­ules made by Hewlett Packard, and are com­posed of mul­ti­ple pre­ci­sion resis­tors on a com­mon sub­strate tai­lored for the require­ments of each cir­cuit board.
The A20 board has two of these mod­ules des­ig­nat­ed as U200 and U500, and the A40 AC board has mod­ule U3.

HP 3456A A20 board relays
HP 3456A A20 board relays and pro­tec­tion com­po­nents. Amplifier input ter­mi­nal at top left
HP 3456A back panel with HPIB connector and Outboard 5 volt regulator
HP 3456A back pan­el with rear ter­mi­nals, HPIB con­nec­tor, Outboard 5 volt reg­u­la­tor, and AC power

So far I am glad that I acquired the HP 3456A instead of the 3457A volt­meter, even if I don’t have the oth­er unit to com­pare it to.
If I found just the right HP 3457A I might be tempt­ed to get it as to be able to com­pare the two meters. Until then I’ll just enjoy this one.

Here is a link to an inter­est­ing PDF arti­cle from the Hewlett Packard Journal from April 1981 (page 23) that dis­cuss­es the design and capa­bil­i­ties of the HP 3456A.
Precision DVM Has Wide Dynamic Range and High Systems Speed

Oshpark C5 Adapter board shared project page

Higher res­o­lu­tion pho­tos are avail­able on my Flickr 3456A album

Leave a Reply

Your email address will not be published. Required fields are marked *