Hewlett Packard 3456A Digital Voltmeter

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). 

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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)

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.

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