Hewlett Packard 3437A System Voltmeter

Hewlett Packard 3437A System Voltmeter ready for ini­tial pow­er up.

Trouble shoot­ing and per­form­ing mod­i­fi­ca­tions on a HP 3437A System Voltmeter from 1984.

The HP 3437A is a micro­proces­sor con­trolled 3 and 1/2 dig­it, suc­ces­sive approx­i­ma­tion volt­meter, capa­ble of sam­pling volt­ages at rates up to 5,700 sam­ples per sec­ond.
It includes chas­sis iso­lat­ed input ter­mi­nals, a wide­band input ampli­fi­er, auto-zero, auto-polar­i­ty, sam­ple and hold, and 100% overange on each of the input volt­age ranges (.1 volt, 1 volt, and 10 volts) over a fre­quen­cy range of DC to 1.0 MHz.
The HP 3437A had a cat­a­log cost of $3,165 in 1986 which is equiv­a­lent to $7,275 in 2019 with infla­tion.

This HP 3437A was an Ebay in state auc­tion pur­chase that cost 8 times more to ship than the final auc­tion price. It was well packed though and arrived in great shape.

The com­plete auc­tion descrip­tion was ” HP 3437A SYSTEM VOLTMETER. Condition is Used. “, and includ­ed one pic­ture of the front pan­el.

Ebay auc­tion pho­to of Hewlett Packard 3437A front pan­el

The front pan­el LED’s that were on in the pic­ture includ­ed “Data Ready”, Invalid Pgm”, “Listen”, “Talk”, “Remote”, “Binary Prgm”, “Enab RQS”, “10V”, “Int”, “Hold/Man”, and “ASCII”. None of the 7 seg­ment dis­plays were illu­mi­nat­ed.
The price was right, so I put in a low bid and fig­ured that it would be a fun win­ter project. I was the only bid­der and picked it up at the start­ing bid price of $3.99. My guess was that the blank dis­play, and the strange com­bi­na­tion of sta­tus LED’s scarred most oth­er bid­ders away.

Before turn­ing it on a thor­ough inspec­tion was in order. I always like to check for loose parts, and cables from ship­ping, along with phys­i­cal signs of burnt parts and dam­age before turn­ing it on for the first time. I also check the trans­form­ers, rec­ti­fiers, and capac­i­tors in the pow­er sup­ply sec­tion for shorts, and gen­er­al oper­a­tional spec­i­fi­ca­tions. All checked well except for the trans­former pri­ma­ry, which seemed to be a bit high­er in resis­tance than I expect­ed. Maybe some dirty volt­age selec­tion switch­es? Nope.….

Hewlett Packard 3437A System Voltmeter back pan­el and volt­age select switch­es.

The select­ed volt­age on the switch­es was 240 volts.
After select­ing 120 volts and replac­ing the fuse with a 500 mA rat­ed part, I was ready to pow­er the unit up.

Hewlett Packard 3437A System Voltmeter Local mode nor­mal indi­ca­tors

The sys­tem came up with a func­tion­al dis­play and work­ing push but­tons. Power sup­ply volt­ages were checked and were all with­in spec­i­fi­ca­tions. The only thing that was­n’t work­ing was the sys­tem fan.

HP for some strange rea­son decid­ed to use a 3‑phase fan, and spe­cial dri­ve cir­cuit­ry with 35 dis­crete com­po­nents to pow­er it at a fixed speed.
Some of the HP 3437A units had fail­ing fans, and HP issued a ser­vice note 3437A-10 to address replace­ment of the 3‑phase AC fan with a DC fan.
I end­ed up using a Sunon axi­al 60x20mm 12VDC fan PN# MF60201V3-1000U-A99
The orig­i­nal screws were too short for the new fan, so I used two stain­less steel 4–40 by 1″ phillips machine screws which fit per­fect­ly. Only two screws are need­ed as the oth­er two holes are used for attach­ing the exter­nal air fil­ter.
For my unit the replace­ment required a 22 ohm 2 watt resis­tor to be fit­ted across the col­lec­tor and emit­ter pads of the removed Q202 tran­sis­tor, and removal of R204.
Instructions for the fan replace­ment and cir­cuit board mod­i­fi­ca­tions start on PDF page 174 of the “Operating and Service man­u­al” linked at the end of the blog post.

Hewlett Packard 3437A System Voltmeter inside of back pan­el wit new fan installed

The 3437A comes stan­dard with 3 lug Triaxial input con­nec­tors on the front and rear pan­els. The price for tri­ax­i­al con­nec­tors and cables is cur­rent­ly out of my hob­by bud­get so I opt­ed to replace the front con­nec­tor with a stan­dard 2 lug iso­lat­ed BNC con­nec­tor.
I know the ben­e­fits of using tri­ax­i­al con­nec­tions, but for my gen­er­al use I am will­ing to trade those off to be able to use my exist­ing cables.
Replacing the con­nec­tor is not easy and took me around 1 hour to com­plete, but is com­plete­ly reversible if I do ever decide to replace the tri­ax­i­al con­nec­tor.

With the new fan and con­nec­tor installed, func­tion­al, and cal­i­bra­tion checks went smooth­ly with every­thing well with­in spec­i­fi­ca­tions.

Hewlett Packard 3437A System Voltmeter with new iso­lat­ed BNC con­nec­tor. 10.00 volt input

After a bit more clean­ing and pic­ture tak­ing I placed the cov­ers back on the unit and added it to my test equip­ment shelve.

Additional pic­tures of inter­nal com­po­nents:
Click on thumb­nails for full sized images.

Hi-Resolution pho­tos on Flickr — Album HP3437A

HP 3437A Operating Manual
HP 3437A Operating and Service Manual

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