Hewlett Packard 34703A DCV/DCA/Ohm Meter

Hewlett Packard 34703A meter plu­g­in mod­ule front panel

Cleaning up and pho­tograph­ing a Hewlett Packard 34703A DCV/DCA/Ω Meter plu­g­in sec­tion of a 3470 mea­sure­ment system.

I recent­ly came across this HP 34703A mod­ule at an excep­tion­al­ly rea­son­able price. The last time I worked with this mea­sure­ment sys­tem was 42 years ago, so I want­ed to doc­u­ment it and see what was inside, and how it worked.

Hewlett Packard 34703A meter plu­g­in mod­ule input side bottom

It is filled with plen­ty of HP good­ness. Six relay mod­ules for auto­mat­ic range switch­ing and auto-zero, laser trimmed resis­tors, pre­ci­sion dis­crete com­po­nents, and a hybrid inte­grat­ed cir­cuit ampli­fi­er with a switched feed­back net­work for estab­lish­ing gain.
Traces on the top side of the board are gold plat­ed for cor­ro­sion resis­tance, and the sol­der mask is removed from bot­tom side areas, along with PTFE insu­la­tors to reduce leak­age cur­rents in high imped­ance areas. It is a won­der­ful piece of engineering.

HP 34703A cir­cuit board with Option H02 A3 log­ic board fold­ed out
HP 34703A Option H02 A3 log­ic board

The 34703A has some impres­sive spec­i­fi­ca­tions for it’s price point and the time it was designed and pro­duced. From the date codes on the IC’s it seems that this unit was built in late 1974.
Some of it’s capa­bil­i­ties include, 10-mV and 1‑ohm full-scale ranges, and also self test, autorang­ing, and cur­rent mea­sure­ments to as low as 1 µA full scale. This is a plug-on mod­ule and is part of the 3470 mea­sure­ment sys­tem. When used with the HP 34750A dis­play sec­tion it can dis­play 5 ½ dig­its on the LED readout.

HP 34703A cir­cuit board bot­tom view with mod­ule connector

I did notice sev­er­al sim­i­lar com­po­nents that are also used in the HP 3456A Digital Voltmeter , such as the large reed relay with met­al mounts. The part num­bers are slight­ly dif­fer­ent, but that may be due to some­thing such as a dif­fer­ent coil volt­age.
Unfortunately, I haven’t been able to find a free down­load­able ver­sion of the user/service man­u­al for this mod­ule. If I can find a rea­son­ably priced 34750A dis­play mod­ule, I might pur­chase one of the man­u­als on CD for this unit.

HP 34703A reed relay, and hybrid ampli­fi­er IC
Close-up view of the HP 34703A hybrid ampli­fi­er module

There are sev­er­al dif­fer­ent revi­sions of the dis­play and Option H02 A3 log­ic boards installed in these units. This unit has a Rev E main board, a Rev B H02 A3 board, and a Rev C dis­play board. There are sev­er­al Service Notes avail­able for this mod­ule with minor upgrades to improve reli­a­bil­i­ty, such as adding a bypass capac­i­tor, or chang­ing a resis­tor val­ue on some of the ear­li­er boards. One item that seems to be com­mon and is evi­dent on my main board is small hair­line cracks trav­el­ing from where the mount­ing brack­ets attach the board to the case. Mine seems to have been patched with small wires sol­dered across the traces in the affect­ed areas.

HP 34703A main board with crack repair and Teflon insulators
HP 34703A main board with crack repair back bracket
HP 34703A Display board with range select micro-switch
HP 34703A main board components
HP 34703A Back pan­el with 10mV zero adjust, and no ser­i­al number ?

I have done some lim­it­ed test­ing on this unit, such as apply­ing 5 volts to the 5 VDC buss with a rea­son­able cur­rent draw for the IC’s on board, and a LED lit up on the front pan­el. I also test­ed most of the capac­i­tors in-place on the board with no issues. So all looks well so far

Hopefully I will have an update at some future date, if I can get this paired up with a work­ing display.

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