Hewlett Packard 34702A Multimeter Module

Hewlett Packard 34702A Multimeter module

When I pur­chased my HP 34750A Display mod­ule, it came with a some­what beat-up 34702A Multimeter mod­ule. I was main­ly inter­est­ed in the dis­play mod­ule to pair up with my exist­ing HP 34703A mod­ule, but this is also an inter­est­ing mod­ule wor­thy of fur­ther exploration.

The Hewlett Packard 34702A Multimeter mod­ule is part of the HP 3470 Measurement System. The mod­u­lar sys­tem con­sists of the HP 34701A DC Voltmeter, this 34702A Multimeter, 34703A DCV/DCA/Ohm Meter, 34740A 4–1/2 dig­it Display, 34750A 5–1/2 dig­it Display, 34721B BCD mod­ule, 34722A pre­am­pli­fi­er mod­ule, and the 34720A Battery module.

HP 34702A top of board with switch­es, and shield­ed atten­u­a­tor section

The 34702A Multimeter mod­ule pro­vides four ranges of DC and AC (1 V to 1,000 V ) plus 6 ranges of resis­tance mea­sure­ment (from 100 Ω to 10 M Ω). The AC mode fre­quen­cy range is 45 Hz to 100 kHz. It does­n’t have a cur­rent mode or auto­mat­ic rang­ing like the HP 34703A though.

HP 34702A bot­tom of board with inter-mod­ule connector

Even though it is a much sim­pler mod­ule com­pared to the HP 34703A, it uses some very high qual­i­ty / pre­ci­sion com­po­nents. C10 is a mas­sive 10,350 pF 1% mica capac­i­tor and is used in the AC atten­u­a­tor cir­cuit. R1 and R2 are a matched set of pre­ci­sion high volt­age resis­tors with a 0.1 % tol­er­ance. There is a matched set of five, 0.05% tol­er­ance resis­tors in the Ohms Converter circuit.

HP 34702A matched pre­ci­sion resis­tors in atten­u­a­tor section
HP 34702A under the switches

I was able to clean up the front pan­el con­sid­er­ably from its orig­i­nal con­di­tion, but there are some deep scratch­es in the paint where cal­i­bra­tion stick­ers had been removed in the past, and some per­ma­nent stain­ing on the switch but­tons and terminals.

HP 34702A PCB trace repairs around mount­ing cutouts

All the capac­i­tors on this board test­ed good, and the only issue was some cir­cuit board cracks pass­ing thru traces at the mount­ing cutouts. This seems to be a typ­i­cal prob­lem with these mod­ule boards.
After a thor­ough clean­ing I used some thin cyano­acry­late glue for the cracks and then fol­lowed up with some thin wire bent to con­form to the exist­ing trace then flow sol­dered and cleaned with iso­propyl alcohol.

HP 34702A atten­u­a­tor, imped­ance con­vert­er, and Ohms con­vert­er sections

All in all the 34702A seems like a very capa­ble mod­ule, even though I spent less than 20 min­utes mak­ing sure that it was oper­a­tional.
If I can find a HP 34740A 4–1/2 dig­it dis­play mod­ule for a rea­son­able price I might pair it up for a com­plete sys­tem. — ( Update, I found a 34740A and will post an update) —

HP 34702A back pan­el and com­po­nents with PTFE standoffs

The oper­a­tion and ser­vice man­u­al for most of the 3470 mea­sure­ment sys­tem except the 34703A, can be found at hparchive.com.
Higher res­o­lu­tion pho­tos are avail­able in my HP 3470 Album on Flickr.

Leave a Reply

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