Hewlett Packard 34750A Display Module Repair

Side view of Hewlett Packard 34750A dis­play mod­ule board

Getting a HP 34750A dis­play mod­ule for the HP 34703A DCV/DCA/Ohm Meter.

After a bit of search­ing, I found a HP 34750A dis­play mod­ule to com­plete a 3470 mea­sure­ment sys­tem using my exist­ing HP 34703A mod­ule.
The mod­ule was sold as “Tested Working” so I thought I would be able to attach it to my exist­ing mod­ule when it arrived, and take some pic­tures of the assem­bly, along with a cal­i­bra­tion check.
The dis­play mod­ule arrived with a some­what beat-up HP 34702A mul­ti­me­ter mod­ule attached, and when pow­ered up, had an odd flick­er to the dis­play which read 0.0000.
Unfortunately that’s about all it would do. When sup­plied a DC or AC input the dis­play still indi­cat­ed 0.0000 on all ranges. A quick check on the Ohms set­ting also showed the same 0.0000 dis­play.
After a quick dis­cus­sion with the sell­er, I found out that it had not been test­ed, and that the per­son he bought it from showed him that it dis­played some­thing oth­er than zeros.
After a sub­stan­tial refund, I opened it up to see what might be caus­ing the issue.

Hewlett Packard 34750A display module circuit board bottom
Hewlett Packard 34750A dis­play mod­ule cir­cuit board bottom

There were no obvi­ous prob­lems that I could see from a visu­al inspec­tion of any com­po­nents on the board, and the volt­age selec­tion card and trans­former both checked out OK. Next thing to check was the pow­er sup­ply volt­ages, all of which were very low with lots of AC rip­ple. So it was time to pull the three elec­trolyt­ic capac­i­tors and check them.

HP 34750A elec­trolyt­ic capac­i­tors removed for testing

The first thing I noticed when remov­ing the capac­i­tors, was that they were extreme­ly light in weight for their size. After dis­charg­ing each capac­i­tor with a jumper lead, and then plac­ing it on my Peak atlas ESR+ meter, the meter indi­cat­ed that the capac­i­tor still had some resid­ual volt­age and auto­mat­i­cal­ly dis­charged it again before the test. All three capac­i­tors test­ed bad and were act­ing more like bat­ter­ies than capac­i­tors by pro­vid­ing a con­tin­u­ous cur­rent out­put even after being short­ed for more than a minute.

HP 34750A board top view with new capac­i­tors installed

I replaced the exist­ing pair of 500 uF 30 volt capac­i­tors with some radi­al lead Nichicon 560 uF 35 volt rat­ed capac­i­tors. The 2200 uF 16 volt capac­i­tor was replaced with a Vishay axi­al capac­i­tor of the same rat­ing.
After pow­er­ing up and adjust­ing vari­able resis­tor AlR69 which is the +12 volt adjust­ment, all the volt­ages were now well with­in spec­i­fi­ca­tions. A quick check of the volts and Ohms inputs dis­play a close val­ue as to the volt­age and resis­tance applied.

HP 34750A front pan­el display
HP 34750A back pan­el with pow­er in and adjustments

After a bit more clean­ing and then allow­ing the sys­tem to warm up for an hour, I per­formed the rest of the main­te­nance adjust­ment pro­ce­dures from the ser­vice man­u­al. Most of the adjust­ments use the four access holes in the back pan­el for the Zero, Hysteresis, and +/- ref­er­ence adjustments.

HP 34750A dis­play mod­ule with cov­er attached

For some of the main­te­nance adjust­ments the man­u­al states that “all cov­ers removed should be reinstalled”.

HP 34750A par­tial view of dis­play board components

The HP 34750A has a five-dig­it res­o­lu­tion dis­play but does­n’t have high accu­ra­cy due to it’s ref­er­ence volt­age sec­tion. But that is fine as it was nev­er meant to act as a cal­i­bra­tion stan­dard at it’s price point. It is very good at mea­sur­ing small dif­fer­ences in read­ings such as check­ing the small vari­a­tions in resis­tance when locat­ing a short­ed capac­i­tor in cir­cuit though.

HP 34750A paired up with a 34703A DCV/DCA/Ohm Meter

So far I am very hap­py with my new acqui­si­tion, and it per­forms very well for a 45 year old sys­tem built in 1976. Now I have to acquire a man­u­al for the HP 34703A mod­ule so that I can cal­i­brate it.

HP 34750A trans­former and pow­er sup­ply section
HP 34750A front com­po­nents and new capacitors

I will check out the HP 34702A mul­ti­me­ter mod­ule in a future post.

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