HP 34740A display and 34702A Multimeter Modules

As men­tioned in a pre­vi­ous post on the Hewlett Packard 34702A Multimeter mod­ule, I have been look­ing for a 34740A dis­play mod­ule to pair with it. I was lucky to find a mod­ule that was in a good cos­met­ic con­di­tion and untest­ed for a rea­son­able price.

The Hewlett Packard 34740A dis­play mod­ule is a four-dig­it 1 V range dc volt­meter with 100% over-range capa­bil­i­ty, it uses a dual-slope inte­grat­ing mea­sure­ment tech­nique, and can dis­play up to 19999 counts. It also sup­plies pow­er to the func­tion mod­ule it is attached to, which in this case is the 34702A mul­ti­me­ter mod­ule. It is the pre­de­ces­sor to the 34750A 5 ½ dig­it display.

Overall the mod­ule was in good con­di­tion, but had a large amount of spi­der webs and molts inside the mod­ule. After a good clean­ing all the spi­ders and webs were evict­ed, with no dam­age found on any com­po­nents or traces. Extra care was giv­en to the A3 board and it’s high imped­ance cir­cuits, which includ­ed sev­er­al 99% iso­propyl alco­hol scrubs and rins­es to remove any slight­ly con­duc­tive materials.

The first checks I made were the sup­ply volt­age set­tings on the rear pan­el, the fuse for cor­rect rat­ing, in-cir­cuit elec­trolyt­ic capac­i­tor checks, and then the pow­er trans­former for opens and shorts. All looked good, so I pow­ered it up for the first time.
Well, at least it had a dis­play indi­ca­tion that read +1.000, and all +5, +12, ‑12, pow­er sup­ply volt­ages were well with­in spec­i­fi­ca­tions. I was hop­ing for a dis­play of .0000 with the input short­ed. I also tried a small pos­i­tive and neg­a­tive volt­age at the input with no changes to the display.

After fir­ing up the oscil­lo­scope and using my HP 548A Logic Clip, I deter­mined that none of the tim­ing sig­nals were active on the A, B, or C cir­cuits. The cul­prit was a bad 74L74 dual D edge-trig­gered flip-flop in U20, HP part num­ber 1820–0596.
Replacing it got the dis­play read­ing some­thing oth­er than 1.0000, but it still was­n’t oper­at­ing correctly.

After some more trou­bleshoot­ing I found that I was miss­ing the +1 volt ref­er­ence volt­age from a cus­tom HP IC. This had me very wor­ried at first as it would be very hard to source a replace­ment. The prob­lem end­ed up being a miss­ing spe­cial 6.2 volt pos­i­tive sup­ply for the ref­er­ence IC. The issue was with Q34 a 30 volt N‑Chan JFET HP part 1855–0062 that was open. I end­ed up replac­ing it with a J211-D74Z rat­ed at 25 volts.
After replace­ment the +6.2 volts was back along with the ref­er­ence +1 volt supplies.

Overall the dis­play mod­ule was func­tion­ing prop­er­ly, but there was still an issue with the dis­play. It was dis­play­ing a short­ened 1 miss­ing the upper and low­er LED dot in the 10,000 dig­it dis­play, when read­ing a val­ue below 10000.
Again the prob­lem IC was a bad 74L74 flip-flop at U8 in the dis­play blank­ing cir­cuit. Once replaced the dis­play blank­ing cir­cuit worked prop­er­ly again.
With so many 74L74 IC’s fail­ing, I pre­emp­tive­ly replaced U16, anoth­er 74L74 in the tim­ing cir­cuit even though it test­ed good. And for good mea­sure I replaced the pow­er sup­ply sec­tion elec­trolyt­ic capac­i­tors which were still bare­ly with­in spec­i­fi­ca­tions, but since this mod­ule was built in 1977 they had a good 45 year life and were ready to retire.

The new elec­trolyt­ic capac­i­tors were Vishay long-life axi­al parts with the same capac­i­tance rat­ing, but one step up from the orig­i­nals in their volt­age rat­ings. I also used some Kapton tape for a bit more insu­la­tion where the capac­i­tors rest on the cir­cuit board.
Another safe­ty upgrade, was some red heat shrink insu­la­tion on the two AC pow­er input tabs, and a piece of Kapton tape above the volt­age selec­tion switches.

Now that the dis­play was work­ing, it was time to turn my atten­tion to the 34702A mul­ti­me­ter mod­ule with min­i­mal test­ing from a pre­vi­ous post.
I was able to ful­ly cal­i­brate the DC volt­age and Ohms func­tions on the mul­ti­mer mod­ule, but no mat­ter what I tried the AC volt­age func­tion was always around 0.18 volts dif­fer­ent than the expect­ed read­ing. After sev­er­al hours of trou­bleshoot­ing, and sev­er­al cups of cof­fee, I decid­ed to lift one of the leads of C19 a bi-polar 100uF capac­i­tor in the “AC Converter” sec­tion and found that it had exces­sive leak­age cur­rent. After check­ing C14 and C21 which were iden­ti­cal capac­i­tors to C19 they were also slight­ly leaky but not near­ly as bad as C19. After replace­ment of these three bi-polar elec­trolyt­ic capac­i­tors I was now able to cal­i­brate the AC function.

I used a 220 uF capac­i­tor in C14, as I did­n’t have enough 100 uF capac­i­tors in stock. Leads of the radi­al capac­i­tors were insu­lat­ed with PTFE wire insu­la­tion to pre­vent short­ing to board traces. A new Vishay TVA series 100 uF axi­al capac­i­tor TVAN1207.1 that fea­tures low leak­age cur­rent, is on order to replace the 220 uF tem­po­rary unit.

After a bit of work the Hewlett Packard 34740A dis­play and 34702A mul­ti­me­ter pair are now ful­ly func­tion­al and cal­i­brat­ed. I have been extreme­ly impressed with the accu­ra­cy and sta­bil­i­ty of this com­bo which exceed­ed my expec­ta­tions.
I would like to replace the front pan­el bind­ing posts $$ at some point in the future, but they are cur­rent­ly func­tion­al, and not in my cur­rent budget.

I am cur­rent­ly work­ing on a replace­ment for the HP 11456–66501 dis­play mod­ule test card for the HP 34740A and 34750A dis­play units, which would have made trou­bleshoot­ing much eas­i­er. I may loan it out to oth­ers in the future if it works as expected.

Gallery pho­tos of resis­tance stan­dard checks, and addi­tion­al 34740A images: —click on images to enlarge.