Hewlett Packard HP 403B AC Voltmeter Restoration

Hewlett Packard HP 403B front reading 0 dB
Hewlett Packard HP 403B front read­ing 0 dB

Another local Ebay find was this Hewlett Packard HP 403B AC Voltmeter in a pro­tec­tive HP ver­ti­cal case. The case did an excel­lent job of pro­tect­ing the meter over it’s now 48 year life.

As with most of my repair finds the first step was to clean it up, and look for poten­tial prob­lems before pow­er­ing it up.

Hewlett Packard HP 403B AC Voltmeter external case
Hewlett Packard HP 403B AC Voltmeter exter­nal case after clean­ing, HP mod­el # 11075A

After remov­ing the meter from the HP instru­ment case, I took off the top, bot­tom, and two side pan­els which gave me some access to the internals.

Hewlett Packard HP 403B AC Voltmeter original NiCD batteries
Hewlett Packard HP 403B AC Voltmeter orig­i­nal NiCD batteries

A quick check of the Nickel-Cadmium bat­ter­ies showed that only one pack was able to take a charge, with the oth­er three packs not accept­ing any charge.
That was pret­ty much what I expect­ed since these cells had most like­ly not been charged in many years. There was also some signs of leak­age, so it was time for them to go.
Unfortunately the meter uses the bat­tery packs as a pow­er sup­ply fil­ter and volt­age divider for the +13V, ‑6.5V, and ‑13V pow­er rails.

Exell Battery 12 volt NiCD 1,000 mAh

After a few quick mea­sure­ments I ordered a pair of Exell 10 cell 12V NiCD bat­tery packs to arrive in a few days. The exist­ing packs were 5 cell 6V units that were con­nect­ed in series with the ground con­nec­tion con­nect­ed in the mid­dle of the packs.
I would have to tap the con­nec­tion between the 5th and 6th cell on one of the new packs for the ‑6.5 sup­ply, but the rest of the con­nec­tions were all to the exist­ing tabs.

Hewlett Packard HP 403B with batteries removed
Hewlett Packard HP 403B with bat­ter­ies removed. Blue wire is the ‑6.5 volt supply

While wait­ing for the bat­ter­ies to arrive I spent some time clean­ing up the cir­cuit boards and check­ing capac­i­tors. The only capac­i­tor that was even slight­ly mar­gin­al was C21 a 65 uF at 60 VDC rat­ed alu­minum elec­trolyt­ic in the pow­er sup­ply AC rec­ti­fi­ca­tion section.

Hewlett Packard HP 403B - 65B power supply board
Hewlett Packard HP 403B65B pow­er sup­ply board
65B power supply board with new Nichicon capacitor
65B pow­er sup­ply board with new Vishay capac­i­tor, top view

There was a small amount of cor­ro­sion on the bot­tom side of the 403B-65A main cir­cuit board, which cleaned up eas­i­ly. The board is posi­tioned direct­ly above the bat­tery packs, so the cor­ro­sion was prob­a­bly from leak­ing electrolyte.

Hewlett Packard HP 403B-65A main board top view
Hewlett Packard HP 403B-65A main board top view
Hewlett Packard HP 403B-65A main board left side view
Hewlett Packard HP 403B-65A main board left side view
Hewlett Packard HP 403B-65A main board right side view
Hewlett Packard HP 403B-65A main board right side view

After work­ing on the cir­cuit boards I opened up the shield­ed and chas­sis ground iso­lat­ed enclo­sure for the atten­u­a­tor 403B-65C board, and the range switch. The range and pow­er switch were cleaned with DeoxIt D5 con­tact clean­er. I also cleaned the front jacks and BNC adapter while I was set­up for mak­ing a mess.
The atten­u­a­tor cov­er is held on with two screws above the side with the adjust­ment holes, one screw in the low­er chas­sis near the meter, and one screw attach­ing the pow­er sup­ply board next to the AC pow­er inlet connector.

Hewlett Packard HP 403B-65C attenuator cover, adjustments, and input overload fuse
Hewlett Packard HP 403B-65C atten­u­a­tor cov­er, adjust­ments, and input over­load fuse

The bat­tery packs that I ordered arrived, so I used some 3M VHB dou­ble sided tape to attach the two packs to the tray. The pack lay­out matched the lay­out of the exist­ing wiring har­ness, with only a few bends need to line up the wires with the ter­mi­nals. The blue wire attach­es to the ‑6.5 volt tap on the oppo­site side of the ‑13V pack on the strap between cells 5 and 6. Be sure to insu­late the new con­nec­tion and also not over­heat the cells when soldering.

Hewlett Packard HP 403B battery pack installation and connections
Hewlett Packard HP 403B bat­tery pack instal­la­tion and connections

I was a lit­tle con­cerned when first turn­ing on the meter, as it pegged the nee­dle of the meter to max­i­mum, with a return to near zero, and then anoth­er excur­sion to max­i­mum before set­tling back to zero and stay­ing there.
After some research, I found that sequence as a nor­mal turn on as long as the nee­dle returns to zero with­in around 20 sec­onds.
After a 20 minute warm up, I was able to com­plete all the cal­i­bra­tion pro­ce­dures and most of the cal­i­bra­tion checks with the excep­tion of the 100 and 300 VAC checks which I did­n’t have the equip­ment to gen­er­ate sig­nals above 50 VAC accurately.

Hewlett Packard HP 403B BNC adapter
Hewlett Packard HP 403B BNC adapter

After 24 hours with the switch held momen­tar­i­ly in the bat­tery check posi­tion the bat­ter­ies had charged up to 27 volts total. Due to the extra capac­i­ty of the new 12 volt packs at 1,000 mAh ver­sus the orig­i­nal pack capac­i­ty of 225 mAh, the meter should be able to oper­ate con­tin­ious­ly for 7 days before need­ing recharged. I did adjust R39 for a charg­ing rate of 10 mA which does­n’t include an addi­tion­al 5 mA cur­rent for meter elec­tron­ics which is always on when charg­ing.
A ful­ly dis­charged bat­tery would take around 120 hours to ful­ly charge back up though.

Hewlett Packard HP 403B AC Voltmeter front
Hewlett Packard HP 403B AC Voltmeter front

This aver­age respond­ing AC volt­meter after it was cal­i­brat­ed, has proven to be very accu­rate on all ranges. It would be a great addi­tion to an audio tech­ni­cians work­bench or any­one work­ing with sub-mega­hertz AC sig­nals. The only thing left to do now is to make some room for it on my workbench.

6 Replies to “Hewlett Packard HP 403B AC Voltmeter Restoration”

  1. I did one not that long ago. The meter read 15% on all ranges, but was fine when not plugged into 120VAC. The C21 cap was dried out…30V rip­ple or more. Once changed out (60uF, 60V) the unit is now good, 3mV rip­ple on the bat­tery ver­sus about 50mV before that.

    Batteries are good for about 10 years… YMMV.
    These are great for wide band AC signals.

  2. Can some­one please inform me upon chang­ing out the 6 ger­ma­ni­um tran­sis­tors for 6 sil­i­con tran­sis­tors if any oth­er cir­cuit mods are needed…maybe ill just have to wing it..thanks.

    1. Hi Jon,
      According to my ser­vice man­u­al and datasheets Q1, Q2, Q3, and Q5 are 1853–0020 PNP Silicon TO-92 tran­sis­tors, and Q4, Q6 are 1854–0071 NPN Silicon TO-92 transistors.
      My ser­vice man­u­al only cov­ers Serial num­bers pre­fixed: 0986A, so if your unit is an ear­li­er pre­fix you might have ger­ma­ni­um transistors.
      If so you might want to com­pare schemat­ics between man­u­als to see what changes were made if any.

      Greg (Barbouri)

  3. This was an excel­lent post and quite help­ful. Since I did­n’t want to use my meter as a portable and did­n’t have the case any­way. I replaced the bat­ter­ies with a diode stack con­sist­ing of 1n4735A 6.2 volt zen­ers. After replac­ing a few capac­i­tors the meter fell right into cal­i­bra­tion. I was care­ful to mark all the bias points when doing the rewire. The meter is much lighter and is a wel­come replace­ment for my old 400D. The charge light stays on but no prob­lem at all.

  4. Hi. I’m a big fan of HP gear and have sev­er­al pieces in my col­lec­tion of test gear . A while back I start­ed work­ing of refur­bish­ing one of two 403B’s in my inven­to­ry. I got side tracked and am just now get­ting back to it. My Nicads were shot. I have new Nicads. I can’t find my note on the col­or cod­ed wiring for con­nect­ing the batteries.Mine has a red, blue and pur­ple. the white with a black trac­er is at ground. HELP!!!

    1. Hi David,
      Red (2) goes to + ter­mi­nal on the pos­i­tive pack.
      Jumper goes from — ter­mi­nal on pos­i­tive pack to + ter­mi­nal on neg­a­tive pack.
      White/Black (90) ground con­nects to pack jumper.
      Violet (7) goes to — ter­mi­nal on neg­a­tive pack.
      Blue (6) con­nects to the ‑6.5 volt tap on the oppo­site side of the neg­a­tive pack, on the strap between cells 5 and 6 for a 10 cell pack.

      Good luck with the repair,
      Greg (Barbouri)

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