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.

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