Hewlett Packard 5306A Multimeter/Counter

I final­ly came across a hard to find HP 5306A Multimeter/Counter paired with the HP 5300B Display mod­ule. It is an inter­est­ing mod­ule as it con­verts resis­tance and DC/AC volt­age to a fre­quen­cy that it then dis­plays on the 5300B dis­play, along with a fre­quen­cy capa­bil­i­ty of 40 Hz to 10 MHz.

This unit had mul­ti­ple issues and kept me busy for sev­er­al weeks, of which most of the time was wait­ing on parts.
The 5300B dis­play mod­ule would pow­er on for a brief sec­ond then the dis­play would dim along with a buzzing sound. My ini­tial guess was a pow­er sup­ply capacitor.

My ini­tial guess was not wrong, and I end­ed up replac­ing 7 out of tol­er­ance and short­ed capac­i­tors. The ‑17 volt sup­ply tan­ta­lum capac­i­tor was short­ed and replaced with the same capac­i­tance val­ue but increased from 25 volt to a 35 unit. It’s +17 com­pan­ion was also replaced pre-emp­tive­ly before it could cause trouble. 

Small DIP pack­age heatsinks were added to some of the IC’s that tend to oper­ate at a high­er tem­per­a­ture, and are hard to find replace­ments for.
I used my DIY 5300 Diagnostic Interface test card for ver­i­fy­ing the prop­er oper­a­tion of the mod­ule, and all tests passed.

It was now time to turn my atten­tion to the 5306A Multimeter/Counter mod­ule.
Did I men­tion that the pre­vi­ous own­er had tried to work on this mod­ule with­out the ser­vice man­u­al and left some notes in the unit, but none of the fas­ten­ing hard­ware.
Upon ini­tial check­out I found that the fre­quen­cy counter work fine, but DC, AC, and Ohms func­tions had no out­put to the dis­play.
The pre­vi­ous notes indi­cat­ed a pos­si­ble prob­lem in the input atten­u­a­tor sec­tion, but that checked out good.

The HP 5306A mod­ule con­sists of two cir­cuit boards, the A2 High Impedance board, and the A1 Logic board. The A2 board requires spe­cial han­dling due to it’s high-imped­ance cir­cuit­ry, and includes the input atten­u­a­tor, DC/AC/Ohms buffer ampli­fi­er, Ohms cur­rent source, and AC to DC con­vert­er. The cir­cuit­ry on the A2 board that is com­mon to all three non-work­ing func­tions are the atten­u­a­tor and switch­es which test­ed good.

On the A1 log­ic board the next com­mon area is the V to F (volt­age to fre­quen­cy) con­vert­er. The V to F con­vert­er out­puts a 0 to 2 MHz sig­nal that is pro­por­tion­al to the volt­age input.
This sec­tion was also work­ing prop­er­ly.
The next com­mon sec­tion is the Voltage or Frequency gat­ing and dif­fer­en­tia­tor cir­cuit where the fre­quen­cy input and the 0 to 2 Mhz sig­nals are select­ed for the out­put to the 5300 dis­play mod­ule.
Ah-Ha … this had to be the faulty area because after this the fre­quen­cy and faulty func­tions share the same path.
NOPE … this sec­tion also worked per­fect­ly, and no it was­n’t the iso­lat­ed pow­er sup­ply on the A1 board. I always check the pow­er sup­plies first before any addi­tion­al troubleshooting.

Out of curios­i­ty I checked out the F1 and F2 sig­nals that are sent to the dis­play mod­ule for noise or dis­tor­tion. The F1 sig­nal is the fre­quen­cy out­put from the V or F gat­ing and dif­fer­en­tia­tor which was good, but when check­ing the F2 out­put I should have been see­ing either a 10 MHz ref­er­ence sig­nal with the fre­quen­cy func­tion select­ed, or a 2 Mhz ref­er­ence sig­nal for the oth­er three non-work­ing func­tions. I was only get­ting the 2 MHz sig­nal when the AC func­tion was select­ed which seemed odd. 

The prob­lem end­ed up being a failed IC U14 which is a dual 2‑wide 2‑input AND-OR-INVERT log­ic gate. I was able to find a OEM HP 1820–0072 IC which is a 7450 on Ebay for a rea­son­able price. A new DIP sock­et was added while wait­ing for the part.
After replac­ing the part all func­tions were now work­ing, but way out of cal­i­bra­tion. It seems like there is a pos­si­bil­i­ty that the pre­vi­ous own­er had tried adjust­ing all the trim­mer poten­tiome­ters in their trou­bleshoot­ing endeav­or, as many of the trim­mers were at their lim­its. I also took the time to thor­ough­ly clean the high-imped­ance A2 board from any con­t­a­m­i­nants as I did­n’t know how it may have been han­dled in the past.

After replac­ing all the miss­ing hard­ware and a full cal­i­bra­tion includ­ing the ref­er­ence oscil­la­tor the sys­tem is work­ing as good as new.
As a mul­ti­me­ter it only has 4 1/2 dig­its, but for it’s time (1979) hav­ing a com­pact fre­quen­cy counter and mul­ti­me­ter in a sin­gle unit must have been fan­tas­tic.
It has been added to my work­bench with it’s broth­ers, sis­ters, and cousins to be used when needed.