
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.

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.

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.

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.


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.

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

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.

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.


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