Hewlett-Packard has built a reputation for quality bench multimeters every bit as impressive as Fluke has for the handheld variety. Nowadays they’re called Agilent, but this unit has the HP nameplate on it, and date codes on the parts inside suggest that it was manufactured in early 1987.
The 3468A and it’s cousin the 3478A are 5½-digit 300,000-count multimeters. Both have instrument interfaces, the former with HP-IL (serial) and the latter with HP-IB (parallel, aka GPIB.) Basic 1-year DCV accuracy is 0.02% of reading + 2 counts for the 3V range. The 3478A has ever so slightly better accuracy figures and adds a 30mV range.
This unit arrives in a completely non-functional state. It is dirty and dusty as are most of these old units, and probably hasn’t seen any use in some time. The remains of a Spanish-language calibration sticker on the back suggests it was maintained in a calibration program at some point in its lifetime.
Made In USA
Looking inside, we find everything in very good condition. The entire analog section is shielded with a metal cover. Under that, there’s an impressive-looking hybrid, a 3V lithium battery to maintain the calibration data in memory, and a bunch of Coto small-signal relays. The analog-to-digital converter is a custom HP part in a ceramic package.
The 12-character alpha-numeric LCD assembly has two COB-style drivers and is connected to the main board with a small ribbon cable. The keypad board is mounted to the main board with a small bracket holding an elastomeric connector strip.
The microprocessor has an HP part number, but looking in the service manual reveals that it is an 8039, which is a RAM-only version of the 8048. A whopping 128 bytes of RAM. Firmware is contained in external ROM. This particular unit is sourced from National Semiconductor and has a 1986 date code.
The transformer-isolated HP-IL interface is in the back alongside the power supply circuitry. The power transformer has four taps to easily change the mains input to different international voltage levels.
Since the unit does not power up, first thing to check is the power supply. A small black mark on the element inside the glass mains fuse is a good clue that something has gone awry. A quick continuity check confirms that it is indeed blown.
Small problem. The fuse is rated 100mA and I do not have an exact replacement handy. Obviously some unknown fault has caused the original fuse to blow, so substituting a higher value and then smoke testing it would be a dumb idea.
Instead, I’ll use a variable auto-transformer (Variac) to slowly ramp up the input voltage while monitoring the current drawn. This will tell me if the fault condition still exists, or whether it was some transient that caused the fuse to blow originally. Starting at 0V on the Variac and ramping up slowly, I get to about 35V and the current is already measuring 100mA. The fault condition must still exist.
Playing the Odds
Instead of studying the schematic like a proper engineer, I just go with probabilities and start checking diodes in the power supply section at the back. Sure enough, there is a group of six diodes surrounding four capacitors, and five of them measure as a dead short. That’s 0.0V on the diode-test range, both ways.
It’s possible some other component or fault could be affecting the in-circuit diode test, so I remove a pair of the diodes and check them again. Now they test OK, out of circuit. I removed another pair and they test OK as well. Checking across the now-empty diode pads with an ohm-meter shows that the short (or shorts) still exist on the board somewhere. Next item in the probability-of-failure category: capacitors.
The four caps in the middle are the likely suspects, so I just remove them all. And 3 out of 4 of them measure as dead shorts. All three are 0.2 ohms or less. This is an unusual failure mode for electrolytic capacitors in my experience, but the ohm-meter does not lie. All four capacitors get replaced.
Another power-up attempt with the Variac shows only a 30 mA current draw for a full 120Vac input. That means I can stick in a 1/16A fuse that I have (62.5mA) and expect it to survive and still have protection in case something else goes wrong.
The unit now powers up and passes the self-test, so hopefully there are no more faults. A quick check-out reveals that it is indeed functional on all ranges, and the calibration looks pretty good, except for AC current. (See note #2 below the calibration check table.) I would not use this meter for small-signal AC currents. In fact, experience has proved that AC or DC current measurements are not strong points for either the 3468A or 3478A.
But this model does have other nifty features. One of them is 4-wire resistance measuring capability. Using separate source and sense test leads nullifies the test lead resistance, and allows for very accurate measurements below one ohm. The easiest way to perform a 4-wire measurement is by using Kelvin clips, as shown in the photo at right.
I cheated to get the display to show a spot-on value. At this resolution, temperature effects on the resistance value are readily apparent. It was reading a few counts high, I just put my fingers on the resistor for a few seconds to warm it up a bit. 🙂
DMMCheck calibration check results for this HP 3468A:
|1mA||0.9993||0.00100A||No 300mA DC range on 3468A|
|1mA||1.0005||.001112A||not very impressive2|
- The Calibration Value is the actual measured value by the makers of the DMMCheck with their equipment at the time it was calibrated.
- The DMMCheck reference signal is a square wave at 100Hz. The lowest AC current range on the 3468A is 300mA. The most applicable accuracy specification is ±(1.1% of reading + 163 counts), which would mean that any reading between 0.826 and 1.174 is acceptable. In addition, the specification is guaranteed only for values above 10% of full-scale (30mA) and only for a sine wave. Neither restriction is satisfied in this case, so we’re lucky that it even reads this close!