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Azur Electronics

The HP 3478A is a versatile multimeter with dc and ac rms volts, dc and ac rms currents, 2 & 4 wire resistance measurement. The reading can be displayed in 5½, 4½, or 3½ digits. Features auto-ranging, auto-zero, HP-IB remote programming and electronic calibration. The Operation, Calibration and Service Manual was downloaded. The HP 3478A cost $1,295 in 1993.

Front view
The push-button switch by the 4mm sockets selects front or rear inputs.

Rear view
Top view with cover removed
Bottom view with cover removed

dc Voltage: 30mV, 300mV, 3V, 30V, 300V ranges.
ac Voltage: 300mV, 3V, 30V, 300V (true rms) ranges.
Resistance: 30Ω, 300Ω, 3kΩ, 30kΩ, 300kΩ, 3MΩ, 30MΩ (2 or 4 wire) ranges.
dc Current: 300mA, 3A ranges.
ac Current: 300mA, 3A (true rms) ranges.

April 2007

Just needed a clean-up and powered-up with 'Self Test OK' displayed. 
Unusually for HP, the line frequency 50Hz/60Hz is set by the left hand switch of the Rear Panel DIP Switch, set to '1' (Up) for 50Hz. Also the line voltage is set by the grey wire, next to the power transformer, set to the appropriate pin.

Power up display
I use the 3478A when I need to check readings on my other multimeters or require extra accuracy. As it has a LCD display it can be difficult to read in low light situations. Apart from this it is an excellent multimeter.

Battery BT701
November 2016

Checking all the specialist batteries in my test equipment, see Batteries In Test Equipment. The 3478A has a 3V 0.95AHr Lithium Battery on A1 Assembly, Reference BT701, Part No 1420-0278. Made by Power Conversion Inc of Elmwood Park N.J. USA, 3V No B9511 Eternacell SDX.

The battery maintains the supply voltage to the CMOS RAM to keep the 3478's calibration constants in the RAM's memory.

October 2020

I hadn't realised that this is a primary cell battery and therefore not rechargeable. If the battery goes flat all calibration data is lost. It is possible to recalibrate by connecting known good voltages, etc. HP-IB commands also exist to read/write data to the SRAM.

Probably a good idea to replace the battery after nearly 30 years?

April 2022