HP Hewlett-Packard 400FL AC Voltmeter 300uV-300V 20Hz-4MHz 1V RMS Output 10MOhm

HP Hewlett-Packard 400FL AC Voltmeter 300uV-300V 20Hz-4MHz 1V RMS Output 10MOhm

The Hewlett-Packard 400FL is a benchtop analog AC voltmeter covering a wide range from 300µV to 300V across a frequency span of 20Hz to 4MHz. It uses an average-responding movement calibrated to read RMS volts, with a large analog meter scale displaying both RMS voltage and decibels (referenced to 1mW into 600Ω). A built-in switchable 100kHz low-pass filter is provided on the front panel to reject high-frequency noise during low-frequency measurements. The unit also supplies a 1V RMS AC output (600Ω source impedance) that can drive a recorder or downstream instrument.

Specifications & What's Included

• Manufacturer: Hewlett-Packard (HP)
• Model: 400FL AC Voltmeter
• Part Number (rear plate): 7120-0855
• Voltage Range: 300µV to 300V (with dedicated 100µV range)
• Frequency Range: 20Hz to 4MHz
• Response: Average responding, RMS calibrated
• Display: Analog panel meter — R.M.S. Volts and Decibels (1mW/600Ω ref)
• RANGE selector: dB (−20 to −80 dB) / mV (100mV to 0.1mV) / Volts (0.3V to 300V) / dB (+scales)
• Input Impedance: 10MΩ / 25pF (100µV–300mV ranges); 10MΩ / 10pF (1V–300V ranges)
• Accuracy — 300µV to 300V ranges: ±4% at 20Hz, 2MHz, 4MHz; ±2% at 40Hz and 1MHz; ±1% at 100Hz
• Accuracy — 100µV range: ±4% at 30Hz and 500kHz; ±2% at 60Hz; +1 to −8% at 100kHz
• Built-in Filter: 100kHz Low-Pass Filter (IN/OUT switch on front panel)
• AC Output: 1V RMS (open circuit), source impedance Zo = 600Ω
• Input Connectors: Banana jacks (front INPUT terminals)
• Rear terminals: Additional banana jack inputs, AC output jacks (1V RMS)
• Power: 115/230V ±10%, 48–440Hz, 6VA MAX, 150mA
• Power Inlet: IEC C14 (rear panel)
• Form Factor: Benchtop portable with top-mounted carrying handle
• Country of Manufacture: United States
• Includes: Unit only — no test leads, cable, or manual

Applications

The HP 400FL belongs to Hewlett-Packard's long-running 400-series AC voltmeter line, instruments that were workhorses of electronics labs, audio engineering shops, and RF/communications test benches from the 1960s through the 1980s. Their wide voltage range (down to the microvolt level) and broad frequency coverage made them especially well suited to audio amplifier testing, filter characterization, signal-level verification, and general AC circuit analysis. The dB display referenced to 1mW/600Ω was directly useful in telephone, broadcast, and audio applications where signal levels are expressed in dBm. The built-in 100kHz low-pass filter lets the instrument ignore high-frequency interference when measuring audio-band signals. The 1V RMS buffered output port allows the voltmeter to drive a strip-chart recorder or level monitor in parallel, making it practical for continuous monitoring or swept-frequency measurements alongside a signal generator. These instruments are still sought by vintage audio enthusiasts, electronics educators, and lab technicians who need a simple, reliable analog AC measurement reference or a low-noise microvolt-capable input stage for signal tracing.