Heath Schlumberger SM-105A SM-104A Solid State Counter TCXO BCD 120/240V

Heath Schlumberger SM-105A SM-104A Solid State Counter TCXO BCD 120/240V

The Heath Schlumberger SM-105A and SM-104A are benchtop solid state frequency counters produced by Heath Co. of Benton Harbor, Michigan. Both models share the same measurement range and numerical readout. The SM-104A variant is distinguished by its research-grade TCXO (Temperature-Compensated Crystal Oscillator) time base, stable to 1 part in 10⁶ per year, and provides 5 digits of TTL-compatible BCD output for interfacing with external equipment or data systems.

Specifications & What's Included

• Manufacturer: Heath Co. (Heath Schlumberger), Benton Harbor, Michigan
• Models: SM-105A / SM-104A (dual-model unit)
• Type: Solid State Frequency / Event Counter
• Time Base (SM-104A): Research-grade TCXO, stable to 1 part in 10⁶ per year
• Output: 5-digit TTL-compatible BCD output
• Front panel: Numeric display, BNC input connector (80V max), Power On indicator
• Rear panel: DB25 parallel output connector, TCXO oscillator module, AC power input
• Power: 120/240 VAC, 50/60 Hz, 12W (per rear label)
• Form factor: Benchtop enclosure
• Power cord included (visible in photos)

Applications

Heath Schlumberger solid state counters from this era — the late 1960s through the 1970s — were widely used in electronics laboratories, university teaching labs, and light industrial test benches for measuring frequency, period, and event counts. The SM-104A's TCXO time base made it suitable for applications requiring tighter frequency reference accuracy than a standard crystal oscillator could provide, such as calibration work, crystal testing, and precision frequency measurement. The 5-digit BCD output allowed the counter to be integrated into semi-automated test setups, driving BCD-to-decimal decoders, printers, or early data loggers. Heath Co. instruments of this generation were popular in academic and small-lab settings both as factory-built units and as Heathkit kits assembled by the end user. By the 1980s this class of instrument was largely superseded by microprocessor-controlled counters with higher gate times and IEEE-488/GPIB interfaces, but units like the SM-104A remain of interest to vintage instrument collectors and those maintaining or restoring period test setups.