PACO's Z-80 is a classic audio + RF signal tracer for tube-era radio and TV servicing. Two probes — an RF/IF demodulator probe and an audio probe — feed an internal high-gain amplifier driving both a speaker and a panel meter. Workflow: inject a known signal at the front end of a chassis, then probe stage by stage with the appropriate probe. Where the trace dies is the broken stage. Also includes a built-in output signal generator function on some variants.
Signal tracing is the dual workflow to signal injection (where you inject at successive stages and listen at the speaker output). For tube-era receiver service, the two techniques together localize almost any signal-path fault in minutes — far faster than trying to brute-force-trace voltages against the schematic. PACO and Eico were the two big names in budget signal-tracer kits of the era; the Z-80 is the higher-end PACO unit. Common service issues: the electrolytics in the audio amp, the speaker cone, and the probe cables (rubber sheaths get brittle and short).
A multi-volume deep dive on this instrument is in planning. When the first volume lands in the source project's 02-inputs/volume_sources/, this page upgrades automatically — no website code change required.