Three Instruments, Three Questions

The bench probing plan lays out four measurement sessions across three instruments. On paper it reads like a checklist, but there is a deeper reason the plan requires three different tools instead of just a good oscilloscope. Each instrument answers a fundamentally different question about the circuit under test, and no single instrument can answer all three.

What does the signal look like over time?

The oscilloscope is the most intuitive of the three. It draws voltage on the Y axis against time on the X axis, and what you see is more or less what the circuit sees. For the 555 this means watching the 0–12V square waves on the key contact bus bars, measuring the TOS master clock amplitude at the crystal, and capturing the analog voicing filter outputs — flute, string, reed — as they transform those square waves into the organ’s characteristic timbres.

The oscilloscope is the safety gate for everything that follows. The bench probing plan requires Session 1 (oscilloscope only) to complete before the logic analyzer touches anything, because the logic analyzer’s absolute max input is 5.25V and most organ signals swing to 12V or higher. The oscilloscope measures first, classifies every signal as safe or unsafe, and the logic analyzer only touches what has been cleared.

When do digital signals switch?

The logic analyzer answers the timing question. It does not care about voltage levels beyond “high” and “low” — it cares about when transitions happen and how often. For the 555 this means confirming the TOS output frequencies across all 13 chromatic notes, verifying the divide-by-two relationships down the octave chain, and — most critically — characterizing the key contact debounce timing that will directly determine the firmware delay constant in the ESP32 MIDI scanner.

Eight channels captured simultaneously at 24 MHz gives microsecond-scale resolution on every transition. That is the kind of measurement the oscilloscope could technically make on one channel at a time, but the logic analyzer captures the full picture — multiple keys, multiple octaves, simultaneously, over long time windows. The debounce measurement alone requires 20+ keystrokes with statistical analysis. That is a logic analyzer job, not an oscilloscope job.

What happens when a signal tries to travel through something?

This is the question the NanoVNA answers, and it is the question neither of the other two instruments can address at all.

The oscilloscope measures what is already there — voltage at a point, right now. The logic analyzer measures when things happen. But the NanoVNA measures the relationship between what goes in and what comes out of a passive network. It sends a known stimulus across a range of frequencies and measures what the network does to it: how much it reflects, how much it passes, how the impedance changes with frequency. This is S-parameter measurement — scattering parameters — and it reveals the frequency-dependent behavior of passive networks in a way that no time-domain instrument can.

For the 555, three measurements depend on this capability:

Bus bar impedance characterization. The key contact bus bars are transmission lines of a sort — long conductors with distributed capacitance from every key contact hanging off them, whether open or closed. The impedance looking into a bus bar determines how much signal a capacitively-coupled sensor can extract without loading the bus enough to affect the organ’s voicing. The oscilloscope can show the voltage on the bus, but it cannot tell you the impedance the bus presents at a given frequency. The NanoVNA can.

Voicing filter frequency response. The tab stop filter networks — the circuits that turn a square wave into a flute voice or a string voice — are passive RC/LC networks whose transfer function defines the organ’s entire tonal character. An S₂₁ (transmission) sweep across the audio band shows the filter’s frequency response directly: which harmonics each voice passes, which it attenuates, where the rolloff frequencies sit. This is the kind of measurement that used to require a sweep generator and a calibrated detector. The NanoVNA does both in one instrument.

Expression pedal potentiometer profiling. The expression pedal’s volume control pot might be linear or logarithmic — the service manual does not specify for this model year. An impedance sweep across the pedal’s mechanical travel reveals the taper law directly, which matters for the MIDI conversion because a logarithmic pot needs different scaling in firmware than a linear one to produce a musically useful expression curve.

One bench, three perspectives

The probing plan organizes these measurements into four sessions — oscilloscope safety pass, logic analyzer digital characterization, oscilloscope analog signals, NanoVNA impedance characterization — because that is the order that makes physical sense. You confirm voltage levels before connecting sensitive instruments. You measure active signals before passive networks. And the NanoVNA session runs with the organ powered off, since it supplies its own stimulus.

All three instruments are USB-controlled. The logic analyzer runs through sigrok and PulseView. The NanoVNA runs through mcnanovna, an MCP server that drives sweeps, calibration, and Touchstone export directly from the workbench. The confirmed ICs on PCB 20918 — the AMI-sourced divider-keyers (658581) and the AY-3-0214 family TOS (142164X) — are the specific circuits these instruments will characterize.

The data from all three feeds into the same design decisions: coupling capacitor values for the CapSense pickup, debounce timing for the shift register firmware, expression pedal scaling curves for the MIDI controller. Three instruments, three questions, one set of answers that the 555’s MIDI conversion needs before a single line of firmware gets written.