# PedalScope Breakout Box — build plan

A small passive box that sits in the measurement loop between an audio
interface's output and the pedal under test. It holds three TS jacks,
three 4 mm posts and one 10 kΩ resistor; everything else is wire. It is
the fixture the [PedalScope](https://pedalscope.com) user guide describes
on its *Bench fixtures* page, and the Reference Box's null runs and
level readings were taken through it.

## What it is for

One box does three jobs. The guide lists them in this order:

1. **A probe point on the loop.** Clip a meter or oscilloscope to **TIP**
   and **SLEEVE** with both plugs seated, and read the signal while a tone
   plays. The loop stays intact, and nothing has to be undone afterwards.
   Black goes on SLEEVE every time, so a reading taken today compares with
   one taken next month.
2. **The 10 kΩ series source, on demand.** Plug the pedal into **OUT 10k**
   instead of **OUT**, and the pedal is driven through the 10 kΩ resistor.
   **TIP +10k** is the probe point after the resistor. Keep the cable from
   OUT 10k to the pedal short: everything after the resistor is
   capacitance a 10 kΩ source has to drive. Moving the pedal from OUT to
   OUT 10k changes what the loop measures. In PedalScope that is a
   recalibration, not a re-patch; the guide gives the steps.
3. **A straight pass-through for null runs.** With no pedal in the chain,
   plug the return cable into **OUT**. IN → OUT is then the same loop the
   pedal will see, cable for cable, joined through the box instead of
   through the device, with a probe point on the join.

**Line-level signal only.** The posts carry the signal between an
interface and a pedal: a couple of volts at most. Never connect this box
to an amplifier's speaker output.

## The parts

`bom.md` lists them: a Hammond 1590B enclosure, three ¼″ non-isolated
TS jacks, three insulated binding posts that take 4 mm banana plugs (two
red, one black), one 10 k 1 % ¼ W resistor, 22 AWG stranded wire, a
ground lug with its screw, nut and star washer, and four rubber feet.

**The jacks must be non-isolated.** A non-isolated jack's sleeve is its
threaded bushing, so bolting the jack through the enclosure grounds that
sleeve to the box. That is how every sleeve reaches the enclosure here.
Isolated jacks would break the enclosure's bond to the circuit's ground.

## Layout, as built

The photos in `photos/` show the box that exists:

- the three posts on the enclosure's top face: **TIP** (red) and
  **SLEEVE** (black) side by side at one end, and **TIP +10k** (red)
  further along;
- **IN** on one long side;
- **OUT** and **OUT 10k** on the opposite long side, with OUT at the TIP /
  SLEEVE end.

The lid is the removable plate opposite the posts; the rubber feet go on
it.

**No drilling template is given.** No hole position on this box has been
measured, and a template drawn from assumed dimensions would state a
hardware specification as fact. Place the holes to suit your parts. Leave
room inside for the jacks' frames and the posts' studs; the inside photo
shows how tight it is.

## Wiring

`schematic.svg` (generated by `schematic.gen.py`) is the circuit. In
words:

- **IN's tip** goes straight to the **TIP** post and to **OUT's tip**. This
  is the direct pass-through.
- **IN's tip** also goes through the **10 kΩ** resistor to the
  **TIP +10k** post and to **OUT 10k's tip**.
- **Every sleeve is common**: IN, OUT and OUT 10k sleeves, and the
  **SLEEVE** post. It all reaches the enclosure through the non-isolated
  jacks' bushings.
- **A lead bonds the lid to that ground.** It runs from the common sleeve
  to a solder lug bolted to the inside of the lid, with the star washer
  under the lug. The closed box is then a shield rather than a lid resting
  on one. Leave the lead long enough to lay the lid flat beside the box.

**The one electrical fact to keep:** every connector is TS, tip and sleeve
and nothing else. The loop is therefore single-ended wherever it meets the
box. PedalScope's rule is that calibration and measurement traverse the
same electrical path, single-ended where it meets the pedal. If the box
is in the loop for a measurement, it belongs in the loop for the
calibration too.

## Meter check, before first use

With the box out of any loop (no interface, no pedal), and a TS plug in
each jack so its tip and sleeve can be reached: a short cable's free
end, or a bare plug.

1. **Continuity, IN tip → TIP post → OUT tip.** Each pair reads a short.
2. **IN tip → TIP +10k post.** It reads the resistor's value, 10 kΩ
   within its tolerance. Record what it reads. **TIP +10k → OUT 10k tip**
   reads a short, and **IN tip → OUT 10k tip** reads the resistor again.
3. **Every sleeve to the SLEEVE post, the enclosure and the lid.** Each
   reads a short: IN, OUT and OUT 10k sleeves to the SLEEVE post; the
   SLEEVE post to bare metal on the enclosure; the SLEEVE post to the lid
   with the lid off and laid flat, through the bond lead.
4. **No tip-to-sleeve continuity anywhere.** IN tip, OUT tip, OUT 10k tip,
   TIP and TIP +10k to SLEEVE each read open.

A short in step 4 is a wiring fault. Find it before the box goes into a
loop.

## Files

- `schematic.gen.py` → `schematic.svg`: the circuit, generated and never
  hand-edited (`python3 schematic.gen.py`).
- `bom.md`: the parts, with what is not recorded stated as such.
- `photos/`: the box from above and opened.
- `LICENSE`, `NOTICE`: CERN-OHL-P-2.0, which covers the photos too.