Awesome Teenage Engineering

Beyond the usual marketplace caution (use buyer protection on Reverb/eBay, beware too-good-to-be-true deals and stolen units), test the things that are expensive to fix. Exercise every key and encoder for dead spots or crackle, check the screen for dead pixels, try the headphone and line jacks, and confirm it charges and holds a charge. Make sure it powers on, takes a firmware update, and isn't iCloud/account-locked or reported lost. New units come with a warranty, which is worth weighing against the second-hand saving.

Pick one master clock and split your gear into two sync worlds. The OP-1 field and the K.O. IIs speak MIDI clock, so run MIDI/USB from your master (the TX-6 works well as the hub) to those devices and set them to follow external clock. Pocket Operators don't do MIDI — they sync to an audio click on the left channel, so feed them a sync signal from a PO set to send sync, a dedicated sync box, or a MIDI-to-audio-sync converter, and chain the POs from there. Decide up front whether your MIDI master or your PO chain is the timing reference and derive the other from it, so everything starts from one tempo.

You can go straight in — no preamp needed. Use a TS (mono) cable from the guitar with a 1/4" (6.3mm) to 3.5mm adapter (or a dedicated guitar-to-3.5mm converter cable) into the OP-1 field's line input, then open the input page and pick the Amp input effect that arrived with the recent firmware. From there you can dial in gain and tone and record straight to tape. A passive guitar's level is a bit low, so push the input/amp gain to taste.

There are two versions: the original with 64MB of sample memory and a newer one with 128MB. They run the same firmware and behave identically — the only difference is how much sampling time you get, so the 128MB holds roughly twice as much material before you have to manage space. For learning the device the 64MB is plenty; the 128MB is worth the extra outlay if you sample a lot or want to future-proof. If you buy used, check the fader feel — some early units had a slightly finicky fader.

The EP-133 updates through the browser using WebUSB, which only works in a Chromium browser (Chrome or Edge) on a desktop — Firefox, Safari, and iOS/iPadOS don't support WebUSB, so the updater simply won't see the device there. Make sure you're on Chrome/Edge, use a USB-C cable that carries data (not a charge-only one), close any other app or browser tab that's connected to the unit, and try a different port. If it still doesn't connect, swap the cable before assuming the device is at fault — bad cables are the most common culprit.