TWIN printer

Restoring the TWIN printers. These are Centronics 306 printers, rebranded in iconic Signetics TWIN orange.

Photo of the Signetics TWIN printer.
  • weight: over 53 kg.
  • dimensions: width 59 cm, height 33 cm, depth 48 cm.
  • 7-pin printhead.
  • character set: standard ASCII, upper-case only.
  • fonts: condensed, normal, or wide characters.
  • tractor feed paper only, no single sheets.

The Centronics 306 is one of the first dot-matrix printers. It is narrower than Centronics’ first printer — the 101 — but mechanically and electronically very similar. This is the printer that gave the name to the “Centronics connector” that was the standard interface for printers until it was superseded by USB.

Both the ECA system and the HM system have their original printers, and both were in poor condition. The coating on the ribbon holder had disintegrated and melted into the ribbon. Sections of the ribbon had to be cut out and discarded. I re-lined the ribbon holders with shrink-wrap tubing.

The drive belts were still in good condition, but the O-rings that drive the paper feed mechanism had just… moulded. I replaced them using a piece of silicone-based drive belt of the correct length. One heats both ends of the silicone using a soldering iron, then press them together to create a new O-ring.

The photo above also shows that the larger white nylon gear has cracked. Both printers showed this defect. This was due to internal stress in the nylon. When I removed the metal hub (by heating it with a soldering iron) the cracked disappeared. It took a lot of force to open it again. I repaired the crack using a staple and reinserted the hub after heating it.

The ribbons appear to be typewriter-style ribbons. I re-inked them using ink for stamp pads, which seemed to do the trick. Here is a short demonstration of the printer in action, with the cover removed.

Information on the Centronics 306 printer

Technical manual at bitsavers.org: full description including electronic and mechanical diagrams (219 pages).

Centronics company history on Wikipedia.

Pinout of the Centronics interface.