The Chronon Blocker was a temporal device constructed by the Doctor on two known occasions. Each time he used different parts.
History
On Kaalann, the Eleventh Doctor collected a Dalekanium coil and a Kontron crystal, using them to construct a chronon blocker to stabilise Amy Pond, who she had became a temporal paradox and started to fade after the Daleks altered history and exterminated humanity before her birth. The Doctor's TARDIS had the same effect, but that could only last for so long. (GAME: City of the Daleks)
When Amy was trapped in a version of the TARDIS a thousand years in the future. The Doctor scavenged parts from the TARDIS to create a tachyon feedback loop to reunite Amy and him. He scavenged three items; his Fob watch, a Oscillator from the distress beacon he had picked up earlier and a Kontron crystal to make a Chronon Blocker; the Doctor made the feedback loop with these. (GAME: TARDIS)