ZTX651 / ZTX751 Transistors
Replacement transistors for the Sinclair ZX Spectrum TR4/TR5 power supply inverter circuit.
These transistors are a common failure point in the original Spectrum design. TR4 and TR5 can fail together due to the way the original circuit operates. If TR5 fails, it can cause TR4 to be driven hard on, resulting in excessive current, rapid overheating and failure of both transistors.
When replacing a failed transistor, it is recommended to check the other transistor, the inverter coil and surrounding components, as an underlying fault can cause the replacement to fail again.
ZTX651 – commonly used as a replacement for TR4 / original ZTX650.
ZTX751 – commonly used as a replacement for TR5 / original ZTX213.