Editing Sinclair ZX Spectrum+ (section)

== Architecture ==

The Spectrum+ is a 48K Spectrum, so:<ref name="sm48">''ZX Spectrum Service Manual'' (48K), Sinclair Research Ltd. Source for the supply test points and tolerances, the clock frequencies, the RAM fault-location table, the keyboard matrix description, the repair data section and the capacitor parts list.</ref><ref name="wp" />

* '''CPU''': Zilog Z80A at 3.5 MHz (IC2).
* '''ULA''': the Ferranti Uncommitted Logic Array (IC1) generates video, handles the keyboard scan, tape I/O and RAM multiplexing. On Issue 4A/4B this is the 6C001E-7.
* '''ROM''': 16 KB containing Sinclair BASIC (IC5).
* '''Lower RAM''': 16 KB of 4116 DRAM (IC6–IC13). These need '''three rails''' — +5 V, −5 V and +12 V — which is why the on-board DC-DC converter exists and why it is such a common source of faults.
* '''Upper RAM''': 32 KB (IC15–IC22), using 4164, TI 4532 or OKI 3732 devices selected by links on the board.
* '''Colour encoder''': LM1889 (IC14).
* '''Clock''': 14 MHz crystal X1, divided in the ULA; the Z80 receives 3.5 MHz.

The Spectrum's split-bus arrangement means external peripherals can drive the keyboard lines, a behaviour later lost on the +2A/+3 machines.<ref name="sfe" />

=== The DC-DC converter ===

The 4116 lower RAM requires −5 V and +12 V, which the Spectrum generates on-board from the single 9 V input using an inverter built around '''TR4 (ZTX650)''', '''TR5 (ZTX213)''' and a small coil.<ref name="ian">Worsley, Ian. ''Repairing Spectrum &amp; Spectrum Plus'', from the Lil Old Sinclair Technical Information Repository. A field-service repair guide; source for the multimeter-only diagnostic sequence, the DC-DC converter fault procedure, the faulty-screen-design catalogue, keyboard and Spectrum+-specific membrane fixes, loading and sound faults, PSU repair and the RAM location chart.</ref>

This circuit is the single most failure-prone part of the machine, and it is the reason so many Spectrums present with a white or patterned screen. It was progressively improved across board issues — Issue 3B was a minor revision specifically to improve DC-DC converter reliability, and Issue 4A incorporated the earlier field modifications directly onto the PCB.<ref name="sfe" />