The SED9420 chip (specifically the SED9420CAC) is a CMOS VFO (Variable Frequency Oscillator) type LSI data separator designed for floppy disk interfaces. In Amstrad CPC and ZX Spectrum +3 systems, this component worked in conjunction with the uPD765 floppy disk controller (or variants such as the Z7065A) to manage data reading from the disk drive.

Below are its technical functions and its role in Amstrad equipment:

Main Functions

• Data Separation: Its primary function is to separate the data signal from the mixed clock signal coming from the floppy drive, allowing the controller (uPD765) to process the information correctly.
• Synchronization Detection: It includes integrated synchronization field (SYNC) detection and loop filtering, reducing the need for many external components.
• Density Support: It is capable of handling single density (FM) and double density (MFM).
• Integrated Timer: It features a timing circuit for functions such as head load time or motor start-up.

Technical Specifications

• Compatibility: Designed to work with popular controllers of the era such as the uPD765, uPD765A, FD1791/93, and MB8876/77.
• Pins and Voltage: It is a 24-pin chip powered by +5V.
• Output Clocks: It generates a clock signal for the floppy disk controller (FDC clock), typically 1 MHz for 5.25“ disks and 2 MHz for 8” disks.

Use in Retro Hardware

• Amstrad CPC: Found in models with integrated disk drives such as the CPC 6128 or CPC 664, as well as in the DDI-1 external interface for the CPC 464.
• Sinclair/Amstrad Spectrum: It was also used in the ZX Spectrum +3, manufactured by Amstrad after the acquisition of Sinclair.
• Rarity: Currently, it is considered a difficult component to find for repairs, sometimes requiring extraction from other vintage equipment (such as the Spectrum +3) to restore an Amstrad CPC.

Chip Details

• Appearance: It is a 24-pin integrated circuit with a DIP (Dual In-line Package) encapsulation. It usually bears the inscription SED9420 or SED9420CAC.
• Board Location: In an Amstrad CPC 6128, PCW, or a ZX Spectrum +3, you will typically find it near the uPD765A floppy disk controller. It is a fundamental part because the disk controller alone cannot directly interpret the analog signal coming from the drive; the SED9420 acts as the “translator” that separates data pulses from clock pulses.
• Availability: Since it was manufactured specifically for these systems in the 80s, it is a scarce component today. Units found are usually salvaged from old boards or New Old Stock (NOS) from specialized retro-computing stores.

If you are performing a repair, it is important to verify that the chip is exactly the CAC model, which is the most common variant used by Amstrad to ensure compatibility with the uPD765.

Documentation

At the following link, you can view the (PDF) documentation regarding the chip.

• SED9420 chip information