File:Ferranti Archives ZX81-UNK-SCH-140 Hand-drawn ZX81 dynamic RAM schematic and FX3280 power converter.pdf
Summary
[edit | edit source]Two A4 sheets of hand-drawn pen schematics. Sheet 1, "SINCLAIR ZX81 DYNAMIC RAM", is a 16K memory expansion using eight 4116 dynamic RAMs: RAS taken from MREQ, a 7451 gate network combining MREQ, REFSH, A14, RD and WR to switch two 74LS157 multiplexers between the two halves of A0 to A13, CAS from a spare multiplexer section through a 220 ohm and 220 pF delay, WR to all eight RAMs, 47 ohm resistors in the data and A0 lines, and RAM SELECT tied to +5 V. Notes say the data lines and each group of address lines can be swapped around. Sheet 2, "FX3280", is a single-transistor (ZTX650) DC-DC converter that makes the +12 V and -5 V the 4116s need from +5 V, with BY206 and 1N4148 rectifiers and windings of 14, 2, 237 and 17 turns; pencil notes read "~200 mA" beside +12 and "~0.4 mA" beside -5. ZX81-UNK-SCH-141 is an annotated copy with pin numbers and design notes, and ZX81-UNK-SCH-135 is the board layout. The catalogue calls both sheets logic diagrams; sheet 2 is a power converter. Part of the Ferranti Archives 2026.
Archive record
[edit | edit source]| Catalogue ID | ZX81-???-SCH-140 |
|---|---|
| Date | Undated |
| Origin | UNKNOWN |
| Document type | Schematic/Diagrams |
| Device or subject | ZX81 Memory Expansion |
| Catalogue description | A pen sketch of "Sinclair ZX81 Dynamic RAM" logic diagrams |
| Original file name | {{{filename|}}}
|
| Found in | ZX81-FER-FOL-142 |
| Pages | 2 |
Source
[edit | edit source]Ferranti Archives 2026, original scan on Google Drive.
Author
[edit | edit source]Unknown
Licence
[edit | edit source]Ferranti Archives 2026: source and terms of use
This scan is part of the Ferranti Archives 2026, a collection of Sinclair-related documents from Ferranti's files, catalogued in 2026 and released through the Happy Little Diodes YouTube channel (video, catalogue). The archive presents the documents for preserving and archiving material of historical interest, and for non-commercial, educational and archival research. Its terms state that consent is strictly limited to non-commercial use, and that any commercial exploitation of the documents or their contents, including use in connection with the manufacture, marketing or sale of goods, is strictly prohibited by Sky UK Limited.
Copyright remains with the rights holders. The file is not freely licensed and is hosted here for preservation and repair reference only, under the archive's terms. Personal information was redacted by the archive before release; if you find any that remains, please tell [email protected].
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|---|---|---|---|---|
| current | 11:00, 18 October 2026 | 0 × 0 (918 KB) | Josh (talk | contribs) | Ferranti Archives 2026: ZX81-UNK-SCH-140 |
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