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Sinclair ZX Spectrum RAM Replacement

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Sinclair ZX Spectrum Issue 4B lower and upper RAM
Issue 4B board: the lower RAM, eight STC 4116 chips (IC6 to IC13), along the bottom left, and the upper RAM, eight OKI M3732H-20RS chips (IC15 to IC22), in two rows of four at the right

The Sinclair ZX Spectrum has two banks of dynamic RAM, and both fail. The lower 16 KB is eight 4116 chips, IC6 to IC13, one per data bit; they need +5 V, −5 V and +12 V. On a 48K machine the upper 32 KB is eight more chips, IC15 to IC22, with IC23 to IC26 as support logic.[1][2] The Sinclair ZX Spectrum+ uses the same boards, so the same procedure applies to it.[3]

The upper chips are TI 4532 or OKI MSM3732 parts: 64 kbit chips of which only one half passed testing, sold as 32 kbit parts with a suffix saying which half works. That is why the board has links to select the half.[4][5] Retroleum lists three causes of lower RAM failure: edge-connector mishaps, a failure of the DC-DC converter that makes the −5 V and +12 V supplies, and age.[6] Repair the converter first: see Sinclair ZX Spectrum DC-DC Converter Repair.

Symptoms

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Symptoms that point at RAM
Symptom Likely bank Source
Does not start; vertical black and white columns, or flashing attribute blocks Lower RAM [7]
Does not start, thick black and white bands, white border Lower RAM [6]
Black screen with one or more thin vertical dashes Lower RAM [4]
Blank black screen, no lines at all Upper RAM [4]
Works only as a 16K machine, or starts but crashes in games Upper RAM (IC15 to IC22) [6]
Black dashes or dots when warm, then a lock-up Lower RAM, or the ULA [4]

If the border is not white when the machine fails to start, the ROM's start-up code is probably not running at all. Retroleum lists a bad CPU, a bad ROM or a RAM chip that has failed badly enough to hold the data bus as the causes.[6]

Thin vertical lines also say which lower RAM chip to suspect. A line at the left of each 8×8 cell is bit 7 (IC13), and each pixel to the right is the next chip down, to bit 0 (IC6). Flashing 8×8 blocks point at IC13, because bit 7 of an attribute is FLASH; bright blocks point at IC12, because bit 6 is BRIGHT.[6]

For faults that are not RAM, see the Sinclair ZX Spectrum Troubleshooting Guide.

Before you start

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  • Check the supplies before changing any chip. At IC6, pin 9 should be +5 V ±0.25 V, pin 1 between −4 V and −5.5 V, and pin 8 +12 V ±1.2 V.[8] A 4116 run without −5 V while +5 V and +12 V are present can be damaged internally.[9]
  • Take normal anti-static precautions and use an earthed, low-power soldering iron.[10]
  • The board's tracks, pads and plated-through holes lift easily when chips are desoldered.[11]

Tools and parts

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RAM and support logic
Position Original parts Replacements in the sources
IC6 to IC13 (lower RAM) 4116, 150 ns; Sinclair's later lists also allow 416-3 and MN5290-2[12][13] New-old-stock 4116 chips (150 ns).[7] A 4164 cannot directly replace a 4116.[7] Retroleum sells a module that replaces all eight 4116 chips with one low-power SRAM; its round pins suit turned-pin sockets[7]
IC15 to IC22 (upper RAM, 48K) TI 4532-3 or 4532-4, or OKI MSM3732 (H or L), 200 ns or 250 ns[2][14] 4164, 150 ns or faster, with 128-cycle refresh; any number can be fitted.[6][7] Retroleum also sells an SRAM module for all eight positions, for Issues 2 to 6[7]
IC23 74LS32[2] 74HCT32[6][15]
IC24 74LS00[2] 74HCT00[6][15]
IC25, IC26 74LS157, not National Semiconductor[2] 74HCT157[6][15]
Sockets 16-pin DIL sockets for the expansion RAM, on 16K machines only[16] Dual-wipe sockets[17] or turned-pin sockets for the Retroleum modules[7]

On Issues 5 and 6A a single gate array, IC27 (ZX 84/01), replaces the six 74LS chips IC3, IC4 and IC23 to IC26.[18]

Procedure

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Open the machine

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  1. Unplug every lead, turn the computer upside down and remove the five self-tapping screws.[19]
  2. Turn it keys-up and lift the top half, minding the two keyboard ribbons.[19]
  3. Remove the single board screw next to the IC22 socket and lift the board out.[19]
  4. On a Sinclair ZX Spectrum+ the base has eight screws, seven 8 mm pan-head screws round the overhang and one 13 mm countersunk screw at the back next to the expansion slot, and the board is held by two 6.5 mm pan-head screws at its front corners.[20]

If the machine starts: Sinclair's memory check

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  1. Type PRINT PEEK 23732+PEEK 23733*256. A good 48K machine returns 65535, a 16K machine 32767.[21]
  2. If the answer is lower, call it n. The first faulty location is n+1. An answer below 32767 puts the fault in the lower 16K.[21]
  3. Type POKE n+1,85: PRINT PEEK n+1 with the real address in place of n+1. If the answer is 85, type POKE n+1,170: PRINT PEEK n+1.[21]
  4. Look the wrong answer up in Sinclair's bit-to-IC table, reproduced in the troubleshooting guide. Each wrong answer is 85 or 170 with one bit changed: bit 0 is IC6 below address 32768 and IC15 above it, up to bit 7, IC13 or IC22.[21] Copies of the table elsewhere have misprints (Worsley's web guide gives 97 for IC7, where Sinclair gives 87), so work from Sinclair's values.[21][4]
  5. Repair the first fault found before testing again; with more than one faulty chip, run the check again after each repair until the machine reports all its memory.[21][4]

If the machine does not start: resistance checks

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Worsley's and Retroleum's method uses a meter on the 20 kΩ range with the power off.[4][6]

  1. Measure from pin 2 of each lower RAM chip, IC6 to IC13, to 0 V. Worsley expects about 500 Ω, and the readings should be practically the same on every chip. Note any chip that reads exceptionally low.[4]
  2. The two banks share the data bus through series resistors, so check the upper chip on the same bit: IC13 pairs with IC22, IC12 with IC21 and so on down to IC6 with IC15. Worsley's rule is to add 9 to the lower chip's number.[6][4]
  3. If the upper chip reads lower still, it (or the CPU or ROM) is the more likely culprit; if the lower chip reads lower, suspect the lower chip.[6]
  4. To prove an upper chip, cut its pins 2 and 14 and measure the stubs on the chip: anything below 20 kΩ means the chip is faulty.[4]
  5. If any of the chips are in sockets, lift them out one at a time; a reading that rises when a chip is removed identifies that chip.[6][4]

The supply-pin checks on IC6 pins 1 and 8, which find a 4116 shorting the −5 V or +12 V rail, are in the DC-DC converter guide.[4]

Substitution

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  • Retroleum's crude test is to hold a known-good chip on top of each suspect chip in turn, power on, and see whether the fault clears or changes.[6]
  • The T.I.S. notes bypass each RAM in turn with a known-good chip of the same type, and wait about ten seconds after each switch-off before moving it.[22]
  • Sinclair's own version is a "test IC": a good 4116 or 4532 on an IC test clip, bridged across each suspect chip in turn. Sinclair says the method is not guaranteed but can save a lot of unnecessary IC changes.[23]
  • A chip that runs hot when the others are cold is shorted.[22][4]

Remove the faulty chip

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  1. Desolder the chip using proper desoldering and heatsinking technique, so that neither the chip nor the board is damaged.[10] Worsley cuts a faulty chip's legs off at the body, removes each leg from the board separately, and then clears the holes.[4]
  2. Fit a socket in its place, with the notch the same way as the other chips.[4][17]
  3. Look over the area for solder splatter. Sinclair warns that hairline splatter can measure several ohms and cause very misleading faults.[10][23]

Fit the replacement

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  1. For the lower bank, fit a 4116 (150 ns).[12] Do not fit a 4164 in a lower RAM position.[7]
  2. For the upper bank, Sinclair's rule for its own parts is that all eight chips must be the same type.[2] Worsley's rules for replacement parts: any 4164 can be mixed with 4532 chips of one suffix, but 4532-3 and 4532-4 cannot be mixed; OKI 3732 chips cannot be mixed with any other type, nor H with L.[4] Retroleum and Spectrum for Everyone use 4164 chips, which can replace any number of the originals.[6][15]
  3. Insert each chip with its notch pointing the same way as the other chips on the board, and check that no leg is bent under or outside the socket.[15][4]
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Sinclair ZX Spectrum Issue 3 upper RAM link settings (Service Manual figure 7.3.4)
Sinclair's figure of the Issue 3 link positions for OKI high-enable and low-enable chips and for TI 4532-4 and 4532-3
Sinclair ZX Spectrum Issue 3 expansion RAM locations (Service Manual figure 7.3.5)
Sinclair's Issue 3 layout, with the expansion RAM and its support logic (IC15 to IC26) shaded

The links tell the support logic which half of each 32 kbit chip works.[15][4]

  • On Issue 1 boards the upper 32K is a plug-in board on special DIL connectors, not part of the main board.[24]
  • Issue 2 boards have a single link with three pads between the ULA and IC5, marked +5 V at the top and 0 V at the bottom. Link the centre pad to 0 V for 4532-3 (or OKI L) chips and to +5 V for 4532-4 (or OKI H) chips. Never link +5 V to 0 V.[24][15][4] Sinclair's text gives the same choice as link 3 (at IC26 pin 10) for 4532-3 and link 4 for 4532-4.[2]
  • Issue 3 and later boards have two links in a small grid between the edge connector and the MIC socket, under the left end of the heatsink. One selects the maker (TI or OKI) and the other the type (3 or 4 for TI, H or L for OKI). The OKI type links sit below the TI side and the TI type links below the OKI side.[2][6] Sinclair's figure 7.3.4 shows the four combinations.[14]
  • With 4164 chips the half does not matter, but one valid combination must still be linked so that the upper RAM does not float between halves. Spectrum for Everyone fits TI and 3, and also accepts TI and 4, OKI and L, or OKI and H.[15][6]
  • Factory links are usually already soldered; clear the holes with braid or a pump before fitting new wire links, and clean the area afterwards.[15]

Close the machine

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  1. Refit the board and its screw, and check that the keyboard ribbons are not trapped when the case is closed.[19]
  2. Do not overtighten the five case screws.[19]

Testing

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  1. Power on. A 48K machine takes a little longer to show the copyright message than a 16K one.[15]
  2. PRINT PEEK 23733 should return 255 on a 48K machine.[15] PRINT PEEK 23732+PEEK 23733*256 should return 65535.[21]
  3. Run the POKE 85 and POKE 170 checks again if anything is still short.[21]
  4. Sinclair's servicing addendum says to "test at elevated temperature" to show up RAM faults.[25] After a few minutes' running, none of the RAM chips should feel warm.[4]
  5. If the RAM checks out but the machine still misbehaves, Retroleum's next suspects are the 74LS157 multiplexers IC3 and IC4 (and IC25 and IC26), TR3, the CPU, the ROM and the upper RAM.[6]

Revision notes

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  • Issue 2 boards were designed for TI 4532 chips; Retroleum describes various modifications made to let them take OKI chips, such as a lifted IC26 leg with a wire link or a small added board, in which case no 0 V or +5 V selection is made.[6]
  • Issues 5 and 6A use the ZX8401 gate array (IC27) in place of IC23 to IC26, so there is no support logic to change there.[18]
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References

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  1. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, sections 1.4 and 1.5 (pp. 9–10) and 1.11 (p. 15): memory organisation, the 4116 lower RAM and its supplies. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 ZX Spectrum Service Manual, Sinclair Research Ltd, section 7.3 "16k to 48k Upgrade", 7.3.1 to 7.3.3 (pp. 37–38): the expansion RAM and support logic types, link 3 and link 4 on Issue 2, and the Issue 3 link grid. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  3. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 2, section 10.1 (p. 57). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 Ian Worsley, "Repairing Spectrum & Spectrum Plus", from the Lil Old Sinclair Technical Information Repository, republished by World of Spectrum (retrieved 2026-10-25): screen symptoms, the pin 2 and pin 14 resistance checks, cutting out faulty chips, the RAM fault chart, mixing rules for upper RAM, and the link settings.
  5. ↑ Adam, "Sinclair ZX Spectrum 48k 'Rubber Key' Restoration", Adam's Vintage Computer Restorations, 6 July 2021: the upper RAM chips as 4164s that partly failed factory testing.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 6.18 "Repairing A ZX Spectrum", Retroleum blog (retrieved 2026-10-25): no-start symptoms, the pin 2 resistance test, vertical lines and attribute blocks, 4164 chips in the upper bank, the link positions, 74HCT logic, and the Issue 2 OKI changes.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 "Spectrum Chips", Retroleum shop (retrieved 2026-10-25): the 4116 lower RAM chip and its failure display, the 4164 upper RAM chip (150 ns or faster, 128-cycle refresh, not a 4116 replacement), and the lower and upper RAM replacement modules.
  8. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 3.1.2 "Voltage Check" (p. 17). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  9. ↑ 1024MAK, post of 15 May 2024 in "Issue 4B, 12V power missing", Spectrum Computing forum: 4116 damage without −5 V.
  10. ↑ 10.0 10.1 10.2 ZX Spectrum Service Manual, Sinclair Research Ltd, section 7.2 "Repair" (p. 37). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  11. ↑ 1024MAK, post of 9 November 2019 in "Two faulty 4A Spectrums", Spectrum Computing forum: the risk to tracks, pads and plated-through holes when desoldering the RAM.
  12. ↑ 12.0 12.1 ZX Spectrum Service Manual, Sinclair Research Ltd, section 8.10 "Integrated Circuits" (p. 44): IC6–IC13 4116 150 ns; IC15–IC22 TI 4532 or OKI. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  13. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 9.19 "Integrated Circuits" (p. 55): IC6–IC13 4116, 416-3 or MN5290-2, 150 ns. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  14. ↑ 14.0 14.1 ZX Spectrum Service Manual, Sinclair Research Ltd, sections 7.3.4 "Issue 3 Links vs IC Manufacturer and Type" (p. 39) and 7.3.5 "Expansion Memory IC Locations" (p. 40). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  15. ↑ 15.00 15.01 15.02 15.03 15.04 15.05 15.06 15.07 15.08 15.09 15.10 "Upgrading a 16K Spectrum to 48K", Spectrum for Everyone (retrieved 2026-10-25): 4164 and 74HCT parts, chip orientation, the Issue 2 link and the Issue 3 link combinations, clearing the link holes, and PEEK 23733.
  16. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 8.13 "Miscellaneous" (p. 46): ten 16-way and two 14-way DIL sockets for the 48K expansion RAM on the 16K model. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  17. ↑ 17.0 17.1 mahjongg, "Sinclair spectrum 16K RAM repair", RevSpace, last updated 25 March 2025: a 48K Spectrum whose 4116 chips were replaced in dual-wipe sockets.
  18. ↑ 18.0 18.1 ZX Spectrum Service Manual, Sinclair Research Ltd, Supplement No. 1, section 9.7 "Issue 5 Board" (p. 50) and 9.19 (p. 55): IC27 (Mullard ZX 84/01) replacing the six 74LS chips. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  19. ↑ 19.0 19.1 19.2 19.3 19.4 ZX Spectrum Service Manual, Sinclair Research Ltd, section 2 "Disassembly/Assembly" (p. 16). Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  20. ↑ Sinclair Research Ltd, ZX Spectrum + Upgrade Instructions, undated leaflet, Step 6 and final assembly: the Spectrum+ case screws and the two 6.5 mm board screws. Hosted as Sinclair ZX Spectrum+ Upgrade Instructions (PDF).
  21. ↑ 21.0 21.1 21.2 21.3 21.4 21.5 21.6 21.7 ZX Spectrum Service Manual, Sinclair Research Ltd, section 5.4 "Memory Check" and 5.4.1 "Example" (pp. 27–28), with the RAM bit-to-IC table. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  22. ↑ 22.0 22.1 ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984), T.I.S. "Repair Data" appended, symptom 3 (PDF p. 64): hot RAM chips and bypassing each RAM with a known-good one. Third-party field-service material. Hosted as ZX Spectrum Servicing Manual (Thorn EMI Datatech, 1984) (PDF).
  23. ↑ 23.0 23.1 ZX Spectrum Service Manual, Sinclair Research Ltd, section 5.1 "Fault Diagnosis Techniques" (p. 25): the test IC on a clip and solder splatter. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  24. ↑ 24.0 24.1 ZX Spectrum Service Manual, Sinclair Research Ltd, section 4.5 "32K Extension Memory" (pp. 23–24): the Issue 1 plug-in board, the RAM select level on Issue 2 (0 V for 4532-3, +5 V for 4532-4) and the Issue 3 link grid. Hosted as Sinclair ZX Spectrum Service Manual (PDF).
  25. ↑ ZX Spectrum Service Manual, Sinclair Research Ltd, section 9.10 "Fault Finding – Servicing Addenda" (p. 51). Hosted as Sinclair ZX Spectrum Service Manual (PDF).


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