ZX Spectrum 48K
Issue 3 — Rubber-key 48K, most common issue
Summary
The Issue 3 is the most common rubber-key 48K board. Eleven electrolytics as built, plus two more if the factory DC-DC service modification has been carried out - which on an Issue 3 it usually has.
Safety & Warnings
Confirm PSU polarity before you power on
Do not assume a modern replacement adapter matches the machine. Check the barrel polarity marked on the Spectrum's case against the adapter before plugging it in - a reversed supply will take out the regulator and more besides.
The 4116 DRAM depends on the on-board DC-DC converter
The 16 4116 DRAM chips need -5 V and +12 V, generated on the board rather than supplied by the PSU. If that converter is sick, the RAM goes with it, so verify those rails before and after the recap rather than assuming new capacitors have fixed it.
Check polarity markings against the board
Sinclair silkscreens are not always reliable on these boards - on the Issue 2 the C46 marking is documented as reversed. Confirm against the copper before soldering rather than trusting the print.
Higher voltage ratings are always safe
The capacitance is what must match. A 22 uF 25 V part is a fine replacement for a 22 uF 10 V original, but never go the other way. Watch physical size - most suitable parts are 10-12 mm long and 5-10 mm in diameter.
All capacitors below are axial types unless noted otherwise. The originals are 85 C parts; modern 105 C parts are a straight upgrade.
C78 and C79 exist only where the DC-DC service modification has been carried out, and C47 is usually removed at the same time. All three are flagged for checking because what is on your board depends on whether a Sinclair service centre ever touched it. The shopping list below assumes the modification is present.
Function and rail annotations are only shown on rows where the service documentation confirms them.
Is This Your Board?
- The issue number is printed on the board itself, usually along an edge: look for "Issue 3" or "Issue 3B" (the 3B shares this capacitor list).
- Rubber-key case, 16 DRAM chips (4116) across the back of the board, and a heatsinked 7805 regulator in the corner.
- Check for the DC-DC service modification before ordering: two added BA157 diodes, often below the right-hand keyboard molex near D15/D17, an added 22 uF at C78, and a 1 uF at C79 in place of D16. Where the mod has been done, C47 is usually removed.
- If your board has a can at C80 and no C47, you are looking at an Issue 4B or 6A rather than an Issue 3.
Verified rows are agreed by two or more independent sources. Check rows are revision-dependent or single-sourced — confirm the value against your own board before ordering.
Mainboard
| Ref | Value | Voltage | Type | Notes | Confidence |
|---|---|---|---|---|---|
C25 |
22 uF | 10 V | Axial | Verified | |
C27 |
1 uF | 50 V | Axial | Verified | |
C28 |
22 uF | 10 V | Axial | Verified | |
C34 |
22 uF | 10 V | Axial | Verified | |
C44 |
100 uF | 16 V | Axial | Verified | |
C45 |
100 uF | 16 V | Axial | Verified | |
C46 |
1 uF | 50 V | Axial | Verified | |
C47 |
22 uF | 10 V | Axial | Deleted on units where the DC-DC service modification has been done. If your board has no C47, do not fit one. | Check |
C50 |
22 uF | 10 V | Axial | Verified | |
C65 |
22 uF | 10 V | Axial | Verified | |
C74 |
4.7 uF | 5 V min | Axial | Rated 5 V minimum. A 4.7 uF 63 V part is the usual replacement. On some boards this position was fitted as a radial as part of a service modification. | Verified |
C78 |
22 uF | 10 V | Radial | Service modification only. Sits below the right-hand keyboard molex near D15/D17, negative lead to the junction of the two added BA157 diodes. | Check |
C79 |
1 uF | 50 V | Axial | Service modification only, fitted in place of D16 with the positive lead to the right. | Check |
Mainboard — shopping list
- 6 x 22 uF 10 V Axial
- 3 x 1 uF 50 V Axial
- 2 x 100 uF 16 V Axial
- 1 x 4.7 uF 5 V min Axial
- 1 x 22 uF 10 V Radial
Suggested Parts
Suggested equivalents from the cited sources, often higher-rated than the originals — confirm physical fit (can diameter, lead spacing, height) against your board before ordering.
| Spec | Qty | Part numbers | Note |
|---|---|---|---|
| 100 uF 25 V | 2 | Vishay MAL202136101E3 | |
| 22 uF 25 V | 7 | Vishay MAL203036229E3 | Six as built, seven where the C78 service modification is present. |
| 4.7 uF 63 V | 1 | Vishay MAL203038478E3 | |
| 1 uF 63 V | 3 | Vishay MAL203038108E3 | Two as built, three where the C79 service modification is present. |
Recap Tips
- Photograph the board before you start. Spectrum service modifications vary from unit to unit and the photo is the only reliable record of what yours had.
- Watch the physical size. These are axial parts in tight runs, and an oversized replacement will fight the keyboard membrane cables.
- The 7805 and its heatsink run genuinely hot on a Spectrum. Renew the thermal paste while you are in there and make sure the heatsink is properly seated.
- Do not fit lower voltage ratings to save space. The Vishay part numbers above are all higher-rated equivalents and are the usual choice for this board.
Related Symptoms
- Low RAM / 16K behaviour on a 48K machine Points at the upper 32K of 4116 DRAM or the DC-DC converter rails rather than the capacitors.
- Black screen / no display Check the 5 V rail and the regulator before assuming a capacitor fault.
Sources & References
- Spectrum for Everyone - Capacitor Lists (48K Range) Per-issue designator lists for the whole 16K/48K range, including the service modification parts.
- Spectrum for Everyone - Recapping your Spectrum Companion guide with the value and quantity summary that covers all 48K types.
- Retro Revival (Mark Morton) - ZX Spectrum Maintenance: Capacitors Independent quantity list with Vishay part numbers; its counts agree with the Spectrum for Everyone designator list, and it documents the Issue 3 DC-DC service modification in detail.
- ZX Spectrum 48K Service Manual (PDF) Primary reference for the board issues, the 4116 expansion RAM links and the official modifications.