Sinclair QL Capacitor Replacement Guide
This guide covers the capacitors on the Sinclair QL. Sinclair published a full component-level parts list for the QL, so unlike most machines of the era every capacitor's designator, value, voltage and type is documented — the tables below are transcribed from Table 5.5 (Final Board Assembly) and Table 5.4 (Microdrive Board) of the Service Manual.[1]
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⚠️ Safety and board handling
editThere are no mains voltages inside the QL — the highest voltage on the board is the 44 V peak-to-peak AC feed from the external power pack — so board-level capacitor work is low voltage.[1]
The real hazard here is to the board, not to you. The QL's PCB is fragile and its pads lift easily. The Service Manual requires recognised desoldering and heatsinking technique and an earthed, low-power iron.[1] Read the soldering rules in the Sinclair QL Maintenance Guide before starting; in summary: temperature-controlled iron, fresh leaded solder and flux added to every old joint first, short dwell times, no flexing of the board, and a careful inspection for solder splatter afterwards.
The external power pack is a sealed mains unit. It can be opened and recapped by someone competent with mains equipment — unplug it, discharge the reservoir capacitor and confirm 0 V before touching the primary side — or simply replaced.[1]
Is the QL a recap candidate?
editThe QL is not a machine with a systematic capacitor-failure problem in the way the later surface-mount Acorn machines are. Its electrolytics are conventional leaded axial and radial parts, and the great majority of the board's capacitors are ceramic decouplers that do not fail.
A recap is nevertheless reasonable on a forty-year-old machine, and there are specific cases where capacitors are directly implicated:
- Supply smoothing — C37, C39 (both 470 µF) and C35 (100 µF) carry the rectified feeds; ageing here shows up as ripple, instability and random crashes.
- Regulator input/output — C41 (0.33 µF) and C42 (47 µF) around the 7805; C38 (4.7 µF) on the +12 V regulator. These are the Service Manual's supply test points, so out-of-spec readings there point straight at them.[1]
- TV picture drifting continuously — the Service Manual says to check C23, C24, C25 (all 22 µF).[1]
- No colour TV output — check C35, C36 along with R85/R86, and C29 with R54 and the components around IC28.[1]
- Real-time clock failure — check C1 (82 pF) and C53 (22 pF, Issue 6 only) along with R91 and R80.[1]
- Microdrive read/write problems — check C15/C16 and C17/C18 on the Microdrive boards along with R35/R34 and R37/R36.[1]
Priority order on an unrestored machine: the supply electrolytics first, then anything visibly aged, then the specific parts named above if the matching symptom is present.
Main board capacitors
editValues are from Table 5.5 of the Service Manual. Where the Issue 6 board differs from Issue 5 it is noted; all other values are common to both.[1]
Electrolytics — the recap targets
edit| Ref | Value | Rating | Mounting | Notes |
|---|---|---|---|---|
| C4 | 4.7 µF | 25 V, −10% +50% | Axial | |
| C21 | 22 µF | 16 V | Radial | |
| C22 | 22 µF | 16 V | Radial | |
| C23 | 22 µF | 16 V, −10% +50% | Axial | Check if the TV picture drifts continuously |
| C24 | 22 µF | 16 V, −10% +50% | Axial | As C23 |
| C25 | 22 µF | 16 V, −10% +50% | Axial | As C23 |
| C35 | 100 µF | −10% +80% | Axial | Check if there is no colour TV output |
| C37 | 470 µF | 25 V, −10% +50% | Axial | Supply smoothing |
| C38 | 4.7 µF | 16 V | Radial | +12 V regulator output — a Service Manual test point |
| C39 | 470 µF | 25 V | Radial | Supply smoothing |
| C40 | 4.7 µF | 16 V | Radial | |
| C41 | 0.33 µF | 50 V | Radial | +5 V regulator input — a Service Manual test point |
| C42 | 47 µF | 10 V | Radial | +5 V regulator output — a Service Manual test point |
| C47 | 22 µF | 16 V | Radial |
Ceramic and film capacitors — normally left alone
edit| Ref | Value | Tolerance / rating | Type |
|---|---|---|---|
| C1 | 82 pF | 5% | Ceramic — real-time clock network |
| C2 | 22 pF | 5% | Ceramic/TB |
| C19, C20 | 47 nF | 50 V | — |
| C26 | 0.01 µF | 20% | Ceramic/TB |
| C27, C28 | 0.1 µF | 50 V, −20% +80% | — |
| C29, C30 | 220 pF | 10% | — (C29: check if there is no colour TV output) |
| C31 | 16 pF (Issue 5) / 22 pF (Issue 6) | 5% | Ceramic — adjusting this slightly can restore TV colour |
| C32 | 0.01 µF | 20% | Ceramic/TB |
| C33 | 0.1 µF | 50 V, −20% +80% | — |
| C34 | 47 pF | 5% | Ceramic/TB |
| C36 | 47 nF | 50 V | — (check if there is no colour TV output) |
| C43, C44, C46 | 0.1 µF | 50 V, −20% +80% | — |
| C45 | 1 nF | 2% | Ceramic |
| C48 | 0.01 µF | 20% | Ceramic/TB |
| C53 | Issue 6 only — 22 pF | 5% | Ceramic — replaced trimmer TC1; real-time clock |
| DC1–DC29 | 0.1 µF | 50 V, −20% +80% | Supply decoupling across the board |
Note on C53 and TC1. Pre-Issue 6 boards have a trimming capacitor TC1 which allows the real-time clock oscillator frequency to be adjusted. At Issue 6 this was replaced by the fixed 22 pF capacitor C53.[1]
Microdrive board capacitors
editEach Microdrive has its own small board. In Table 5.4 the first column is the MDV1 designator and the second is the MDV2 equivalent — so, for example, C5 on MDV1 corresponds to C6 on MDV2.[1]
| MDV1 | MDV2 | Value | Rating | Type / notes |
|---|---|---|---|---|
| C5 | C6 | 330 pF | 2.5% | Suflex HS30 |
| C9 | C10 | 0.47 µF | 35 V | ITT |
| C11 | C12 | 0.22 µF | 35 V | ITT |
| C13 | C14 | 330 pF | 2.5% | Suflex HS30 |
| C15 | C16 | 220 pF | 2.5% | Suflex HS30 — check on "Failed microdrive test" |
| C17 | C18 | 220 pF | 2.5% | Suflex HS30 — check on "Failed microdrive test" |
| C49 | C50 | 47 nF | 50 V | — |
| C51 | C52 | 47 nF | 50 V | — |
| DC28 | DC29 | 100 µF | Decoupling |
The 220 pF and 330 pF Suflex parts are stable polystyrene-type capacitors in the read/write signal path and rarely fail; they are named in the Service Manual's "Failed microdrive test" remedy list mainly so that their values can be verified.[1]
Choosing replacements
edit- Use 105 °C low-ESR aluminium electrolytics of equal capacitance and equal-or-higher voltage.
- Several originals are axial (C4, C23, C24, C25, C35, C37). Axial parts are still available; if substituting radial parts, form the leads neatly and insulate any lead extension with heat-shrink so nothing shorts against neighbouring tracks.
- Note the wide original tolerances (several parts are specified −10% +50% or −10% +80%). Modern ±20% parts of the nominal value are comfortably within the original specification.
- Do not substitute ceramics for the Suflex parts in the Microdrive read path — match the type as well as the value.
Replacement procedure
edit- Unplug the power pack and open the case (see the Sinclair QL Maintenance Guide).
- Remove the Microdrives and lift the main board out, so it can be supported flat.
- Record the value, voltage and polarity of each electrolytic before removing it. Cross-check against the PCB component layout drawing on this page.
- Add fresh flux and leaded solder to both joints of the part being removed, and let it mix with the old solder.
- Lift one lead at a time with the joint fully molten, then clear the holes with solder wick.
- Fit the replacement with the correct polarity, solder both leads and trim flush.
- Inspect for solder splatter and bridges before reassembly — the Service Manual specifically warns that hairline bridges can read as several ohms and cause very misleading faults.[1]
Post-recap verification
edit- Reconnect the power pack and switch on. Confirm the F1...MONITOR / F2...TELEVISION screen appears.
- Measure the supply test points:
| Function | Test point | Expected |
|---|---|---|
| ±12 V regulator input | + side of D26 | 15.6 V AC ± 2.0 V |
| +5 V regulator input | + side of C41 | +9.0 V ± 2.0 V |
| +12 V regulator output | + side of C38 | +12.0 V DC ± 0.25 V, no discernible ripple |
| +5 V regulator output | + side of C42 | +5.0 V ± 0.15 V, no discernible ripple |
- Check both monitor and TV output, and verify TV colour.
- Verify both Microdrives read and write a known-good cartridge.
- Run an RS-232 loopback test between J5 and J6 as an overall system check.[1]
If a previously-working function fails after the recap, re-check the polarity of every replaced electrolytic first.