Sinclair QL Capacitor Replacement Guide

Sinclair QL PCB component layout, Issue 5 (Figure 5.1 of the Service Manual). Every capacitor designator in the tables below can be found on this drawing; the Issue 6 layout is also on the wiki.

Sinclair's Service Manual gives a component-level parts list for the Issue 5 and Issue 6 boards of the Sinclair QL, with each capacitor's designator, value and rating, and its type where the manual states one. The tables below are transcribed from Table 5.5 (Final Board Assembly) and Table 5.4 (Microdrive Board), and the circuit positions are read from the Issue 5 circuit diagram, Figure 1.4.

Safety and board handling

There is no mains voltage inside the QL. The external power pack supplies 9 V DC and 44 V peak-to-peak AC, so work on the main board is low voltage. The power pack itself contains the mains side; unplug it before opening it.

The Service Manual asks for recognised desoldering and heatsinking techniques to avoid damaging components or the board, and for an earthed, low-power soldering iron. The soldering notes in the Sinclair QL Maintenance Guide apply here too.

Which capacitors matter

The QL's electrolytics are leaded axial and radial parts; most of the other capacitors on the board are ceramic decouplers. On the Figure 1.4 circuit diagram the electrolytics sit in these positions:

• C37 and C39 (470 µF 25 V) are the reservoir capacitors after the half-wave rectifiers D25 and D26, which take the AC input to the +12 V regulator IC37 (78L12) and the −12 V regulator IC36 (79L12). C38 and C40 are on those regulators' outputs; the parts list gives them as 4.7 µF and the circuit diagram as 47 µF (see below).
• C41 (0.33 µF) is on the 9 V DC input to the +5 V regulator IC35 (7805), and C42 (47 µF 10 V) is on its output. There is no larger reservoir capacitor on the 9 V input on the board.
• C35 (100 µF) decouples the 5 V supply to the UHF modulator M1.
• C23, C24 and C25 (22 µF) couple the red, green and blue signals into IC28, the MC1377P PAL encoder.
• C21 (22 µF) sets, with R28, the run-on time that stops the Microdrive motor cutting out too quickly and damaging the tape.

The Service Manual's fault-finding table names capacitors in these remedies:

• TV picture drifting continuously: check C23, C24 and C25.
• No colour TV output: check C35 and C36 with R85 and R86, and C29 with R54 and the components around IC28.
• Real-time clock test fails: check C1 and C53 with R91 and R80.
• "Failed Microdrive test": check C16 (C15) and C18 (C17) with R35 (R34) and R37 (R36), the bracketed parts being the other drive's.

C38, C41 and C42 are also supply test points listed under post-recap checks below.

Main board capacitors

Values 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. QLs were also built on earlier ISS.2 and ISS.4 boards, a later ISS.7, and Samsung's US-series boards, which have extra capacitors fitted piggyback on some chips and lines for FCC and VDE compliance. The manual has no list for these boards, so read the fitted parts.

Electrolytic capacitors

Sinclair QL main board: electrolytic capacitors (Table 5.5)

Ref	Value	Rating	Mounting
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
C24	22 µF	16 V, −10% +50%	Axial
C25	22 µF	16 V, −10% +50%	Axial
C35	100 µF	−10% +80% (no voltage given)	Axial
C37	470 µF	25 V, −10% +50%	Axial
C38	4.7 µF	16 V	Radial
C39	470 µF	25 V	Radial
C40	4.7 µF	16 V	Radial
C41	0.33 µF	50 V	Radial
C42	47 µF	10 V	Radial
C47	22 µF	16 V	Radial

The circuit diagram prints C23 to C25 as 22 µF 10 V and C38 and C40 as 47 µF 15 V. The parts list gives 16 V for all five and 4.7 µF for C38 and C40. Read the value of the fitted parts at C38 and C40 before ordering; a rating of 16 V or more is the safe choice for all five.

Ceramic and other capacitors

Sinclair QL main board: non-electrolytic capacitors (Table 5.5)

Ref	Value	Tolerance / rating	Type as listed
C1	82 pF	5%	Ceramic
C2	22 pF	5%	Ceramic/TB
C19, C20	47 nF	50 V	Not stated
C26	0.01 µF	20%	Ceramic/TB
C27, C28	0.1 µF	50 V, −20% +80%	Not stated
C29, C30	220 pF	10%	Not stated
C31	16 pF (Issue 5), 22 pF (Issue 6)	5%	Ceramic
C32	0.01 µF	20%	Ceramic/TB
C33	0.1 µF	50 V, −20% +80%	Not stated
C34	47 pF	5%	Ceramic/TB
C36	47 nF	50 V	Not stated
C43, C44, C46	0.1 µF	50 V, −20% +80%	Not stated
C45	1 nF	2%	Ceramic
C48	0.01 µF	20%	Ceramic/TB
C53	Not fitted (Issue 5), 22 pF (Issue 6)	5%	Ceramic
DC1–DC29	0.1 µF	50 V, −20% +80%	Not stated

C31 is in the crystal circuit of the PAL encoder. Section 4 says that if chroma is present at the video output but the television shows no colour, changing the value of C31 slightly may bring the colour back.

Pre-Issue 6 boards have a trimming capacitor, TC1 (540 pF), on the real-time clock oscillator. It is factory-set to give 32.768 kHz at IC23 pins 30 and 31 and should not need adjustment. At Issue 6 it was replaced by the fixed 22 pF capacitor C53.

Microdrive board capacitors

In Table 5.4 the first column is the MDV1 designator and the second is the MDV2 equivalent, so C5 on MDV1 corresponds to C6 on MDV2. The table states that all its capacitors are axial unless noted.

Sinclair QL Microdrive circuits: capacitors (Table 5.4)

MDV1	MDV2	Value	Rating	Type as listed
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
C17	C18	220 pF	2.5%	Suflex HS30
C49	C50	47 nF	50 V	Not stated
C51	C52	47 nF	50 V	Not stated
DC28	DC29	100 µF	50 V, −10% +80%	Not stated

Table 5.5 lists "DC1–29" as 0.1 µF decouplers, but DC28 and DC29 are the MDV1 and MDV2 positions above, which Table 5.4 gives as 100 µF 50 V; on the Issue 5 component layout they sit in the Microdrive areas beside the tape-head connections. Table 5.4 gives no type for them, so check the fitted part and its polarity before removing it.

The 220 pF and 330 pF Suflex parts are 2.5% tolerance capacitors in the Microdrive read and write path; paragraph 7.3.6 of Section 1 reads the replayed signal across C15 and C17. Replace them with parts of the same value and tolerance.

Choosing replacements

• Use aluminium electrolytics of the same capacitance and the same or a higher voltage rating, rated for 105 °C.
• C4, C23, C24, C25, C35 and C37 are axial on the original board. If you fit radial parts in their place, form the leads neatly and sleeve any long lead so it cannot touch a neighbouring track.
• Several originals were specified with wide tolerances (−10% +50% or −10% +80%), so the exact capacitance of a removed part is not a reliable guide to its health.
• Match the 2.5% tolerance of the Suflex parts in the Microdrive circuits.

Replacement procedure

1. Unplug the power pack and open the case (see the Sinclair QL Maintenance Guide).
2. Remove the Microdrives and lift the main board out so it can be supported flat.
3. Record the value, voltage and polarity of each electrolytic before removing it, and check it against the component layout drawing.
4. Add fresh flux and leaded solder to both joints of the part being removed.
5. Lift one lead at a time with the joint fully molten, then clear the holes with solder wick.
6. Fit the replacement with the correct polarity, solder both leads and trim them.
7. Inspect for solder splatter and bridges before reassembly. The Service Manual warns that hairline solder splatter can measure several ohms and cause very misleading faults.

Post-recap checks

1. Reconnect the power pack and switch on. The F1...MONITOR / F2...TELEVISION screen should appear.
2. Measure the supply test points from the Service Manual's power-up table:

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

1. Check both monitor and TV output, and check the TV colour.
2. Check that both Microdrives read and write a known-good cartridge.
3. Run an RS-232 loopback test between J5 and J6. The Service Manual treats a working loopback as close to a guarantee that the whole system is working.

If a function that worked before the recap now fails, check the polarity of every replaced electrolytic first.

Related pages

• Sinclair QL
• Sinclair QL Maintenance Guide
• Sinclair QL Troubleshooting Guide
• Sinclair QL Microdrive Troubleshooting Guide
• Sinclair ZX Spectrum Capacitor Replacement Guide
• Capacitor Failure Symptoms