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Rewrite from Sinclair ZX80 circuit diagram: every value in the old table was wrong (C1/C2/C4/C5/C6 are 47nF ceramics, not electrolytics; the 22uF reservoir is C8; reset cap is C10 1uF; no C3/C7 exist). Add refs, clear Unverified data
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[[File:Sinclair ZX80 circuit diagram.png|thumb|420px|Sinclair's ZX80 circuit diagram. Every designator and value on this page is read from it; the power supply section is at the top left, above the edge-connector tables.]]


'''Replacing the electrolytic capacitors (“recapping”) in your Sinclair ZX80 is a crucial preventative maintenance step.'''
The '''[[Sinclair ZX80]]''' has very few capacitors, and only '''two''' of them are electrolytic. It is therefore not a "recap on sight" machine in the way that a machine full of low-voltage electrolytics is. The two electrolytics are worth attention on a forty-five-year-old board, and the rest of the board is ceramic and does not normally need to be touched.
Original capacitors from 1980 are now far beyond their expected lifespan, and failure can cause instability, video issues, or even prevent startup. Fresh, quality capacitors restore reliable operation and protect logic chips.


== Visual Inspection & Failure Signs ==
== Visual inspection and failure signs ==
* '''Leaking or crusty capacitors''' – Look for brown, green, or white residue at the base of any can.
* '''Bulging or tilted cans''' – Even small axial capacitors may bulge or lean if failing.
* '''Unstable video output''' – Flickering, rolling, or missing sync often traces to dried-out C5 or C6.
* '''Boot failures or random resets''' – Power supply filter caps (C1, C2) with high ESR can cause unreliable startup.


If '''any''' capacitor shows trouble, replace '''all''' electrolytics on the board.
* '''Leaking or crusty electrolytics''' — brown, green or white residue at the base of C8 or C10.
* '''Bulging or split cans''' on either of those two parts.
* '''Machine dead with a cold regulator.''' If the 7805 (IC22) never warms up, check the supply and C8 before anything else: C8 is the only reservoir between the unregulated input and the regulator.<ref name="zx80cd" />
* '''No cursor at switch-on, but the screen is otherwise live.''' The Z80's reset is held by R21 (220 k) charging C10 (1 µF). A failed C10 attacks this directly.<ref name="zx80cd" />
* '''Horizontal black and white stripes drifting through the picture.''' Sinclair attributed this to the power supply — either insufficient smoothing or too low an output on load.<ref name="zx81man" />


== Sinclair ZX80 Capacitor List ==
== Capacitors on the ZX80 board ==


The ZX80 uses a small number of electrolytic capacitors, all easily accessible on the main PCB.
Sinclair's circuit diagram is the source for the list below.<ref name="zx80cd">Sinclair Research Ltd, ''ZX80 Circuit Diagram''. Hosted on this wiki as [[:File:Sinclair ZX80 circuit diagram.png]]. Source for every ZX80 capacitor designator, value, IC type and pin number on this page.</ref> Grant Searle's 2009 vector redraw of the same diagram carries identical designators and values, which is a useful independent check.<ref name="searle">Grant Searle, ''ZX80 hardware page'', http://searle.x10host.com/zx80/zx80.html — accurately redrawn ZX80 schematic, January 2009. Used here only as independent confirmation of the designators and values on Sinclair's own diagram.</ref>
Always confirm values against your board’s silkscreen and schematic, as some early kits may differ.


{| class="wikitable styled-table" style="width:100%; text-align:center;"
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Sinclair ZX80 Electrolytic Capacitors'''
|+'''Sinclair ZX80 capacitors, from Sinclair's circuit diagram'''
! Ref !! Capacitance !! Voltage !! Function / Notes
! Ref !! Value !! Type !! Where it sits on the diagram
|-
|-
| C1 || 22 µF || 16 V || +5V input smoothing (main power filter)
| C1, C2, C4, C5, C6 || 47 nF || Ceramic || Five decouplers across the regulated +5 V rail at the output of IC22
|-
|-
| C2 || 1 µF || 16 V || +5V decoupling (logic rail)
| C8 || 22 µF, 16 V || '''Electrolytic''' || Across the unregulated DC input, ahead of the IC22 regulator. The board's only reservoir capacitor
|-
|-
| C3 || 1 µF || 16 V || Video modulator coupling
| C9 || 47 pF || Ceramic || With R2 (470 R) in the ∅2 clock path feeding IC16
|-
|-
| C4 || 1 µF || 16 V || Video sync coupling
| C10 || 1 µF || '''Electrolytic''' || With R21 (220 k) to +5 V on IC1 (Z80) pin 26, /RESET — the power-on reset network
|-
|-
| C5 || 10 µF || 16 V || Video composite output smoothing
| C11 || 47 pF || Ceramic || With R25 (470 R) in the /MREQ timing network feeding IC11
|-
|-
| C6 || 22 µF || 16 V || Reset circuit (power-on reset timing)
| C12 || 47 nF || Ceramic || In series with R29 (180 R) at the EAR socket — the tape input path
|-
| C13 || 47 nF || Ceramic || In the MIC socket network with R34 (1 k), fed from SYNC
|-
| C14 || 47 pF || Ceramic || In the same MIC socket network, with R35 (1 M)
|-
| C15 || 0–330 pF trimmer || Trimmer || In the 6.5 MHz master oscillator round X1, with R27 (1 k)
|}
|}


''All original capacitors are axial-lead types. Modern radial types may be used if carefully fitted and insulated.''
'''There is no C3 and no C7 on the ZX80 circuit diagram.''' Designators were not reused from a ZX81 or a Spectrum; if your board is silkscreened with a designator not in the list above, work from your own board and post the finding rather than assuming this table is incomplete.


== ZX80 Recapping Procedure ==
Only '''C8 and C10''' are electrolytic. If you are ordering parts, those are the two to order.
# '''Unplug and disassemble''' – Remove the case screws and carefully lift the top shell.
# '''Remove the PCB''' – Disconnect the keyboard membrane and gently free the main board.
# '''Desolder old capacitors''' – Use a temperature-controlled iron (300–350 °C) and desoldering braid or pump. 
# '''Clean pads''' – Remove old solder and residue; inspect for lifted traces.
# '''Fit new capacitors''' – Match polarity (long lead = +). Axial types fit the original holes; radial types can be bent to fit, but insulate leads with heatshrink or sleeving.
# '''Solder and trim leads''' – Ensure solid joints and no solder bridges.
# '''Clean flux residue''' – Use IPA and a soft brush.
# '''Reassemble and test''' – Power up and confirm stable video and reliable boot.


== Post-Recap Voltage Checks ==
=== Choosing replacements ===
After recapping, measure the +5V rail at the 7805 regulator output and at the RAM/CPU pins:


{| class="wikitable styled-table" style="width:60%; text-align:center;"
* '''C8''' — 22 µF at 16 V or higher. The rail it sits on is the raw supply, nominally 9–11 V DC and rising to about 14 V open-circuit, so do not fit anything rated below 16 V.<ref name="zx80cd" /><ref name="kio80" />
|+'''ZX80 Power Rail Checkpoints'''
* '''C10''' — 1 µF. It sits on the +5 V rail; 16 V or higher is ample.
! Test Point !! Expected Voltage !! Max Ripple (p-p)
* Use 105 °C aluminium electrolytics from a recognised manufacturer. Equal capacitance, equal or higher voltage.
* The ceramics are stable parts and are not a normal service item. Replace one only if it has been physically damaged.
 
== Recapping procedure ==
 
# '''Unplug the supply''' and remove the case screws.
# '''Free the board.''' Disconnect the keyboard tails from their connectors. The tails are the most fragile thing in the machine — pull them straight out, never at an angle, and never fold them.<ref name="tv2" />
# '''Note the polarity''' of C8 and C10 before removing anything. The circuit diagram marks the positive end of both.<ref name="zx80cd" />
# '''Add fresh flux and solder''' to the old joints and let it mix before applying wick or a pump. The ZX80 board is single-sided and the pads lift if you work them hot and dry.
# '''Fit the replacements''' the right way round. The originals are axial; radial parts are acceptable if you form and sleeve the leads so nothing can short to an adjacent track.
# '''Clean the flux residue''' with isopropyl alcohol and a soft brush, then inspect under magnification for splashes and bridges — a hairline solder splash reads as a few ohms and produces very misleading symptoms.<ref name="tv2">"Servicing Sinclair Computers, Part 2", ''Television'' magazine, 1986. Trade servicing article covering the ZX80/ZX81 family; mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source, used here for handling practice, keyboard tails and solder-splash faults.</ref>
# '''Reassemble and test.'''
 
== Checks after the work ==
 
The ZX80 has a single internal rail. There is no −5 V and no +12 V on this machine — those belong to the [[Sinclair ZX Spectrum Capacitor Replacement Guide|ZX Spectrum]], whose 4116 RAM needs them.
 
{| class="wikitable styled-table" style="width:90%; text-align:left;"
|+'''ZX80 supply checkpoints'''
! Point !! Expect !! Source
|-
| Power jack, 3.5 mm, '''centre pin positive''' || Nominal 9 V DC unregulated, about 14 V open-circuit; the circuit diagram marks the input "9 V – 11 V" || <ref name="zx80cd" /><ref name="kio80">''Sinclair ZX80 Connectors'', The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity and open-circuit voltage.</ref>
|-
| IC22 (7805) output || +5 V || <ref name="zx80cd" />
|-
|-
| 7805 output tab || 4.95 – 5.10 V || < 50 mV
| IC1 (Z80) pin 11 Vcc, pin 29 ground || +5 V || <ref name="zx80cd" />
|-
|-
| RAM/CPU Vcc pin || 4.90 – 5.05 V || < 50 mV
| IC2 (8332 ROM) pin 24 Vcc, pin 12 ground || +5 V || <ref name="zx80cd" />
|-
| IC3, IC4 (2114 RAM) pin 18 Vcc, pin 9 ground || +5 V || <ref name="zx80cd" />
|-
| IC1 (Z80) pin 6 || 3.25 MHz CPU clock, derived from the 6.5 MHz oscillator X1 || <ref name="zx80cd" />
|}
|}


''Excessive ripple or voltage drop may indicate a faulty regulator or incorrect capacitor installation.''
Sinclair did not publish a ripple figure for the ZX80, and none is given here. What the manual for the closely related [[Sinclair ZX81]] does say is that the regulator will not work correctly below about 7 V on load, and that drifting horizontal stripes point at the supply rather than the board.<ref name="zx81man">Sinclair Research Ltd, ''ZX81 Service and Assembly Manual'', section 6 (fault finding) and section 9 (power supply). Hosted on this wiki as [[:File:Sinclair ZX81 Service and Assembly Manual.pdf]]. Cited here because Sinclair published no equivalent document for the ZX80 and the two machines share the same external supply arrangement.</ref>


== Recommended Tools & Parts ==
== Tools and materials ==
* Temperature-controlled soldering iron (fine tip, 1–2 mm)
* Desoldering braid and/or pump
* Leaded 63/37 or quality lead-free solder
* 105 °C, low-ESR capacitors (Nichicon, Panasonic, Rubycon)
* Isopropyl alcohol (IPA) and antistatic brush
* Heatshrink tubing (for radial cap lead insulation)


== Extra Tips ==
See [[Recommended Tools]] for the general list. For this job specifically:
* '''Double-check polarity!''' The ZX80 board is single-sided and traces lift easily.
* '''Use axial capacitors''' for easiest fit, but radial types are fine if insulated and laid flat.
* '''Check the 7805 regulator''' – If it runs hot or output is low, consider replacing it along with the capacitors.
* '''Clean the edge connector''' while the board is out for best RAM pack reliability.
* '''Store the ZX80 in a dry place''' – moisture accelerates capacitor aging.


== Related Pages ==
* Temperature-controlled iron with a fine tip
* Desoldering braid and/or a pump
* 63/37 leaded solder and fresh no-clean or rosin flux
* Two 105 °C electrolytics: 22 µF 16 V (or higher) and 1 µF 16 V (or higher)
* Isopropyl alcohol and a soft antistatic brush
* Heatshrink or sleeving if you are fitting radial parts in axial positions
 
== Notes ==
 
* '''Check the power supply before you blame the board.''' A supply that has been re-terminated with the polarity reversed is a known way to destroy the regulator. On this family the regulator usually takes the hit and protects the rest of the board, but do not rely on that.<ref name="tv2" />
* '''The ZX80 has no ULA.''' All of its logic is standard 74LS parts; the ULA arrived with the ZX81, which combined them into one chip. Advice written for a ZX81 or a Spectrum does not transfer.<ref name="kiodiff">''Differences between ZX80, ZX81 and Timex 1000'', The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: the ZX80's standard TTL logic was combined into a single ULA for the ZX81.</ref>
* '''Clean the edge connector''' while the board is out. Contact trouble there is a common cause of unreliable behaviour with a RAM pack fitted.
* See [[Capacitor Failure Symptoms]] for what failing electrolytics look like in general, and [[Battery Explosion, Capacitor or Corrosion Damage]] if there is existing corrosion to deal with.
 
== Related pages ==
 
* [[Sinclair ZX80]]
* [[Sinclair ZX80 General Maintenance]]
* [[Sinclair ZX80 Troubleshooting Guide]]
* [[Sinclair ZX80 Troubleshooting Guide]]
* [[Sinclair ZX81 Capacitor Replacement Guide]]
* [[Sinclair ZX81 Capacitor Replacement Guide]]
* [[Sinclair Power Supply Refurbishment]]
* [[Capacitor Failure Symptoms]]
 
== References ==
 
<references />


[[Category:Sinclair Computers]]
[[Category:Sinclair Computers]]
[[Category:Capacitor Replacement Guides]]
[[Category:Capacitor Replacement Guides]]

Revision as of 17:30, 22 September 2026

Sinclair's ZX80 circuit diagram. Every designator and value on this page is read from it; the power supply section is at the top left, above the edge-connector tables.

The Sinclair ZX80 has very few capacitors, and only two of them are electrolytic. It is therefore not a "recap on sight" machine in the way that a machine full of low-voltage electrolytics is. The two electrolytics are worth attention on a forty-five-year-old board, and the rest of the board is ceramic and does not normally need to be touched.

Visual inspection and failure signs

  • Leaking or crusty electrolytics — brown, green or white residue at the base of C8 or C10.
  • Bulging or split cans on either of those two parts.
  • Machine dead with a cold regulator. If the 7805 (IC22) never warms up, check the supply and C8 before anything else: C8 is the only reservoir between the unregulated input and the regulator.[1]
  • No cursor at switch-on, but the screen is otherwise live. The Z80's reset is held by R21 (220 k) charging C10 (1 µF). A failed C10 attacks this directly.[1]
  • Horizontal black and white stripes drifting through the picture. Sinclair attributed this to the power supply — either insufficient smoothing or too low an output on load.[2]

Capacitors on the ZX80 board

Sinclair's circuit diagram is the source for the list below.[1] Grant Searle's 2009 vector redraw of the same diagram carries identical designators and values, which is a useful independent check.[3]

Sinclair ZX80 capacitors, from Sinclair's circuit diagram
Ref Value Type Where it sits on the diagram
C1, C2, C4, C5, C6 47 nF Ceramic Five decouplers across the regulated +5 V rail at the output of IC22
C8 22 µF, 16 V Electrolytic Across the unregulated DC input, ahead of the IC22 regulator. The board's only reservoir capacitor
C9 47 pF Ceramic With R2 (470 R) in the ∅2 clock path feeding IC16
C10 1 µF Electrolytic With R21 (220 k) to +5 V on IC1 (Z80) pin 26, /RESET — the power-on reset network
C11 47 pF Ceramic With R25 (470 R) in the /MREQ timing network feeding IC11
C12 47 nF Ceramic In series with R29 (180 R) at the EAR socket — the tape input path
C13 47 nF Ceramic In the MIC socket network with R34 (1 k), fed from SYNC
C14 47 pF Ceramic In the same MIC socket network, with R35 (1 M)
C15 0–330 pF trimmer Trimmer In the 6.5 MHz master oscillator round X1, with R27 (1 k)

There is no C3 and no C7 on the ZX80 circuit diagram. Designators were not reused from a ZX81 or a Spectrum; if your board is silkscreened with a designator not in the list above, work from your own board and post the finding rather than assuming this table is incomplete.

Only C8 and C10 are electrolytic. If you are ordering parts, those are the two to order.

Choosing replacements

  • C8 — 22 µF at 16 V or higher. The rail it sits on is the raw supply, nominally 9–11 V DC and rising to about 14 V open-circuit, so do not fit anything rated below 16 V.[1][4]
  • C10 — 1 µF. It sits on the +5 V rail; 16 V or higher is ample.
  • Use 105 °C aluminium electrolytics from a recognised manufacturer. Equal capacitance, equal or higher voltage.
  • The ceramics are stable parts and are not a normal service item. Replace one only if it has been physically damaged.

Recapping procedure

  1. Unplug the supply and remove the case screws.
  2. Free the board. Disconnect the keyboard tails from their connectors. The tails are the most fragile thing in the machine — pull them straight out, never at an angle, and never fold them.[5]
  3. Note the polarity of C8 and C10 before removing anything. The circuit diagram marks the positive end of both.[1]
  4. Add fresh flux and solder to the old joints and let it mix before applying wick or a pump. The ZX80 board is single-sided and the pads lift if you work them hot and dry.
  5. Fit the replacements the right way round. The originals are axial; radial parts are acceptable if you form and sleeve the leads so nothing can short to an adjacent track.
  6. Clean the flux residue with isopropyl alcohol and a soft brush, then inspect under magnification for splashes and bridges — a hairline solder splash reads as a few ohms and produces very misleading symptoms.[5]
  7. Reassemble and test.

Checks after the work

The ZX80 has a single internal rail. There is no −5 V and no +12 V on this machine — those belong to the ZX Spectrum, whose 4116 RAM needs them.

ZX80 supply checkpoints
Point Expect Source
Power jack, 3.5 mm, centre pin positive Nominal 9 V DC unregulated, about 14 V open-circuit; the circuit diagram marks the input "9 V – 11 V" [1][4]
IC22 (7805) output +5 V [1]
IC1 (Z80) pin 11 Vcc, pin 29 ground +5 V [1]
IC2 (8332 ROM) pin 24 Vcc, pin 12 ground +5 V [1]
IC3, IC4 (2114 RAM) pin 18 Vcc, pin 9 ground +5 V [1]
IC1 (Z80) pin 6 3.25 MHz CPU clock, derived from the 6.5 MHz oscillator X1 [1]

Sinclair did not publish a ripple figure for the ZX80, and none is given here. What the manual for the closely related Sinclair ZX81 does say is that the regulator will not work correctly below about 7 V on load, and that drifting horizontal stripes point at the supply rather than the board.[2]

Tools and materials

See Recommended Tools for the general list. For this job specifically:

  • Temperature-controlled iron with a fine tip
  • Desoldering braid and/or a pump
  • 63/37 leaded solder and fresh no-clean or rosin flux
  • Two 105 °C electrolytics: 22 µF 16 V (or higher) and 1 µF 16 V (or higher)
  • Isopropyl alcohol and a soft antistatic brush
  • Heatshrink or sleeving if you are fitting radial parts in axial positions

Notes

  • Check the power supply before you blame the board. A supply that has been re-terminated with the polarity reversed is a known way to destroy the regulator. On this family the regulator usually takes the hit and protects the rest of the board, but do not rely on that.[5]
  • The ZX80 has no ULA. All of its logic is standard 74LS parts; the ULA arrived with the ZX81, which combined them into one chip. Advice written for a ZX81 or a Spectrum does not transfer.[6]
  • Clean the edge connector while the board is out. Contact trouble there is a common cause of unreliable behaviour with a RAM pack fitted.
  • See Capacitor Failure Symptoms for what failing electrolytics look like in general, and Battery Explosion, Capacitor or Corrosion Damage if there is existing corrosion to deal with.

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Sinclair Research Ltd, ZX80 Circuit Diagram. Hosted on this wiki as File:Sinclair ZX80 circuit diagram.png. Source for every ZX80 capacitor designator, value, IC type and pin number on this page.
  2. ↑ 2.0 2.1 Sinclair Research Ltd, ZX81 Service and Assembly Manual, section 6 (fault finding) and section 9 (power supply). Hosted on this wiki as File:Sinclair ZX81 Service and Assembly Manual.pdf. Cited here because Sinclair published no equivalent document for the ZX80 and the two machines share the same external supply arrangement.
  3. ↑ Grant Searle, ZX80 hardware page, http://searle.x10host.com/zx80/zx80.html — accurately redrawn ZX80 schematic, January 2009. Used here only as independent confirmation of the designators and values on Sinclair's own diagram.
  4. ↑ 4.0 4.1 Sinclair ZX80 Connectors, The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity and open-circuit voltage.
  5. ↑ 5.0 5.1 5.2 "Servicing Sinclair Computers, Part 2", Television magazine, 1986. Trade servicing article covering the ZX80/ZX81 family; mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source, used here for handling practice, keyboard tails and solder-splash faults.
  6. ↑ Differences between ZX80, ZX81 and Timex 1000, The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: the ZX80's standard TTL logic was combined into a single ULA for the ZX81.
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