Sinclair ZX80 General Maintenance: Difference between revisions

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[[File:ZX80 PCB Top.jpg|thumb|right|300px|Sinclair ZX80 main board]]

[[~~File:Sinclair_ZX80_Thumbnail.jpg|thumb|~~Sinclair ZX80 ~~Mainboard~~]]

The '''[[Sinclair ZX80]]''' is a simple machine to look after. It has one internal supply rail, two electrolytic capacitors, no custom silicon and no battery. Most of what goes wrong with one is mechanical — keyboard tails, edge-connector contacts, dry joints — or comes in from the external power supply.

== ~~Board Revisions~~ ==

'''Observe ESD precautions''' when the board is out of the case. Use a wrist strap and an antistatic mat.

{| class="wikitable styled-table" style="width:~~100~~%; text-align:~~center~~;"

|+ ~~Board Revisions~~

== What the ZX80 actually is ==

! ~~ASSY Number~~ !! ~~Year~~ !! ~~Key Differences~~

Knowing this saves you from applying ZX81 or Spectrum advice to a machine that does not work that way.

* '''There is no ULA.''' The ZX80's logic is twenty standard 74LS parts. Sinclair combined them into a single ULA to make the [[Sinclair ZX81|ZX81]]; nothing on a ZX80 board can be diagnosed as "a failed ULA".<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 one ULA for the ZX81; the ZX80's display blanks while a program runs.</ref><ref name="zx80cd" />

* '''The RAM is static.''' IC3 and IC4 are 2114 (1K × 4) static RAMs, giving the standard 1 KB. There is no DRAM, so there is no −5 V and no +12 V rail anywhere on the machine.<ref name="zx80cd" />

* '''The regulator is IC22''', a 7805 bolted to an aluminium heatsink. It is not "IC3" — IC3 is a RAM chip.<ref name="zx80cd" />

{| class="wikitable styled-table" style="width:90%; text-align:left;"

|+'''ZX80 integrated circuits, from Sinclair's circuit diagram'''<ref name="zx80cd">Sinclair Research Ltd, ''ZX80 Circuit Diagram''. Hosted on this wiki as [[:File:Sinclair ZX80 circuit diagram.png]]. Source for the IC type list, the capacitor designators and values, the supply rails and every pin number quoted on this page.</ref>

! Ref !! Type !! Ref !! Type

|-

| IC1 || 780-1 (Z80 CPU) || IC12 || 74LS00

|-

| IC2 || 8332 (4 KB mask ROM) || IC13 || 74LS04

|-

| IC3 || 2114 RAM || IC14 || 74LS05

|-

| IC4 || 2114-1 RAM || IC15 || 74LS05

|-

| IC5 || 74LS373 || IC16 || 74LS10

|-

| IC6 || 74LS157 || IC17 || 74LS32

|-

| IC7 || 74LS157 || IC18 || 74LS74

|-

| IC8 || 74LS157 || IC19 || 74LS74

|-

| ~~001~~ || ~~1980~~ || ~~Original design with basic components.~~

| IC9 || 74LS166 || IC20 || 74LS86

|-

| ~~002~~ || ~~1981~~ || ~~Improved power supply circuit and minor layout changes.~~

| IC10 || 74LS365 || IC21 || 74LS93

|-

| ~~003~~ || ~~1982~~ || ~~Enhanced memory management and additional ROM features.~~

| IC11 || 74LS00 || IC22 || 7805 regulator

|}

~~== Power Supply & Regulator Care ==~~

'''Board revisions.''' No Sinclair-published list of ZX80 board issues has been found, and the unsourced revision table that used to appear here has been removed rather than guessed at. Photographs of Issue 3 and Issue 3A boards are held in the Sinclair ZX Computers Archive.<ref name="kiopcb">''Sinclair ZX80 — PCB'', The Sinclair ZX Computers Archive, k1.spdns.de: photographs of ZX80 Issue 3 and Issue 3A boards. Secondary source; the only board-issue evidence located.</ref> Your board's issue number is silkscreened on it — work from what is in front of you.

~~The~~ Sinclair ZX80 ~~utilizes a simple power supply~~ and ~~regulator setup~~. ~~Below~~ are the ~~recommended PSU models~~, ~~common failure modes~~, and on-board ~~regulator capacitor information~~.

==~~= PSU Models =~~==

== Power supply ==

* Sinclair ZX80 Power Supply (Original)

* Third-party 9V DC adapters (ensure correct polarity)

~~=== Common Failure Modes ===~~

The ZX80 runs from an external, unregulated DC adaptor through a '''3.5 mm mono jack'''.

* Overheating due to inadequate ventilation.

* Voltage drop under load leading to unstable operation.

=== On-~~board Regulator Capacitors~~ ===

{| class="wikitable styled-table" style="width:90%; text-align:left;"

* C1: 100µF, ~~16V (Electrolytic)~~

|+'''ZX80 external supply'''

* C2: 10µF, 16V (Electrolytic)

! Property !! Value !! Source

|-

| Connector || 3.5 mm mono jack — the same size as the EAR and MIC sockets, so it is easy to plug into the wrong one || <ref name="kio80" />

|-

| Polarity || '''Centre pin positive''', shaft ground || <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><ref name="zx80cd" />

|-

| Voltage || Nominal 9 V DC unregulated; about 14 V open-circuit. Sinclair's circuit diagram marks the input "9 V – 11 V" || <ref name="kio80" /><ref name="zx80cd" />

|-

| Current || Sinclair published a figure for the [[Sinclair ZX81|ZX81]] — not less than 600 mA, or 1.2 A if the ZX printer shares the supply — but no equivalent published figure for the ZX80 has been found || <ref name="zx81man" />

|}

== ~~Capacitor Replacement (Recap)~~ ==

'''Get the polarity right.''' A supply re-terminated the wrong way round is a known killer of the 5 V regulator on this family. On these machines the regulator usually dies and saves the rest of the board, but that is luck, not protection.<ref name="tv2" />

{| class="wikitable styled-table" style="width:~~100~~%; text-align:~~center~~;"

~~|+ Capacitor Replacement (Recap)~~

The adaptor itself contains a mains transformer and rectifier. There is no reason not to open one and service it — that is ordinary repair work — but the primary side is at mains potential and the reservoir capacitor holds charge after unplugging. Unplug it, wait, and verify the reservoir is discharged before putting a hand or a probe inside. On the closely related Spectrum supplies, shorted leads have been known to set the rectifier diodes on fire, so a lead in poor condition is worth replacing on sight.<ref name="clinic">"Micro Clinic", ''Television'' magazine, 1986 — collected repair notes on Sinclair machines, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: Sinclair power-supply lead failures and their consequences.</ref>

! ~~Board Position~~ !! ~~Capacitance~~ !! ~~Voltage !! Notes~~

== Supply and clock checkpoints ==

{| class="wikitable styled-table" style="width:90%; text-align:left;"

! Point !! Expect !! Source

|-

| Positive end of C8 || The incoming unregulated supply, nominally 9 V DC || <ref name="zx80cd" />

|-

| IC22 (7805) output || +5 V || <ref name="zx80cd" />

|-

| IC1 (Z80) pin 11 Vcc / pin 29 ground || +5 V || <ref name="zx80cd" />

|-

| C1 || ~~100µF~~ || ~~16V || Replace with low-ESR 105°C part.~~

| IC2 (8332 ROM) pin 24 Vcc / pin 12 ground || +5 V || <ref name="zx80cd" />

|-

| C2 || ~~10µF~~ || ~~16V || Replace with low-ESR 105°C part.~~

| IC3, IC4 (2114 RAM) pin 18 Vcc / pin 9 ground || +5 V || <ref name="zx80cd" />

|-

| C3 || ~~100µF~~ || ~~16V || Replace with low-ESR 105°C part.~~

| IC1 (Z80) pin 6 || 3·2 MHz, as Sinclair's diagram labels it — the 6.5 MHz oscillator X1 divided by two, i.e. 3.25 MHz || <ref name="zx80cd" />

|-

| C4 || ~~1µF || 50V~~ || ~~Replace with low-ESR 105°C part.~~

| IC1 (Z80) pin 26 || /RESET — held low briefly at switch-on by R21 (220 k) and C10 (1 µF), then high || <ref name="zx80cd" />

|}

== ~~Voltage & Clock Test Points~~ ==

The edge connector carries +5 V, +9 V and 0 V alongside the Z80 bus; the full pinout is printed on Sinclair's circuit diagram.<ref name="zx80cd" /> Note that /ROMCS does '''not''' exist on the ZX80 bus — that input was added for the ZX81, so ZX81-era expansion hardware that relies on it will not behave the same way.<ref name="kio80" />

{| class="wikitable styled-table" style="width:100%; text-align:~~center~~;"

|+ ~~Voltage & Clock Test Points~~

== Capacitors ==

~~! Test Point !! Location~~/~~Pin !! Expected Values~~

There are exactly two electrolytics: '''C8''' (22 µF, 16 V) across the unregulated input, and '''C10''' (1 µF) in the power-on reset network. Everything else is ceramic and is not a service item.<ref name="zx80cd" />

Full detail, including replacement guidance, is in the [[Sinclair ZX80 Capacitor Replacement Guide]]. See also [[Capacitor Failure Symptoms]].

== Preventive maintenance ==

# Unplug the supply. Measure its output and check its polarity '''before''' plugging it into the machine.

# Open the case and inspect the board for corrosion, cracked tracks and burnt components.

# Inspect C8 and C10 for leakage or bulging.

# Inspect the keyboard tails at the connectors. They fail by cracking where they bend. Remove them straight, never at an angle, and never fold them double — a trapped, folded tail will go open circuit sooner or later.<ref name="tv2" /><ref name="smspec">''ZX Spectrum / ZX Spectrum+ Service Manual'', Sinclair Research Ltd, section 2.2 (assembly). Hosted on this wiki as [[:File:Sinclair ZX Spectrum Service Manual.pdf]]. Cited here for Sinclair's own warning that a trapped, folded keyboard ribbon will go open circuit — the same ribbons and the same failure occur across the ZX80, ZX81 and Spectrum.</ref>

# Clean dust with a soft brush. Clean the edge connector fingers.

# Reseat any socketed ICs.

# Inspect solder joints under magnification, particularly around the power jack, the regulator fixings and the edge connector. Look for splashes as well as dry joints: a hairline solder splash measures a few ohms and produces very misleading symptoms.<ref name="tv2">"Servicing Sinclair Computers, Part 2", ''Television'' magazine, 1986. Trade servicing article on the ZX80/ZX81 family, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: keyboard tail handling, solder-splash faults, reverse-polarity damage, piggy-backing technique.</ref>

# Check the regulator is still firmly bolted to its heatsink before reassembly.

== Things that are not faults ==

* '''The picture disappears while a program runs.''' The ZX80 cannot generate a TV signal and execute a program at the same time. The screen blanks while the program works and flickers when a key is pressed. This is how the machine was designed; the ZX81's SLOW mode is what fixed it.<ref name="kiodiff" />

* '''The regulator runs warm.''' It should. On the ZX81, Sinclair told kit builders that a regulator which never warms up means the supply is not reaching it; a regulator so hot you cannot bear to touch it means it is not properly bolted to its heatsink.<ref name="zx81man">Sinclair Research Ltd, ''ZX81 Service and Assembly Manual'', section 6 (fault finding) and section 9 (power supply specification). Hosted on this wiki as [[:File:Sinclair ZX81 Service and Assembly Manual.pdf]]. Cited on this page because Sinclair published no equivalent service document for the ZX80 and the two machines share the same supply arrangement and regulator; each such use is identified as a ZX81 figure.</ref>

== Common faults and first checks ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"

! Symptom !! Where to look first

|-

| Completely dead, regulator cold || Supply output and polarity; the power jack solder joints; C8; IC22<ref name="zx81man" /><ref name="zx80cd" />

|-

| Works then cuts out, regulator very hot || Regulator not properly bolted to the heatsink<ref name="zx81man" />

|-

| No picture at all || Tuning first — sweep UHF around channel 36. Then the +5 V rail, then the UM1233 modulator and the R30–R33 network feeding it<ref name="zx81man" /><ref name="kio80" /><ref name="zx80cd" />

|-

| Cursor on screen, keyboard does nothing || Keyboard tails seated in their connectors and not twisted; the matrix diodes D3–D10; IC10 (74LS365)<ref name="zx81man" /><ref name="zx80cd" />

|-

| ~~TP1~~ || ~~Near C1 || +5V~~

| Screen clears but no cursor || Unplug the supply, wait a few seconds, try again — then suspect the R21/C10 reset network<ref name="zx81man" /><ref name="zx80cd" />

|-

| ~~TP2~~ || ~~Near IC1 || +12V~~

| Horizontal black and white stripes through the picture || The external supply: insufficient smoothing, or output too low on load<ref name="zx81man" />

|-

| ~~TP3~~ || ~~Near IC2 || Clock signal: 1.77 MHz~~

| Random resets or lockups with a RAM pack fitted || Edge-connector contacts and mechanical movement of the pack, before anything on the board

|}

For a symptom-by-symptom walk-through, see the [[Sinclair ZX80 Troubleshooting Guide]].

== Tools ==

See [[Recommended Tools]]. The ZX80-specific additions are a multimeter (for the supply check that should precede every session) and, if you intend to chase clock or video faults, an oscilloscope.

== Related pages ==

* [[Sinclair ZX80]]

* [[Sinclair ZX80 Capacitor Replacement Guide]]

* [[Sinclair ZX80 Troubleshooting Guide]]

* [[Sinclair ZX81 General Maintenance]]

* [[Capacitor Failure Symptoms]]

* [[Recommended Tools]]

== References ==

[[Category:Sinclair Computers]]

[[Category:Maintenance Guides]]

@@ Line 1: / Line 1: @@
 <templatestyles src="Template:StyledTable/styles.css" />
+[[File:ZX80 PCB Top.jpg|thumb|right|300px|Sinclair ZX80 main board]]
-[[File:Sinclair_ZX80_Thumbnail.jpg|thumb|Sinclair ZX80 Mainboard]]
+The '''[[Sinclair ZX80]]''' is a simple machine to look after. It has one internal supply rail, two electrolytic capacitors, no custom silicon and no battery. Most of what goes wrong with one is mechanical — keyboard tails, edge-connector contacts, dry joints — or comes in from the external power supply.
-== Board Revisions ==
+'''Observe ESD precautions''' when the board is out of the case. Use a wrist strap and an antistatic mat.
-{| class="wikitable styled-table" style="width:100%; text-align:center;"
-|+ Board Revisions
+== What the ZX80 actually is ==
-! ASSY Number !! Year !! Key Differences
+Knowing this saves you from applying ZX81 or Spectrum advice to a machine that does not work that way.
+* '''There is no ULA.''' The ZX80's logic is twenty standard 74LS parts. Sinclair combined them into a single ULA to make the [[Sinclair ZX81|ZX81]]; nothing on a ZX80 board can be diagnosed as "a failed ULA".<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 one ULA for the ZX81; the ZX80's display blanks while a program runs.</ref><ref name="zx80cd" />
+* '''The RAM is static.''' IC3 and IC4 are 2114 (1K × 4) static RAMs, giving the standard 1 KB. There is no DRAM, so there is no −5 V and no +12 V rail anywhere on the machine.<ref name="zx80cd" />
+* '''The regulator is IC22''', a 7805 bolted to an aluminium heatsink. It is not "IC3" — IC3 is a RAM chip.<ref name="zx80cd" />
+{| class="wikitable styled-table" style="width:90%; text-align:left;"
+|+'''ZX80 integrated circuits, from Sinclair's circuit diagram'''<ref name="zx80cd">Sinclair Research Ltd, ''ZX80 Circuit Diagram''. Hosted on this wiki as [[:File:Sinclair ZX80 circuit diagram.png]]. Source for the IC type list, the capacitor designators and values, the supply rails and every pin number quoted on this page.</ref>
+! Ref !! Type !! Ref !! Type
+|-
+| IC1 || 780-1 (Z80 CPU) || IC12 || 74LS00
+|-
+| IC2 || 8332 (4 KB mask ROM) || IC13 || 74LS04
+|-
+| IC3 || 2114 RAM || IC14 || 74LS05
+|-
+| IC4 || 2114-1 RAM || IC15 || 74LS05
+|-
+| IC5 || 74LS373 || IC16 || 74LS10
+|-
+| IC6 || 74LS157 || IC17 || 74LS32
+|-
+| IC7 || 74LS157 || IC18 || 74LS74
+|-
+| IC8 || 74LS157 || IC19 || 74LS74
 |-
-| 001 || 1980 || Original design with basic components.
+| IC9 || 74LS166 || IC20 || 74LS86
 |-
-| 002 || 1981 || Improved power supply circuit and minor layout changes.
+| IC10 || 74LS365 || IC21 || 74LS93
 |-
-| 003 || 1982 || Enhanced memory management and additional ROM features.
+| IC11 || 74LS00 || IC22 || 7805 regulator
 |}
-== Power Supply & Regulator Care ==
+'''Board revisions.''' No Sinclair-published list of ZX80 board issues has been found, and the unsourced revision table that used to appear here has been removed rather than guessed at. Photographs of Issue 3 and Issue 3A boards are held in the Sinclair ZX Computers Archive.<ref name="kiopcb">''Sinclair ZX80 — PCB'', The Sinclair ZX Computers Archive, k1.spdns.de: photographs of ZX80 Issue 3 and Issue 3A boards. Secondary source; the only board-issue evidence located.</ref> Your board's issue number is silkscreened on it — work from what is in front of you.
-The Sinclair ZX80 utilizes a simple power supply and regulator setup. Below are the recommended PSU models, common failure modes, and on-board regulator capacitor information.
-=== PSU Models ===
+== Power supply ==
-* Sinclair ZX80 Power Supply (Original)
-* Third-party 9V DC adapters (ensure correct polarity)
-=== Common Failure Modes ===
+The ZX80 runs from an external, unregulated DC adaptor through a '''3.5 mm mono jack'''.
-* Overheating due to inadequate ventilation.
-* Voltage drop under load leading to unstable operation.
-=== On-board Regulator Capacitors ===
+{| class="wikitable styled-table" style="width:90%; text-align:left;"
-* C1: 100µF, 16V (Electrolytic)
+|+'''ZX80 external supply'''
-* C2: 10µF, 16V (Electrolytic)
+! Property !! Value !! Source
+|-
+| Connector || 3.5 mm mono jack — the same size as the EAR and MIC sockets, so it is easy to plug into the wrong one || <ref name="kio80" />
+|-
+| Polarity || '''Centre pin positive''', shaft ground || <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><ref name="zx80cd" />
+|-
+| Voltage || Nominal 9 V DC unregulated; about 14 V open-circuit. Sinclair's circuit diagram marks the input "9 V – 11 V" || <ref name="kio80" /><ref name="zx80cd" />
+|-
+| Current || Sinclair published a figure for the [[Sinclair ZX81|ZX81]] — not less than 600 mA, or 1.2 A if the ZX printer shares the supply — but no equivalent published figure for the ZX80 has been found || <ref name="zx81man" />
+|}
-== Capacitor Replacement (Recap) ==
+'''Get the polarity right.''' A supply re-terminated the wrong way round is a known killer of the 5 V regulator on this family. On these machines the regulator usually dies and saves the rest of the board, but that is luck, not protection.<ref name="tv2" />
-{| class="wikitable styled-table" style="width:100%; text-align:center;"
-|+ Capacitor Replacement (Recap)
+The adaptor itself contains a mains transformer and rectifier. There is no reason not to open one and service it — that is ordinary repair work — but the primary side is at mains potential and the reservoir capacitor holds charge after unplugging. Unplug it, wait, and verify the reservoir is discharged before putting a hand or a probe inside. On the closely related Spectrum supplies, shorted leads have been known to set the rectifier diodes on fire, so a lead in poor condition is worth replacing on sight.<ref name="clinic">"Micro Clinic", ''Television'' magazine, 1986 — collected repair notes on Sinclair machines, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: Sinclair power-supply lead failures and their consequences.</ref>
-! Board Position !! Capacitance !! Voltage !! Notes
+== Supply and clock checkpoints ==
+{| class="wikitable styled-table" style="width:90%; text-align:left;"
+! Point !! Expect !! Source
+|-
+| Positive end of C8 || The incoming unregulated supply, nominally 9 V DC || <ref name="zx80cd" />
+|-
+| IC22 (7805) output || +5 V || <ref name="zx80cd" />
+|-
+| IC1 (Z80) pin 11 Vcc / pin 29 ground || +5 V || <ref name="zx80cd" />
 |-
-| C1 || 100µF || 16V || Replace with low-ESR 105°C part.
+| IC2 (8332 ROM) pin 24 Vcc / pin 12 ground || +5 V || <ref name="zx80cd" />
 |-
-| C2 || 10µF || 16V || Replace with low-ESR 105°C part.
+| IC3, IC4 (2114 RAM) pin 18 Vcc / pin 9 ground || +5 V || <ref name="zx80cd" />
 |-
-| C3 || 100µF || 16V || Replace with low-ESR 105°C part.
+| IC1 (Z80) pin 6 || 3·2 MHz, as Sinclair's diagram labels it — the 6.5 MHz oscillator X1 divided by two, i.e. 3.25 MHz || <ref name="zx80cd" />
 |-
-| C4 || 1µF || 50V || Replace with low-ESR 105°C part.
+| IC1 (Z80) pin 26 || /RESET — held low briefly at switch-on by R21 (220 k) and C10 (1 µF), then high || <ref name="zx80cd" />
 |}
-== Voltage & Clock Test Points ==
+The edge connector carries +5 V, +9 V and 0 V alongside the Z80 bus; the full pinout is printed on Sinclair's circuit diagram.<ref name="zx80cd" /> Note that /ROMCS does '''not''' exist on the ZX80 bus — that input was added for the ZX81, so ZX81-era expansion hardware that relies on it will not behave the same way.<ref name="kio80" />
-{| class="wikitable styled-table" style="width:100%; text-align:center;"
-|+ Voltage & Clock Test Points
+== Capacitors ==
-! Test Point !! Location/Pin !! Expected Values
+There are exactly two electrolytics: '''C8''' (22 µF, 16 V) across the unregulated input, and '''C10''' (1 µF) in the power-on reset network. Everything else is ceramic and is not a service item.<ref name="zx80cd" />
+Full detail, including replacement guidance, is in the [[Sinclair ZX80 Capacitor Replacement Guide]]. See also [[Capacitor Failure Symptoms]].
+== Preventive maintenance ==
+# Unplug the supply. Measure its output and check its polarity '''before''' plugging it into the machine.
+# Open the case and inspect the board for corrosion, cracked tracks and burnt components.
+# Inspect C8 and C10 for leakage or bulging.
+# Inspect the keyboard tails at the connectors. They fail by cracking where they bend. Remove them straight, never at an angle, and never fold them double — a trapped, folded tail will go open circuit sooner or later.<ref name="tv2" /><ref name="smspec">''ZX Spectrum / ZX Spectrum+ Service Manual'', Sinclair Research Ltd, section 2.2 (assembly). Hosted on this wiki as [[:File:Sinclair ZX Spectrum Service Manual.pdf]]. Cited here for Sinclair's own warning that a trapped, folded keyboard ribbon will go open circuit — the same ribbons and the same failure occur across the ZX80, ZX81 and Spectrum.</ref>
+# Clean dust with a soft brush. Clean the edge connector fingers.
+# Reseat any socketed ICs.
+# Inspect solder joints under magnification, particularly around the power jack, the regulator fixings and the edge connector. Look for splashes as well as dry joints: a hairline solder splash measures a few ohms and produces very misleading symptoms.<ref name="tv2">"Servicing Sinclair Computers, Part 2", ''Television'' magazine, 1986. Trade servicing article on the ZX80/ZX81 family, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: keyboard tail handling, solder-splash faults, reverse-polarity damage, piggy-backing technique.</ref>
+# Check the regulator is still firmly bolted to its heatsink before reassembly.
+== Things that are not faults ==
+* '''The picture disappears while a program runs.''' The ZX80 cannot generate a TV signal and execute a program at the same time. The screen blanks while the program works and flickers when a key is pressed. This is how the machine was designed; the ZX81's SLOW mode is what fixed it.<ref name="kiodiff" />
+* '''The regulator runs warm.''' It should. On the ZX81, Sinclair told kit builders that a regulator which never warms up means the supply is not reaching it; a regulator so hot you cannot bear to touch it means it is not properly bolted to its heatsink.<ref name="zx81man">Sinclair Research Ltd, ''ZX81 Service and Assembly Manual'', section 6 (fault finding) and section 9 (power supply specification). Hosted on this wiki as [[:File:Sinclair ZX81 Service and Assembly Manual.pdf]]. Cited on this page because Sinclair published no equivalent service document for the ZX80 and the two machines share the same supply arrangement and regulator; each such use is identified as a ZX81 figure.</ref>
+== Common faults and first checks ==
+{| class="wikitable styled-table" style="width:100%; text-align:left;"
+! Symptom !! Where to look first
+|-
+| Completely dead, regulator cold || Supply output and polarity; the power jack solder joints; C8; IC22<ref name="zx81man" /><ref name="zx80cd" />
+|-
+| Works then cuts out, regulator very hot || Regulator not properly bolted to the heatsink<ref name="zx81man" />
+|-
+| No picture at all || Tuning first — sweep UHF around channel 36. Then the +5 V rail, then the UM1233 modulator and the R30–R33 network feeding it<ref name="zx81man" /><ref name="kio80" /><ref name="zx80cd" />
+|-
+| Cursor on screen, keyboard does nothing || Keyboard tails seated in their connectors and not twisted; the matrix diodes D3–D10; IC10 (74LS365)<ref name="zx81man" /><ref name="zx80cd" />
 |-
-| TP1 || Near C1 || +5V
+| Screen clears but no cursor || Unplug the supply, wait a few seconds, try again — then suspect the R21/C10 reset network<ref name="zx81man" /><ref name="zx80cd" />
 |-
-| TP2 || Near IC1 || +12V
+| Horizontal black and white stripes through the picture || The external supply: insufficient smoothing, or output too low on load<ref name="zx81man" />
 |-
-| TP3 || Near IC2 || Clock signal: 1.77 MHz
+| Random resets or lockups with a RAM pack fitted || Edge-connector contacts and mechanical movement of the pack, before anything on the board
 |}
+For a symptom-by-symptom walk-through, see the [[Sinclair ZX80 Troubleshooting Guide]].
+== Tools ==
+See [[Recommended Tools]]. The ZX80-specific additions are a multimeter (for the supply check that should precede every session) and, if you intend to chase clock or video faults, an oscilloscope.
+== Related pages ==
+* [[Sinclair ZX80]]
+* [[Sinclair ZX80 Capacitor Replacement Guide]]
+* [[Sinclair ZX80 Troubleshooting Guide]]
+* [[Sinclair ZX81 General Maintenance]]
+* [[Capacitor Failure Symptoms]]
+* [[Recommended Tools]]
+== References ==
+<references />
+[[Category:Sinclair Computers]]
+[[Category:Maintenance Guides]]

Sinclair ZX80 General Maintenance: Difference between revisions

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