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Correct CPU clock attribution: Sinclair's ZX80 circuit diagram labels the CPU clock 3·2 MHz, not 3.25 MHz; both figures now given with the derivation
 
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[[File:Sinclair ZX80 circuit diagram.png|thumb|420px|Sinclair's ZX80 circuit diagram, the reference for every designator and pin number below.]]


{{DISPLAYTITLE:Sinclair ZX80 Troubleshooting Guide}}
Component-level fault finding for the '''[[Sinclair ZX80]]'''. Every designator, pin number and value below is taken from Sinclair's own circuit diagram.<ref name="zx80cd">Sinclair Research Ltd, ''ZX80 Circuit Diagram''. Hosted on this wiki as [[:File:Sinclair ZX80 circuit diagram.png]]. Source for every ZX80 designator, IC type, pin number and component value on this page.</ref>


This guide provides detailed, component-level troubleshooting for the '''Sinclair ZX80''' home computer. It covers all major PCB revisions and notes differences between early and late production models where relevant.
'''Read this first.''' Sinclair never published a service manual for the ZX80, only the circuit diagram. Where a published procedure exists it is for the [[Sinclair ZX81|ZX81]], and where one is used here it is labelled as such. Much of the ZX80 "repair advice" in circulation is really ZX81 or Spectrum advice, and it does not fit:


Common failure symptoms, diagnostic procedures, and fixes are outlined for:
* '''The ZX80 has no ULA.''' Its logic is twenty standard 74LS packages. The single-chip ULA was introduced with the ZX81.<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 blanks its display while a program runs.</ref>
* '''The RAM is static.''' IC3 and IC4 are 2114 devices (1K × 4), 1 KB in total. There is no DRAM, so there is no −5 V or +12 V rail to lose and no DRAM refresh circuit to fail.<ref name="zx80cd" />
* '''There is no video output transistor.''' The VIDEO and SYNC logic outputs are summed through R30–R33 straight into the UM1233 modulator.<ref name="zx80cd" />
* '''The regulator is IC22.''' IC3 is a RAM chip.<ref name="zx80cd" />
* '''There is no /ROMCS on the edge connector.''' That signal exists only on the ZX81.<ref name="kio80">''Sinclair ZX80 Connectors'', The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity, open-circuit voltage, and the absence of /ROMCS from the ZX80 bus.</ref>


* Power supply faults
== Diagnostic techniques ==
* “No video” or “garbage video” scenarios
* Memory (RAM) issues
* ROM faults
* ULA (Uncommitted Logic Array) failures
* Z80 CPU faults
* Keyboard matrix problems
* Video sync and display issues


Diagnostic techniques (chip substitution, piggy-backing, logic probing, thermal checks), voltage test points (+5 V), and critical signals (RESET, clock, chip-enable lines) are all explained.
=== Visual inspection ===


''Note: This guide does not cover ZX81 or ZX80-to-ZX81 upgrade kits.''
Examine for burnt or cracked components, corrosion, and dry joints — especially at the power jack, the regulator fixings, the IC sockets and the edge connector. Look for solder ''splashes'' as well as dry joints: a hairline splash measures a few ohms and produces convincingly 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: solder-splash faults, track cracks under sockets, piggy-backing technique, reverse-polarity damage, logic-probe practice.</ref>


== 🧪 Diagnostic Tools & Techniques ==
Cracked tracks '''underneath IC sockets''' are a recognised nuisance on this family, and they are invisible until you check continuity from the IC pin through to the next component.<ref name="tv2" />


=== Visual Inspection ===
=== Thermal checks ===
* Remove the top cover; examine for burnt or cracked components, corrosion, or '''cold solder joints'''—especially around the power jack, ULA, and edge connector.
* Re-flow or re-solder any suspect joints to cure intermittent power or I/O issues.


=== Thermal Checks ===
After a minute or so of running, feel the ICs. One package noticeably hotter than its neighbours is a strong suspect. Freeze spray is useful: if behaviour changes as a chip cools, that chip is worth changing.
* Gently touch (or use an IR thermometer) on chips after ~60 seconds of power-on.
* A '''too-hot-to-touch''' ULA or RAM IC often indicates an internal short.
* Use freeze spray: if behaviour changes while cooling/heating, that IC is likely faulty.


=== Power & Signal Probing ===
The regulator (IC22) is supposed to run warm. See "Things that are not faults" below.
* '''+5 V DC''' at multiple IC Vcc pins (e.g. Z80 pin 11, RAM, ULA).
* '''RESET''': low for a moment at power-on, then high (5 V).
* '''System clock''': logic-level 3.25 MHz at Z80 pin 6.
* '''ROM chip-enable''', '''RAM chip-enable''', and address/data bus activity.


=== Chip Substitution ===
=== Signal probing ===
Swap socketed chips '''one at a time''' with known-good parts:
* ULA (IC1) – highest failure rate.
* Z80 CPU, ROM, RAM.
* Observe orientation (notch/pin 1) and always power off before removal/insertion.


=== Piggy-back Testing ===
A logic probe is sufficient for most ZX80 work; an oscilloscope is needed for the clock and video sections.
* Press a good logic or RAM IC on top of each suspect chip (pins aligned).
* If symptoms change or machine boots, the underlying IC is bad.
* '''Warning:''' Won’t help if the bad IC is shorted; never piggy-back the ULA.


=== Minimal-Configuration Boot ===
{| class="wikitable styled-table" style="width:95%; text-align:left;"
You can power up '''without''' the following:
|+'''Primary checkpoints'''<ref name="zx80cd" />
* Keyboard membrane—system will boot, but no input.
! Point !! Expect
* RAM—system will display a blank or unstable screen, but should still show sync.
|-
 
| Positive end of C8 || Incoming unregulated supply, nominally 9 V DC
Remove one at a time; if the ZX80 suddenly boots, the removed IC was dragging the bus down.
|-
 
| IC22 (7805) output || +5 V
== 🔌 Power Supply & Voltage Checks ==
|-
 
| IC1 (Z80) pin 11 / pin 29 || +5 V / ground
Always verify the PSU '''before''' blaming main-board silicon.
 
{| class="wikitable" style="text-align:left"
! Test Point !! Expected Voltage !! Purpose / Notes
|-
|-
| Edge connector pin 7 (+5 V) ↔ pin 8 (GND) || +5 V DC (±5%) || Main logic rail
| IC2 (8332 ROM) pin 24 / pin 12 || +5 V / ground
|-
|-
| ULA pin 24 (Vcc) ↔ pin 12 (GND) || +5 V DC || ULA supply
| IC3, IC4 (2114 RAM) pin 18 / pin 9 || +5 V / ground
|-
|-
| Z80 pin 11 (Vcc) ↔ pin 29 (GND) || +5 V DC || CPU supply
| IC1 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
|-
|-
| RAM pin 8 (Vcc) ↔ pin 16 (GND) || +5 V DC || RAM supply
| IC1 pin 26 (/RESET) || Low briefly at switch-on via R21 (220 k) and C10 (1 µF), then high
|}
|}


=== Common PSU Faults ===
=== Chip substitution and piggy-backing ===
* '''5 V missing/high''' → no boot or chip death (>6 V destroys RAM/ULA quickly).
 
* '''Intermittent drop-outs''' via dirty power switch or cracked power-jack solder.
Swap socketed devices one at a time with known-good parts, powered off, watching pin 1 orientation.
* '''Ripple/hum bars''' = dried-out PSU capacitors.
 
Piggy-backing — pressing a known-good device on top of a suspect one with the legs sprung in — is worth trying on soldered-in logic. It is particularly effective against open-circuit chips and useless against shorted ones.<ref name="tv2" />
 
Where a track cut is needed to isolate part of the circuit, plan it so the cut will not end up hidden under a socket.<ref name="tv2" />
 
== Power supply ==
 
Check the supply before suspecting anything on the board.
 
* Connector: '''3.5 mm mono jack, centre pin positive'''. Nominal 9 V DC unregulated, about 14 V open-circuit. Sinclair's diagram marks the input "9 V – 11 V".<ref name="kio80" /><ref name="zx80cd" />
* A supply re-terminated with reversed polarity is a known killer of the regulator on this family. The regulator usually takes the damage and spares the rest of the board — but treat that as luck.<ref name="tv2" />
* Sinclair's guidance for the ZX81, which uses the same arrangement: below about 7 V on load the regulator will not function correctly, and drifting horizontal black-and-white stripes through the picture indicate a supply that is inadequately smoothed or sagging under load.<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]]. Used on this page where Sinclair published a procedure for the ZX81 and none for the ZX80; each such use is identified.</ref>
 
The adaptor is an ordinary mains transformer unit and can be opened and repaired. The primary side is at mains potential and the reservoir capacitor holds charge after unplugging — unplug, wait, and confirm the reservoir is discharged before working inside. Replace a perished or damaged lead rather than patching it; on the related Spectrum supplies a shorted lead has been known to set the rectifier diodes alight.<ref name="clinic">"Micro Clinic", ''Television'' magazine, 1986 — collected Sinclair repair notes, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: Sinclair power-supply lead failures.</ref>
 
== No picture, or unusable picture ==


Never run a ZX80 on an unstable PSU; modern regulated supplies are highly recommended (≥500 mA @ 5 V).
# '''Tune the television first.''' The modulator is set to UHF channel 36 but sweep the band in case it has drifted or been altered. With no signal from the board the modulator emits carrier only, and the screen shows almost nothing.<ref name="zx81man" />
# '''Check the +5 V rail''' at IC1 pin 11, then at the RAM and ROM.
# '''Check the clock''' at IC1 pin 6 and the oscillator round X1.
# '''Check /RESET''' releases high at IC1 pin 26.
# '''Check the modulator supply.''' The UM1233 takes +5 V and ground. On the ZX81, Sinclair's servicing note is that the tuning screw should sit about 3 mm down inside the former — worth knowing before assuming a modulator is dead.<ref name="tv2" />
# '''Check the VIDEO and SYNC logic outputs''' and the R30–R33 network feeding the modulator.<ref name="zx80cd" />
# '''Disconnect the keyboard tails''' and retry. The machine initialises without the keyboard connected.<ref name="zx81man" />


== ⚠️ “No Video” or “Garbage Video” Flowchart ==
== RAM faults ==


# Check PSU rails, RESET, and system clock first.
The standard machine has 1 KB: IC3 (2114) and IC4 (2114-1), each 1K × 4, together forming an 8-bit-wide kilobyte.<ref name="zx80cd" />
# Swap/feel ULA (dead clock = blank display).
# Replace/logic-probe '''ROM''' (statistically #2 cause).
# Swap CPU (Z80) '''only after''' ULA & ROM ruled out.
# Test/replace '''RAM''' (blank screen or random characters).
# Inspect video output transistor (TR1) and modulator.
# Remove keyboard membrane if stuck in reset.


== 💾 RAM Failures ==
Symptoms of a bad RAM are a corrupted or unstable display and lock-ups. Because the two devices split the byte, a failure in one affects only the low or high nibble of every location.


* '''2114 (1K x 4-bit) RAM'''—two chips for 1 KB ZX80, four for 2 KB.
Diagnosis is by substitution or piggy-backing. If the fault survives both RAM devices being changed, move to the address and data path — IC5 (74LS373) and the 74LS157 multiplexers IC6, IC7 and IC8.<ref name="zx80cd" />
* Expansion RAM packs connect via edge connector.


=== Symptoms ===
Sinclair published a RAM bit-to-IC fault table for the Spectrum but not for the ZX80, and none is invented here.
* Blank screen (lower RAM failure).
* Garbage characters, freezes, or random resets.
* One RAM chip hotter than neighbours.


=== Diagnosis / Fix ===
== ROM faults ==
* Swap suspected chip, or socket & replace.
* If still faulty after RAM swap, check address and data bus logic (74LS157, 74LS00, etc.).


== 📺 Video Output & ULA Faults ==
The ROM is '''IC2, an 8332''' — a 24-pin 4 KB mask ROM holding the BASIC interpreter and operating system.<ref name="zx80cd" />


{| class="wikitable"
Symptoms: no cursor with an otherwise live display, or a display of random characters.
! Symptom !! Likely Cause !! Notes
 
|-
If you substitute an EPROM, check the pinout carefully. 2532 and 2732 devices differ at pins 18, 20 and 21, and the Sinclair ZX Computers Archive keeps a comparison figure in its ZX80 technical section for exactly this reason.<ref name="kio2532">''2532 vs. 2732'', pinout comparison figure in the ZX80 technical section of The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: the pin differences between the two EPROM types.</ref>
| No video at all || Dead ULA, bad modulator, or no +5 V || Confirm ULA pin 24 = 5 V
 
|-
== CPU faults ==
| Rolling, unstable, or no sync || ULA or clock circuit || Check crystal, C4, C5, and associated logic
|-
| Black/white bars only || ROM failure or ULA || Try known-good ROM first
|-
| Weak/ghosted video || Faulty TR1 transistor or modulator || Replace as needed
|}


=== ULA Failure Signs ===
IC1 is a 780-1 — a Z80. CPU failure is uncommon but does happen after over-voltage or static damage.<ref name="zx80cd" />
* No video output, or random patterns regardless of ROM/RAM/CPU swaps.
* ULA runs very hot, or draws excessive current.
* Replace with same ULA type (early and late variants differ).


== 🧩 ROM Faults ==
* Clock present at pin 6 but the address lines static: suspect the CPU.
* No response to /RESET releasing at pin 26: suspect the CPU, after confirming R21 and C10.
* Before condemning it, confirm nothing else is holding the bus down. Removing expansion hardware and the keyboard is the quickest way to rule that out.


* 4K mask ROM (IC3) – contains BASIC interpreter and OS.
== Keyboard ==
* Early ZX80s use 2364 ROM; later ones may use 2716 EPROM.


=== Symptoms ===
The keyboard is a membrane matrix. Address lines A8'–A15' drive the matrix through diodes D3–D10; the return lines are read through IC10 (74LS365), and R13–R17 (47 k) pull them up.<ref name="zx80cd" />
* Blank screen with border, or random characters.
* No response to keyboard.
* System boots with cartridge/ROM swap but not with original.


=== Repair ===
* '''A whole row or column dead''' points at one membrane tail or one of its connector contacts, not at the board.
* Replace with known-good ROM or compatible EPROM adapter (e.g. 2732 with adapter).
* '''The whole keyboard dead''' — check the tails are fully seated and not twisted, then check the diodes and IC10.<ref name="zx81man" />
* Ensure correct ROM type for board revision.
* On the ZX81 Sinclair's own test is to bridge a contact on the small keyboard connector to one on the large connector and watch for a character. Shorting more than one contact does no damage. The same technique works on the ZX80's two connectors.<ref name="zx81man" />


== ⚙️ Z80 CPU Faults ==
Membrane tails fail by cracking where they bend. Withdraw them straight, never at an angle, and never fold one double.<ref name="tv2" />


Rare, but possible (over-voltage, ESD).
== Other logic ==


* Address lines static despite proper clock ⇒ dead CPU.
Twenty 74LS packages do the work that a ULA does on later machines: 74LS00, 74LS04, 74LS05, 74LS10, 74LS32, 74LS74, 74LS86, 74LS93, 74LS157, 74LS166, 74LS365 and 74LS373. The full IC-to-type map is in [[Sinclair ZX80 General Maintenance]] and on the circuit diagram.<ref name="zx80cd" />
* No data bus activity with working ULA/ROM ⇒ suspect CPU.
* Replace with Z80A or compatible.


== ⌨️ Keyboard Matrix Issues ==
Any of them can produce video, timing or bus faults. Substitution and piggy-backing are the practical tools; the circuit diagram tells you what each one is doing.


* Membrane cable tears or corrosion at connector.
== Edge connector and expansion ==
* Key rows or columns dead = membrane or PCB trace open.
* No keys work = membrane not seated, or ULA pin 14/15/16/17/18/19 failure.


Test continuity from each keyboard pad to ULA; repair or replace membrane as needed.
* Contact trouble at the edge connector causes more symptoms than anything else on the expansion side. Clean the fingers and inspect for pitting and bent contacts.
* Random resets with a RAM pack fitted are usually mechanical movement of the pack, not a board fault.
* Remember /ROMCS is absent from the ZX80 bus — ZX81-era hardware that expects it will not behave as documented.<ref name="kio80" />


== 🔠 Other Logic ICs ==
== Things that are not faults ==


* 74LS00, 74LS157, 74LS32, 74LS04—glue logic for address decoding, video timing, and bus control.
* '''The screen blanks while a program runs, and flickers on every keypress.''' The ZX80 cannot generate the TV signal and run your program at the same time. This is the machine working as designed; it is the reason the ZX81 gained SLOW mode.<ref name="kiodiff" />
* Failure can cause random boot, video, or RAM issues.
* '''The regulator runs warm.''' It should. Sinclair's guidance for the ZX81 is that a regulator which never warms up means the supply is not reaching it, and one that is unbearably hot is not properly bolted to its heatsink.<ref name="zx81man" />
* Piggy-back or swap with known-good chips to confirm.


== 🖧 Edge Connector & Expansion Issues ==
== Related pages ==


* No response from RAM pack or printer = dirty or bent edge connector.
* [[Sinclair ZX80]]
* Random resets with RAM pack = “RAM pack wobble”; secure with tape or support bracket.
* [[Sinclair ZX80 General Maintenance]]
* Check for shorts or broken tracks near the edge connector.
* [[Sinclair ZX80 Capacitor Replacement Guide]]
* [[Sinclair ZX81 Troubleshooting Guide]]
* [[Recommended Tools]]


== Final Notes ==
== References ==


* '''Start with power checks.'''
<references />
* ULA is statistically the most common failure, followed by ROM and RAM.
* Always fit sockets on replacement parts and monitor PSU health.
* Avoid static discharge when handling the ULA or RAM chips.


[[Category:Sinclair Computers]]
[[Category:Sinclair Computers]]
[[Category:Sinclair Troubleshooting Guides]]
[[Category:Troubleshooting Guides]]
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