Editing Sinclair ZX80 Troubleshooting Guide

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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.]]

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>

'''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:

* '''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>

== Diagnostic techniques ==

=== Visual inspection ===

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>

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" />

=== 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.

The regulator (IC22) is supposed to run warm. See "Things that are not faults" below.

=== Signal probing ===

A logic probe is sufficient for most ZX80 work; an oscilloscope is needed for the clock and video sections.

{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''Primary checkpoints'''<ref name="zx80cd" />
! Point !! Expect
|-
| Positive end of C8 || Incoming unregulated supply, nominally 9 V DC
|-
| IC22 (7805) output || +5 V
|-
| IC1 (Z80) pin 11 / pin 29 || +5 V / ground
|-
| IC2 (8332 ROM) pin 24 / pin 12 || +5 V / ground
|-
| IC3, IC4 (2114 RAM) pin 18 / pin 9 || +5 V / ground
|-
| IC1 pin 6 || 3.25 MHz CPU clock, from the 6.5 MHz oscillator X1
|-
| IC1 pin 26 (/RESET) || Low briefly at switch-on via R21 (220 k) and C10 (1 µF), then high
|}

=== Chip substitution and piggy-backing ===

Swap socketed devices one at a time with known-good parts, powered off, watching pin 1 orientation.

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 ==

# '''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" />

== RAM faults ==

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" />

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.

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" />

Sinclair published a RAM bit-to-IC fault table for the Spectrum but not for the ZX80, and none is invented here.

== ROM faults ==

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

Symptoms: no cursor with an otherwise live display, or a display of random characters.

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>

== CPU faults ==

IC1 is a 780-1 — a Z80. CPU failure is uncommon but does happen after over-voltage or static damage.<ref name="zx80cd" />

* 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.

== Keyboard ==

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" />

* '''A whole row or column dead''' points at one membrane tail or one of its connector contacts, not at the board.
* '''The whole keyboard dead''' — check the tails are fully seated and not twisted, then check the diodes and IC10.<ref name="zx81man" />
* 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" />

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

== Other logic ==

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" />

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.

== Edge connector and expansion ==

* 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" />

== Things that are not faults ==

* '''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" />
* '''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" />

== Related pages ==

* [[Sinclair ZX80]]
* [[Sinclair ZX80 General Maintenance]]
* [[Sinclair ZX80 Capacitor Replacement Guide]]
* [[Sinclair ZX81 Troubleshooting Guide]]
* [[Recommended Tools]]

== References ==

<references />

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