Editing Sinclair ZX80 Troubleshooting Guide (section)

== 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 || 3·2 MHz, as Sinclair's diagram labels it — the 6.5 MHz oscillator X1 divided by two, i.e. 3.25 MHz
|-
| 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" />