Editing Atari Mega ST Troubleshooting (section)

=== U2 (74LS07) — Most Common Failure ===

The 74LS07 hex buffer at U2 drives both /RESET and /HALT to the 68000 CPU. The same IC also serves the MIDI In/Out ports; external voltage spikes on MIDI lines (not fully opto-isolated on some lines) can damage U2, causing one or both outputs to fail.<ref name="atari-forum-repair"/>

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:80%; text-align:center;"
|+ '''U2 (74LS07) Pin Functions'''
|-
! Pin !! Function !! Connected To
|-
| 1 || Input A (gate 1) || U1 reset delay output (active-high)
|-
| 2 || Output A (gate 1) || /RESET line to 68000 pin 18
|-
| 3 || Input B (gate 2) || Tied to pin 1
|-
| 4 || Output B (gate 2) || /HALT line to 68000 pin 17
|-
| 5–12 || Gates 3–6 || MIDI I/O circuitry
|}

'''Diagnosis:''' With the machine powered, check that pin 1/3 go high after the reset delay. If the inputs are correct but pin 2 or pin 4 remain low, U2 has failed. Desolder and replace with a known-good 74LS07.