Repair Log Missile Command

Problem:

Black screen, plays blind

Examining L7 a 82S25 ram

Looking at the color ram with a scope shows the unlatched colors are not
changing (5:7:9:11)

The BDx inputs (4:6:10:12) appear good, and !COLRAM (3)is signaling

Ax inputs are not changing (1:14:15)

Backing up a chip to M7 74LS157

VBlank is good (1)

COLNUM2 (5), COLNUM1 (14) and COLNUM0 (2) are not signaling.

Backing up to N7 to track COLNUM2 74LS195

COL10-13 are signaling (4:5:6:7)

!5MHZ (10) is good

LD2COL (9) is stuck

Checked M6 LD2COL (9) also stuck, and P6 LD3COL (15) stuck

Backing up to the source for these two signals C7 74LS175

3D (12) and 4D (13) inputs are signaling

1D (4) is signaling and 1Q output is working

5MHZ (9) clock is good

Piggybacked chip onto it no change

Attached my B&K logic comparator to it, shows 10,11 & 14,15 as bad.

Replaced chip, full function.

Resolution:

Replaced 74LS175 @ C7

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Received on Sun Apr 7 04:42:57 2013