The IC3 – 74LS157N – is for the lower 16K of RAM, a bad chip or trace will usually manifest itself as corrupt graphics on the screen
A modern replacement can be found here:
| Shop | Link |
|---|---|
| RetroRevivalShop | https://retrorevivalshop.co.uk/index.php?main_page=product_info&products_id=35 |
| Retroleum | https://www.retroleum.co.uk/zx-spectrum-chips |
Using a multimeter check the traces for IC3, the the diagram below for chip pin position
| Pin | Traces to | Voltage |
|---|---|---|
| 1 | IC4 Pin 1 | |
| 2 | IC2 Pin 2 | |
| 3 | IC25 Pin 3 | |
| 4 | R22 Right | |
| 5 | IC25 Pin 5 | |
| 6 | IC25 Pin 6 / IC5 Pin 6 | |
| 7 | R21 Right | |
| 8 | IC25 Pin 8 / IC5 Pin 14 | |
| 9 | R20 Right | |
| 10 | IC25 Pin 10 | |
| 11 | IC25 Pin 11 / IC2 Pin 40 | |
| 12 | R23 Right | |
| 13 | IC25 Pin 13 / IC2 Pin 35 | |
| 14 | IC25 Pin 14/ IC5 Pin 26 | |
| 15 | IC25 Pin 15 / IC5 Pin 14 | |
| 16 | IC25 Pin 16 / IC5 Pin 1 |
END