Editing Sinclair ZX Spectrum 128 Troubleshooting Guide (section)

== RAM faults ==

RAM is the most common electronic failure on a surviving 128.

* All devices must be '''4164, 150 ns, 128-row refresh'''. Mixing speed grades or fitting slower parts causes instability that looks like a logic fault.<ref name="sm128" />
* '''Shorted RAM pulls down the 5 V rail.''' Powering the board from a '''current-limited bench supply''' is the fastest way to find it: the board goes straight into limiting and the offending devices get hot. Remove the hottest first; it is common to find several.<ref name="tyne">Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2017/06/spectrum-128k-2-grey-repairs.html "Spectrum 128K +2 Grey Repairs"]. Source for shorted-RAM behaviour on current-limited supplies, diagnostic ROM practice, RAM socketing and the ZTX651 substitution — the +2 shares the 128's architecture.</ref>
* After the rails are proven, feel each RAM device. '''None should be warm.''' A warm RAM chip is a failing one.<ref name="ian" />
* '''Diagnostic ROMs are far quicker than probing.''' Brendan Alford's ZX Diagnostics runs from the ROM socket and reports failing data bits with beeps and coloured bars even when the display RAM itself is faulty; Paul Farrow's 128K RAM tester then exercises all eight banks.<ref name="tyne" />
* Where several devices in a bank have failed, '''socketing and replacing the whole bank''' is usually faster and kinder to the board than chasing them one at a time.<ref name="tyne" />

The 48K machine's POKE-based RAM location test also works on a 128 running in 48 BASIC and will identify a failed device in the lower 16 K — see the [[Sinclair ZX Spectrum+ Troubleshooting Guide]] for the table.<ref name="ian" />