🇬🇧 Sinclair ZX Spectrum: Part 4 (Troubleshooting Begins) [TCE #0491]
26Work continues on this Sinclair ZX Spectrum (Issue 2 board) that came in a Fuller FDS case with a ZX Interface 1.
Unfortunately it seems to be deteriorating ... it was originally fine, then I fiddled with it and it was still okay, then the text became corrupt (same corruption between re-boots), and even the diagnostic ROM had similar corrupted text.
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From Part 3:
Ivan very kindly donated one of his 128KByte RAM expansion boards so I am looking forward to trying that out - my first package from 🇷🇸 Serbia! In order for it to work you need to disable the upper 32KBytes of internal RAM, which I've done by adding a switch to inhibit the !CAS signal for that bank of RAM (make one OR gate input on IC23 always HIGH by connecting to +5V thus forcing the output HIGH, thus disabling !CAS).
I've verified the switch works by executing "PRINT PEEK 23732 + 256 * PEEK 23733" : I get 65535 (full 48KBytes), or 32767 (upper 32KBytes disabled).
However, after this it seems things started to go awry (nothing to do with Ivan's board).
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Continuing on with today's experience ...
But now it's not even booting - it did initially and then failed permamnently.
The three voltages for the 4116 DRAMs look okay ... ±5V and +12V.
Some address lines are entirely quiet. And all the data lines appear to be a mess. The clock signal looks okay.
Swapping the original ULA (6C001E-6) with vRetroDesign's vLA82 makes no difference, nor does swapping the ROM or using a cartridge.
The Z80 isn't socketed so I can't easily try replacing that.
Ivan's 128KByte RAM expansion that he kindly sent me:
https://github.com/goloskokovic/zx16to128upgrade
Tynemouth Software's always excellent repair blog (Dave):
http://blog.tynemouthsoftware.co.uk/2023/08/zx-spectrum-issue-2-future-proofing.html
Composite modification:
https://spectrumforeveryone.com/technical/composite-mod-for-the-48k-range/
vRetro for replacement ULAs & other goodies (Charlie):
https://www.vretrodesign.com
ZX Interface 2 cartridges:
https://github.com/mosaicmap/ZXS_I2_kartridz
Music by Karl Casey @ White Bat Audio
"Disorder" on White Bat 37
https://karlcasey.bandcamp.com