Hardware Weekend – Modding a Spectrum

This weekend rather than fiddling around with software of some sort I decided to get the old soldering iron out and have a go at a job I’ve been meaning to do for ages. I have a sinclair ZX Spectrum which of course under normal circumstances uses a RF signal with my TV. This works ok, but is often fuzzy and has interference. So I decided to perform a simple mod in order to bypass the RF modulator on board. I had previously replaced the keyboard membrane because the old one had failed.

ZX Spectrum mainboard

ZX Spectrum mainboard

Fortunately there are many websites around the web that tell you exactly how to do this. So after dismantling my speccy, and reading up about what to do I began by desoldering the wires of the RF modulator. These seemed to be harder to get out than I had anticipated partly because they seemed to be soldered in from both sides of the board and my solder sucker didn’t seem to be very effective.

RF Modulator

RF Modulator

I did however eventually get the wires disconnected and the board ready. I soldered in the new wire and prepared to test it out. The TV display was wonderful and clear, but unfortunately it seems my poor speccy has acquired a new problem since I last used it.

Mod Complete!

Mod Complete!

The display appears to be corrupted and I believe the CPU is hanging, after a bit of cleaning of the socketed chips I think I’ve narrowed down the problem to the display ram chips that are socketed. The ULA and other parts of the board all appear to work fine within the limitations of what I can test, and swaping the socketed ram chips seems to merely change where the corruption is on screen.

Image clearer - but corrupted

Image clearer – but corrupted

The main problem is sourcing the replacement chips! Video memory on the speccy is in the form of some 4116-n2 chips that are notorious for failure. Looking around online in Australia has turned up no obvious source of getting new chips. I may have to find a similar chip (say a 4164 or something similar) to replace it, but that also will be a difficult task of comparing data sheets. My best hope at the moment is to find another speccy for parts (or working) in order to repair this one.

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