Spectrum DIY upgrade to 48K

Question

The ZX Spectrum was initially sold in 16 and 48K versions. Every owner of the former version who continued using it, inevitably ended up wanting the upgrade to the latter. (I get the impression it wasn't long before published games routinely required 48K.) This involves installing eight additional RAM chips (4164 half-bad). In all cases I have read about, this involved shipping the machine back to Sinclair.

https://rudedogretros.co.uk/product/spectrum-16k-boxed-with-power-supply-and-manuals/ shows a picture of the mainboard, which seems to have eight empty sockets installed, so it would just be a matter of opening the case and plugging in chips, no soldering needed:

Now, many non-technical consumers would still not feel up to that, but was it a thing some people did? Was it officially acknowledged as an option?

Answer 1

If you had one of the 60,000 (or so) issue 1 boards then memory expansion was a little different, as issue 1 boards are, essentially, just 16 kB boards, with no room for expansion - that is to say, there weren't any empty RAM IC sockets waiting to be populated.

A special daughter board was required, from Sinclair themselves (although there were probably third parties that sold similar kits):

^{Image source: 32K RAM pack for Issue One Spectrum}

Note that this solution is not compatible with issue two (or onwards) boards, which did have empty RAM IC sockets.

On the daughter board, note that there are two "up-ended" 16 pin sockets (with turned pins) - one on the left hand side and the other towards the right hand side, respectively. The board would be flipped over and the pins of these two sockets plugged into the two sockets marked 16 pin EXT (highlighted in red) on the issue 1 board layout diagram below:

^{Image source: ZX Spectrum Issue 12 - Schematics1}

Note that the right hand socket seems to be labelled 14 pin EXT - I am not sure if that is an error, or a handwriting issue, or whatever.

These two sockets appear to have a sole purpose of providing RAM expansion, as they are usually unpopulated in the 16k models.

This board was fitted towards the rear of the case, like so:

^{Image source: Sinclair ZX Spectrum 16K & 48K}

However, whether this "daughter board" was a part that was freely available from Sinclair, which could be fitted by the user, or whether the unit had to be returned for a factory install, is unclear.

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I have found an early ad for a 32 KB RAM upgrade, from a third-party supplier (Downsway), that looks suspiciously like the Issue 1 daughter board above, in Your Computer, 1982/11², page 124, which cost £42.50+£2 (p+p)

Downsway first offered this Spectrum RAM expansion in the September 1982 issue of Your Computer. Prior to that, Downsway ads only offered ZX81 RAM expansion packs.

However, in the January 1983 edition of Your Computer, on page 77, Downsway also start offering issue 2 board memory upgrades, that appear to be just a bunch of loose ICs:

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¹ From ZX Spectrum PCB Schematics and Layout ² Your Computer archive on the retro 8-bit computers site

Answer 2

The only thing I can add is that I have seen this upgrade documented in Elektuur (Dutch Elector) magazine (most probably). The additional 32kx8 memory was added on top of the original 16kx8 chips, and some of the pins needed to be bent, so that CS worked not for both banks at the same time. I don't think any additional circuitry was needed. I suppose that A15 was used to switch between the two banks. It seems the most logical.

However, it is a long time ago. I think I saw this article before I bought my ZX Spectrum in 1984, and decided to go immediately for the 48k version (hah, this year in September it is 40 years since I bought my first computer).

Answer 3

I had a 16K Spectrum, bought the additional chips and plugged them in to upgrade to 48K. I don't remember having to bend any pins, but it was a long time ago!

Answer 4

I just opened my ZX spectrum 48K, it is an Issue 4B (D01-249720).

Seems that some IC's need to be added to the empty sockets.

p.s. Ignore the wires...I replaced the broken membrane with micro switches (and now the keyboard clicks).

Answer 5

There was the Indescomp external 32 RAM pack, from Spain.

• Name: Indescomp 32K Ram Pack
• Manufacturer: Indescomp S.A. (MHT ingenieros)
• Year: 1983/1984
• Type: RAM/ROM
• Price: £39.95
• Blurb: Upgrade 16K Speccy to 48K

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As a side note, there might not be any empty IC slots, on an Issue 2 (or beyond) board:

There is anecdotal evidence that 48K machines that failed factory testing with faulty upper RAM were packaged and sold as 16K models. Open the machine up to check first – if the upper RAM is present and not socketed, then a repair of the faulty RAM is more appropriate, and will be covered in another article.

Source Upgrading a 16K Spectrum to 48K.

What isn't clear is whether an external RAM pack would work, in this case. Would it "switch out" or "page out" the bad 32k RAM?

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References

• Spectrum Hardware Index - I
• Indescomp 32K Ram Pack
• External expansion 32K
• Upgrading a 16K Spectrum to 48K