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Sinclair ZX Spectrum +3 128K

Introduction

Development now was at a much slower pace and the next Sinclair machine, the Spectrum +3, didn’t appear until 1987.  This had Amstrad's own 3” disk drive from the CPC6128 built in and an upgraded version of BASIC complete with a new operating system, +3 DOS (developed by one of Amstrad's favourite development partners - Locomotive Software Ltd).

It sported a completely redesigned motherboard based around the new Amstrad +3 4.1 ROM model with a much reduced chip count and two more 16K ROMs. 

One held the second part of the reorganised 128 ROM and the other contained the +3's disk operating system (+3 DOS).  The two new ROMs and the original two 16K ROMs were physically combined into two 32K chips. 


The keypad scanning routines were also removed (not used since the days of the Spanish Spectrum+ 128).

Bank switching of memory was improved allowing the ROM to be paged out for another 16K of RAM at the bottom of the address space to permit the use of CP/M.

However, several lines were removed from the expansion bus edge connector which caused many problems with existing external devices.  Memory timings changed which caused problems with high speed colour changes , the reading of I/O data caused problems with some games, as did the ROM tweaks.
All of this little lot caused many more incompatibilities with older software - even some written specifically for the 128K machines!

On the positive side, the Amstrad FD-1 external drive could be used directly with the +3 by just plugging it straight into the "Disk B Port".  A standard parallel printer could also be connected directly making this model the ultimate Spectrum.

The new model didn’t sell well though and the +2 remained the punters choice of Sinclair machine. It was difficult to get software for the +3 (90% of the disk versions of games were simply 48K games transferred to disk) and even more difficult to transfer tape based games to disk (unless you had a Multiface 3).  Maybe for this reason an updated version of the +2, the +2A also appeared later in 1987.  The main board and ROM were very similar to the +3 and it continued selling well beyond the +3 into the early 90’s.

All in all though, the Spectrum +3 it was launched too late, just as the far superior 16 bit Atari ST and Commodore Amiga were fighting for people's money. 

It was the last official Sinclair Spectrum model ever made.

Technical Details

  • CPU: Z80A
  • Speed: 3.5MHz
  • ROM: 64KB (32K for 128K mode, 16K for 48K mode, 16K for +3DOS)
  • RAM: 128KB (8 x 16K banks)
  • Full-stroke 58 key Keyboard (same layout as Spectrum+)
  • Colour Graphics
  • Sound - 3 Channels, 7 Octaves (Yamaha AY-3-8912)
  • Expansion I/O, RGB, RS232/Midi, Printer port, Disk B port, 2 x Joystick ports
  • 3" Hitachi Floppy Disk Drive - single sided
  • Local price at launch: £249

Problems and Issues

Disk Drives
With Amstrad's production techniques, the Spectrum +3's were pretty robust machines and are very reliable, even now.

However, 90% of all +3s purchased from lofts, car boot sales or eBay will have non-working disk drives.  The drive belts completely self-destruct over time and very often turn into a sticky gooey mess all around the drive motor capstan.
This takes time and patience to remove (but must be done completely) and a new belt must of course be fitted.  There is a comprehensive guide on how to do this in my repairs section.

Unfortunately, many have had their knackered old drive belts replaced by any old elastic band prior to them being offered on eBay as "tested and working".  Whilst this might well work for a short period, often these "bands" are too tight and I have had many drives sent to me for repair with the 25 year old motor bearings damaged irrepairably.

Dirty read/write heads, dried out worm drives and missing read/write pins (see the drive repair guide for details of what these are) are also common, but repairable faults.

Sometimes a fault is present characterised by a machine being able to format a disk, but then the CAT command not be able to CATalogue it and display its content.  The error message "Missing address mark" may be reported.
This is usually caused by a head alignment problem which is very difficult indeed to cure (usually impossible without an oscilloscope).  If you have this problem, send the drive to me and I'll do my best with it, but the problem is usually terminal.

Finding replacement drives now is just about impossible, so make sure you look after the one you've got!

Tape / Sound Socket

Another common problem is a damaged tape/sound socket.
Damage is usually the result of years of loading games from cassette and over-zealous removal of the cassette leads.

These are fairly easily replaced if you're good at de-soldering and re-soldering and I usually have plenty of new sockets.  Email me if you need one, or check the shop.

I may put a guide to replacing them in the Repairs section in the future if there is a demand for it.

Built-in Test ROM
If you have random, or intermittent faults with your +3, the built-in ROM test routines can help to diagnose the problem.




To start the test suite, hold down the [BREAK], keep it pressed, then press and release the reset button on the left side of the +3.  This will bring up the colour and sound test screen which is detailed in the manual.

 


Whilst the screen is displaying the test card and is still beeping, press the following keys simultaneously:

[Q] [A] [Z] and [P] [L] [M]

This is fairly easy to do as the keys are down the left and right side of the keyboard.

A set of brief instructions will now be displayed, including a warning that disks will be corrupted.

You will need a formatted, blank disk later in the tests.

If any of the tests fail, then special equipment will be needed for an accurate diagnosis and this of course is no longer available, but it will at least give an idea of what components are faulty.

When you get to the disk drive speed test, don't be alarmed that the test fails.  This test routine was designed for new machines and drives (even then many failed) and an figure below 1% is fine.  A variance higher than 1% is usually an indication of a worn drive belt (or an old sticking disk).

 

 

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