Sinclair ZX Spectrum +2A and +2B

Introduction

The Spectrum +2A replaced the previous Spectrum +2 model in 1987 (actually coming to market just after the Spectrum +3).  The case was restored to the traditional Spectrum black making it easily distinguishable from the old grey model.

Although it kept the same name, it sported a completely redesigned motherboard based around the new Amstrad +3 4.1 ROM model with a much reduced chip count.

Essentially the +2A was a Spectrum +3 (as can be seen from the reference to +3 BASIC in the start up menu) with a cassette deck replacing the disk drive and so had two more 16K ROMs.  One held the second part of the reorganised 128 ROM and the other contained the +3's disk operating system which went unused of course on the the +2A and +2B.  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 (on the Spectrum +3).

Several changes were also made on the bus and of course all this little lot caused many more incompatibilities with older software - even some written specifically for the 128K machines!

The Spectrum +2B did have some minor revisions over the +2A (the main board was at Issue 4 for the +2B), but the main thing of any significance was a move of manufacture to from Hong Kong to Taiwan.

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)
  • RGB, RS232/Midi
  • Local price at launch: £200


Problems and Issues

The cassette mechanism on the +2A and +2B models was superior to that used on the earlier grey +2 model and generally more reliable, however, the drive belt does need changing as it suffers the same "kink" and stretching with age as its predecessor.  See the Spectrum +2 Technical Information page here.
Similarly, the read and write heads will almost certainly need a good clean with alcohol and a chamois stick.
The +2A and +2B models only have one belt however, which is identical to the large belt on the Spectrum +2.

Programs will not load at all
Another problem which I see more and more frequently with the +2 and +2A units is a complete failure to load a program and no visible loading border or loading sound.
This can be a faulty play head, but most often the problem is temporarily cured by sharply banging the spectrum sharply again the table it is sat on.  The loading border then appears and the usual sound can be heard.  Pressing down with your thumb on the case at the bottom left of the cassette flap during loading also often temporarily cures the problem, but standing there with your thumb aching for 10 minutes is hardy a cure...

The problem is due to a "dry" solder joint in one or more of the soldering lines on the cassette drive controller board - usually the joints for the red and white wires which connect the read and write heads. 

 

The standard of soldering on assembled items like the cassette on these machines was variable to say the least, and at the point where the leads from the play head meet the cassette drive controller board, the solder is a dull grey colour, rather than the shiny metallic colour it should be.  Poor solder technique using insufficient flux often causes this "oxidation" effect and over time, the joint breaks down and electrical contact is lost.

To do this, add a little flux with a fine children's paint brush to the grey solder blob, then hold the tip of a soldering iron against the poor joint.  The solder will melt fairly quickly - take away the iron and the solder will set again, usually repairing the joint.  Remake the joints for both the red and white leads as shown in the photograph.

 

Crackling interference
A static-like, noisy crackle heard through the TV speaker during loading is a similar issue to the dry joint problem described above.
In this case, the dry-joint issue is still there, but the joint hasn't broken down completely.  However, it is still usually enough to badly impair loading performance.  Again, try pressing down on the case again around the cassette unit whilst loading - the crackle may stop, or more likely get worse.

The cure is the same, BUT, the sound may be coming from other solder joints on the cassette controller board.  The "crackle" problem seems to be more of an issue on the +2A boards rather than the +2's's - possible because the +2A, which has more soldered cable joints. 

If you have this problem, re-make the joints shown on the photograph to the right.

Fuller instructions and guidance are available in the Repairs and Servicing section for the Spectrum +2 models here.


If the cassette drive does prove to be faulty, don't be fooled by the the misleading labelling of the "Tape / Sound" port at the back of the +2A.  This suggests that an external cassette player can be connected via this port in the same way as it can with a Spectrum +3.  It is not the case - the port on the +2A (and +2B) is wired only for sound out on the main board - it will not accept sound in.  There are ways around this with a hardware hack if you have decent soldering and de-soldering skills and I may cover this in a future article in the "Repairs" section.


If you do try and find that you have damaged the Sound socket, it is identical to the +3 socket and a replacement is available in the Repairs & Servicing pages of the Sinclair Sales section of the website.

 

Disk Drives
Don't expect to be able to connect an Amstrad external FD-1 or DDI-1
 to the back of the computer as the interface advertised by Amstrad before its launch was never implemented.  The external interface which was available for the Amstrad CPC464 is not compatible either, so there is no way to attach a 3" disk drive to this model.

All is not lost though if you're desperate for a disk drive - the Spectrum +3e ROMs from Garry Lancaster will work with the +2A which (with a suitable interface such as the DIVIDE or DIVMMC units) will allow you to connect a standard density IDE 3.5" disk drive, or even a hard drive. 

Built-in Test ROM
If you have random, or intermittent faults with your +2A, 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 +2A.  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 warnings that disks may be corrupted during testing!  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, you will be informed that there is no disk interface fitted - true of course, as there is no disk drive but proof again though that the +2A , +2B and the +3 shared the same ROMs.