Sunday, 9 November 2014

ZX Spectrum Plus Keyboard Restoration

I seem to be doing quite a lot of keyboard cleanups recently, the Pet, C128 and BBC that I've written up, and the usual collection of Commodore 64's and Spectrums I seem to do quite regularly. This is one such Spectrum, and it strikes me just how many parts there are in the thing.
So you start with you standard Spectrum Plus, complete with 30 years of dust and dirt, and in this case, missing a key. When opening these up, the membranes tails tend to be very fragile, and if they haven't already failed, opening the up without due care and attention tends to snap them. This seems to have happened to most of the 'untested' ones on ebay, usually just before or just after they went up the loft.
This is one of the better ones, these clear membranes seem to be a lot more resilient than the brittle opaque ones.
With the tails carefully removed, you get to the main board. This is the same board as used in the original rubber keyed ZX Spectrum, with the addition of the reset switch (wired across a capacitor in the reset circuit). The big lump of metal at the top is the heatsink.
The keyboard is stripped down and the keys washed and cleaned.
The case is also given a clean up.
With the keys drying, they do seem reminiscent of a certain game.
Maybe a new logo?
Maybe not. However, as a side note, when designing a new TARDIS console for the 20th Anniversary Doctor Who story in the 1983, they did the reverse of this, and stuck on upside down scabble tiles to look like buttons.....
The process of putting this back together shows the number of layers involved in the Spectrum Plus's keyboard.
So first you have the case, then the keys, then the plungers, slowly being repopulated (and the broken one replaced).
Once they keys are all in place, there is a rubber mat.
After that comes the membrane.
This is a multilayer membrane, and the layers need to be squeezed together by clamps. Sometimes a keyboard which doesn't seem to be working can be fixed by tightening these up. Although usually it means a visit to SellMyRetro to buy another RWAP replacement membrane.
After the membrane, comes a sheet of cardboard, or sometimes plastic.
And after that a sheet of metal and then it can finally be screwed in place.
With everything all put back together, the final product is a much nicer looking Spectrum Plus.
If the Spectrum is going to be used a lot, I recommend replacing the voltage regulator with a modern switch mode device. I usually have a number of refurbished computers available, contact me if you are looking for one.

ZX Spectrum Voltage Regulator Replacement

One of the things the Sinclair ZX81 and ZX Spectrums were known for was getting rather warm. If you (carefully) open one up, you'll see where the heat comes from.
This is a Spectrum Plus, but it is the same board as used in the original rubber keyed ZX Spectrum. The big lump of metal at the top is the heatsink.
The original circuit uses a 7805 linear regulator. The input for the Spectrum is marked 9V, although most of the Sinclair supplies give around 12V output. The current used depends of the version of the ULA, the manufacturer of the RAM etc, but is around 650mA. Original Spectrum's came with UK700 model supplies (700mA), later ones with UK1400, 1.4A supplies. The regulator is rated 1A, and generated 5V used by the chips on board. The -5V and +12V used by the RAM and video circuits are also derived from the input supply. This is a purely linear shunt regulator, so 5V at 1A is used by the board and eventually converted to heat by the chips (usually the ULA gets the hottest unless something is wrong). The rest, up to 7V and 1A is converted to heat in the regulator, so there could be 7W of heat dissipated by the heat sink, hence why these things get hot. This works out about 40% efficient.
This is a Traco Power TSR 1-2450 switch mode replacement for a 7805. It takes in the DC in and generates 5V DC out using a switch mode power supply design. This is around 90% efficient, and so generates little heat. It also puts less strain on the power supply unit as it draws around half the power of the original for the same output. (7805, 12V @1A to get 5V @ 1A, TSR1-2450, 12V @ 400mA to get 5V @ 1A). The same Spectrum that used about 650mA dropped to 400mA with the new regulator.
These modules are drop in replacements, so just remove the 7805 and metalwork, and install the switch mode unit. You now have a much cooler running Spectrum.
Usually a good idea at this point to reflow the solder on the DC jack as they can start to become dry joints over time. Even replaced the jack if the connector is tarnished or worn looking.
The same approach can be used on many other devices of that era which use linear regulators and big heatsinks. The ZX81 and Spectrums, the Atari 2600, the NES, most of the Commodore computers etc.
I use one on my Spectrum test rig as removing the heatsink also gives easier access to the chips behind.
Since this was used without a lid, I used the hole where the heatsink had been bolted to add a power LED.

Saturday, 1 November 2014

USB to 1541 Adapter

Most 1980's home computers that had disk drive options used an internal disk controller with external drives attached under the control of the computer. The BBC, Spectrum and Apple were like this. Commodore and Atari disk drives are rather different. They are in fact computers in their own right, connected via a serial bus. So in modern terms, more like a network server that a USB hard drive.
The most popular of the Commodore drives is the 1541. These things are about the size and weight of a concrete breeze block, mainly due to the internal linear power supply.
Inside there is a 6502 processor, two 6522 VIA's, 2K RAM and 16K ROM. This has a custom drive controller chip, rather than the VID video and sound chip, but otherwise not for off the spec of the VIC20 computer. The serial bus connection has a rather useful benefit, without too much work, the serial protocol can be implemented on another computer, giving full access to the drive.
This was done many years ago on the PC, and simple parallel port adapters gave access to the drives. I've build a few versions of the X cables over the years. This is quite a nice little XA1541V2 board I picked up this one on ebay a few years ago. It has transistor drivers to boost the drive capabilities. The only problem with these is the lack of a parallel port on modern computers. I have an old XP laptop I keep for this sort of task, which has built in parallel and serial ports.
I have a few disk drive projects coming up, so I wanted to get this up and running again, ideally on my main development computer, without a parallel port. I looked at various options, including a pci express parallel card which didn't work too well, it seems the older the better for this task. I also looked at USB-parallel options, then thought why not cut out the middle man and drive it direct. Turns out, this too has already been done. xum1541 is a USB to 1541 interface, and ZoomFloppy is an nice implementation of that. I don't know if these are still current, these seem to date to 2011. All the information is there, so I set about building one.
This chip used, an ATmega32U2 is the same foorprint as the chip I used on my smaller USB keyboards, so I adapted one of my ZX81 boards for this purpose. It had all the USB interface and the crystal etc, so that was the USB side sorted. The firmware was loaded using the Atmel Flip interface, as I think was the plan with xum1541.
The IEC side is similar to the output stage of the Commodore computers it was designed to talk to, so in a slight change to the design, I used a 7406 chip. The red LED is activity, I later added a green power LED on the right.
Those two sandwich together to implement something like the xum1541. The design can also include a parallel connection to the 1541 drive to speed it up, I'm not adding that at this stage as I don't mind it running a little slower. There is also the option to talk to IEE488 drives, which is something I'm going to be looking at next, I have an 8250 drive to refurbish to go with the Pet.
That all fits nicely into a small box, firmly held in place with too much hot melt glue as usual
With that complete, the 6 pin IEC cable connects to the drive and the USB side to the PC. Time to fire it up. As usual with IEC devices, best to connect everything up, then power on, drive first then computer. A good way to fry the 7406 chips is to plug the drive into a Commodore 64 when powered on. In this case, connect to the drive, power on the drive, then plug in the USB.
The software I used with the parallel cable was a collection of command line utilities, opencbm. These are still there, and still work. This is cbmctrl, which is used for sending commands to the drive such as reset, status and directory.
But things have progressed, and there is now a nice user interface which uses these programs behind these scenes. This is CBMXfer.
Very nice to use, this also ties in a number of other tools such as d64 disk image creation (disk to image and image to disk). There are also tools to view source files, with the basic tokens correctly displayed.
And integration with vice, the commodore emulator, so you can run the files you have just transferred or are about to transfer to check them out.
There are a number of elements involved in this process, and this is the work of many different people. All credit to them, and I am grateful to them all for making it available, it's going to be very useful.

Sunday, 26 October 2014

Commodore 128 Repair - Part 4 Commodore 128 Modes

Having completed testing the Commodore 128, it's time to run through a few of it's features.
It's a bit of a strange beast the Commodore 128, two CPUs, two video processors, four types of memory, three operating systems and two versions of BASIC. Not bad for a single computer.
The default mode on power on is Commodore 128 mode. This has 128K RAM and BASIC V7.0 and uses the 8502 processor, based on the same 6502 as the Commodore 64's 6510 and the 264 series 7501. The default is 40 column mode, it can also work in 80 column mode, but you need to connect the monitor to the 9 pin D connector, rather than the normal 7 pin DIN.
If you type GO64, or hold down the C= key on startup, it goes into C64 mode.
This is limited to 40 columns, and 64K of RAM and runs BASIC 2.0, as did the Commodore 64, again using the 8502 processor.
The third mode, uses the Z80 processor to run CP/M. To use this, you need a CP/M system disk in a 1570 or 1571 disk drive. I don't have one of those at the moment (watch this space), so I'm loading the disk image from an SD2IEC.
This loads up (after a while) into CP/M version 3.0.
It also works in 80 column mode.
That's quite a lot of options.

  • 40 Column Commodore 128 mode
  • 80 Column Commodore 128 mode
  • 40 Column Commodore 64 mode
  • 40 Column CP/M mode
  • 80 Column CP/M mode

I wonder how many C128 owners used all the modes?

It seemed compulsory in the Commodore Pet repair articles to show space invaders, so why not continue that, here in an 80 column Commodore 128 version.


Saturday, 25 October 2014

Commodore 128 Repair - Part 3 Testing

This is the third part in the Commodore 128 repair. Initially, the whole system was dead, but now, the power supply has been fixed. After that. some keys were not working, which was resolved when the keyboard was refurbished. The mainboard has been cleaned and checked over, and there doesn't seem to be anything wrong with it.
Nice to start off with a working board for a change. One of the things the C128 offers is 'full' C64 compatibility, so I tested it with the Commodore 64 Diagnostic Cartridge and Test Harness.
This shows an error on the user port, even after swaping over the two 6526 PIA chips the error stays on the user port. I also tried two known working chips with the same results.
I've seen this problem on a couple of Commodore 64's as well, I think it may just be an issue with the test algorithm or the loopback, but I've seen it pass on enough machines to know it is wired up correctly and the PIA chips do work.
I've cleaned the contacts and checked for loopback continuity. I think I'll build something with LED's on and a simple BASIC test program to be sure.
This is only testing the lower 64K of memory, all that is visible in Commodore 64 mode. There are Commodore 128 Diagnostics, but I haven't had much success with those. I've found three versions, all 8K cartridge images, so I programmed the latest one in the same way as I had for the other carts, onto an MCM68766 EPROM.
No luck. I tried again with a different EPROM. Still nothing, so I tried the other two versions, and also with a 2764 EPROM in an adapter. I even tried a new C64 cart which takes 2764 EPROMS. All did the same. With the jumpers set for normal mode, I get the C64 screen, with them set for ultimax mode, I get a blank screen?
When it did load up to the C64 BASIC screen (the C128 goes into C64 mdoe when it detects a cartridge), it showed less available memory. 30719 bytes free, rather than the normal 38911 (slightly more than 8K difference).The reduction in memory seemed to imply it's address space was reserved, so I tried executing the code at 0x8000, using 'SYS 32768', I also tried 'SYS 32778', which should have been the start of the code.
That sort of worked, but not particularly well. All the external tests failed? I'll come back to this at a later date.
So there is the Commodore 128. Time to give it a bit of a run through and try out all the modes.