Wednesday, 19 October 2016

ICL One Per Desk Microdrive Repair

The previous blog post on the repair of a BT Merlin Tonto (ICL One Per Desk) covered the cleanup and monitor / power supply.
Once it was up and running, it was clear that neither of the two microdrives were working. When attempting to access either drive there was no response, no motor noise, no activity lights.
The microdrive was a cassette tape loop based storage device, with 5m of 1.9mm tape spooled in a small cartridge, like an 8 track cartridge. I don't know why I think that will help, if you don't know what a microdrive is, you probably don't know what an 8 track cartridge is either.
There is no spool, the tape is pulled around in a big loop by a capstan from the side, the tape head sticks in from the top. Those foam pads tend to wear out, so need to be replaced to keep the tape pressed against the head.
These were introduced for the ZX Spectrum, as a replacement for standard audio cassette storage and a much cheaper alternative to disk based solutions. You could chain up to 8 of these drives together to provide massive storage capability for the Spectrum. (or why not just buy 8 cartridges and swap them over in the one drive?)
It may look like the microdrive just plugs into the side of the Spectrum, but there is in fact a larger interface unit the 'Interface 1' which the Spectrum is sitting on.This contains the drive circuits and ROM patches for the additional commands necessary to use Microdrive storage.
This provided the Spectrum with 'up to' 100K per cartridge, although in reality this was a lot less. As the cartridges aged, the tape would stretch, so you could format it again and the capacity would increase slightly (probably in proportion to a decrease in reliability).
The microdrives were again used in the Sinclair QL, with two of them intregrated into the QLs case and intended as the QLs main storage device. The BT Merlin Tonto was based on parts of the QL and included two microdrives as well.
One of the articles I have read say that ICL re-engineered these microdrives to be better than the Sinclair versions, I'm not sure about that. Here is a Spectrum microdrive.
Theses are the QL ones.
And these are the ICL ones.
The plastic chassis looks identical, same motors, same switches,  the PCB was laid out differently, but basically the same. Comparing one of those to the Spectrum one, I can't see much difference. Sinclair on the left, ICL on the right.
On the rear, again I'm struggling to see any mayor mechanical differences.
The main electronic part on these is, true to form for Sinclair, a Ferranti ULA, here in the Sincalir microdrive. You can see the two edge connectors on the rear PCB, one on each side.
These are chained in series. Most of the pins pass through from one microdrive to another. The exception is the data signal. This goes into the first device, is processed and then passed onto the next device. Presumably removing anything addressed to it, and passing on the rest.
The same part 2G007E is used on the ICL version, and like the Sinclair versions they are chained in series. When I checked, there was data going into the first chip, but it was doing nothing with it. It was also not passing on any data to the second drive, so it looks like a faulty ULA.
This could mean the second drive is actually fine, and just wasn't getting the data passed to it from the first drive. To test this, I rewired the connectors so the first drive was disconnected, and the second was connected to the main board.
Success, that drive was fine, One cartridge formatted with 204 x 512 byte blocks (102K). It should be noted that although they appear mechanically and electronically very similar, they use a different logical format, so you can't actually interchange date between the three system.
That would confirm data wasn't passing through the first drives ULA, which does look to be faulty. I was able to get a new old stock ULA and replace it. Not trivial as the 6 pads you can see below the socket are the back of the tape head which has to be desoldered before the board can be accessed to remove the ULA.
I fitted a socket and the new ULA and inserted the module back into the chain. I was going to place this as the second drive, but I found that although the modules looked identical, one PCB was slightly different, the input connector is positioned differently, which meant the linking cable wouldn't reach. (UPDATE: I've just noticed the left hand board is part number PWB 5400054, and the right is PWB 5400055, so yes they are different.)
With both modules reconnected and the new ULA in place, time to test again.
All looking good, both drives are formatting and reading back, and it has successfully copied from one drive to another.
All finished and ready to go back into service.

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Friday, 14 October 2016

Acorn Electron RISC OS Pico

This is an Acorn Electron, but not any Acorn Electron. You might notice the cable coming out of the back, that's HDMI.
It boots up with a beep (a BBC style double beep) and within a few seconds, you get this mode 7 boot screen.
As you can see, it's got a touch more than the original 32K, about 6000 times more. The processor is also a lot faster (700MHz vs 1MHz).
OK, it's got a Raspberry Pi model A+ inside, but that's a RISC based chip, derived from the Acorn family, so it counts, doesn't it. The original power supply and keyboard are used, with a USB hub and one of my Acorn Electron USB keyboard controllers. More info on the original conversion of this Electron on the previous post, Acorn Electron with Raspberry Pi.
The Pi is now loaded with RISC OS Pico, a cut down version of RISC OS specifically for programming. It is running BBC BASIC, so you can just type in good old BASIC programs.
You can download the files and copy to a microSD card, or buy a version from them, I always try to support open source projects, so I got one of the official ones.
These now run quite a bit faster, so it is flying around the screen. On an original Electron, you can see the pause between printing each phrase, and then scrolling the page up, here you can't see it scrolling any more.
This is a more powerful version of BASIC which adds some new features, including more modes.
I'll zoom into the bottom corner of the screen, yes that's full HD mode.
The graphics can also make use of the full screen resolution.
You can also use the editor to create your BASIC programs.
The normal file access commands work to access files on the SD card, directly from the FAT32 folders rather than a separate partition or filing system as the Pi normally has. You can use *SCSI to change to reading a USB memory stick and *SDFS to change back to the microSD card.
The Electron keyboard is well suited to this as all the usual symbols are there, as well as the arrow keys and copy key etc., all working as the original. As with the Electron, shift + key gives the left hand symbol, ctrl+key gives the right hand. The function keys use function + number keys, with FUNC+0 for F10, FUNC+ -/= for F11 and FUNC+Break for F12.
The Electron had a unique feature in the Acorn range of having keywords printed on the keys, so originally, you could just press FUNC + C and COLOUR would appear on the screen as if you had typed it.
There is no support for that in RISC OS Pico, so I have added it to the USB keyboard controller. If you press FUNC+F, it will send the keys F, then O, then R, so FOR will appear on the screen. These are on all the letter keys, and some of the symbols. I've also added two more to the unused keys, FUNC with ;/+ now gives EDIT and return, to start the editor, and FUNC with :/* give *. and return to give a file listing. These keywords always need to be in upper case, so if caps lock is off, it holds down shift whilst typing the letters.

If you want to try this yourself, you can buy a suitably programmed Acorn Electron USB keyboard controller kit, or contact me if you are interested in a complete Acorn Electron Pi preloaded with RISC OS Pico.

My thanks to John Dale who introduced me to RISC OS Pico and suggested pairing it with the Acorn Electron USB keyboard.

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Sunday, 9 October 2016

BT Merlin Tonto (ICL One Per Desk) Repair

This is a BT Merlin Tonto M1800, a rebadged version of the ICL One Per Desk, which itself was basically a Sinclair QL with built in telephone and monitor.
The monitor is a separate unit that rests on top of the main unit. The telephone handset is built in on the side of the keyboard
The main unit has the same 128K RAM and 68008 CPU as the QL, as well as two microdrives. Amongst the chipset are the QLs ZX8301 (CLA2310) and ZX8302.
There are a surprisingly large number of ROM chips in here, 4 on the main board, 5 on a plug in module, and one more which plugs into that.
The status of all of this is unknown, as I am told the monitor 'went up in smoke' when the owner plugged it in to test it.
There is a single mains connection into the monitor, and connects to the main unit via a 15 way D plug. This supplies power and takes the video signal out of the unit to display. There was a version with a larger colour monitor, so presumably there are RGB signal available on the connector. This is the monochrome version. Inside thee are two board, one power supply and one to drive the 9" monochrome tube.
Like many Astec brand power supplies from the 1980's, this uses RIFA mains filter capacitors. The type X capacitors, usually 100nF sit across the mains input to reduce mains interference. They have a habit or exploding when not used for a while. Spotted the problem yet?
I've only had one of those blow up on me once, many years ago, and it make a lot of acrid smoke that lingers for ages. I've seen evidence of several others like this which have gone bang, and these days I always check over anything before plugging it in, and if I find these I replace them immediately before applying mains power.
These are used in BBC micros (whose exploding power supplies are well known), and later Acorn machines, some Apple IIs and IIIs, anywhere you find an Astec branded mains supply.
They are not the only ones, see a previous post on exploding Commodore PET mains filters.
I replace these with modern X2 rated capactors. these are smaller, but the PCBs often have multiple pads to accommodate smaller devices.
I checked over the other capacitors and they all seemed fine, values within range and no high ESR readings.
With that replaced and everything reconnected, time to test it out. No smoke, always a good sign.
The picture was initially faint and flickering, but that was cleared up by cleaning the brightness control, a slider variable resistor on the front. With some contact cleaner and working it back and forth a few times, it was able to smoothly adjust the brightness, and the picture was nice and stable
On power on, the screen showed various tests, clearing the screen, filling it up, and various patterns of lines. These can be used to diagnose faults. In this case, this pattern tell me I hadn't plugged in the telecoms module.
This is a removable section, and I had removed it whilst testing. It has the handset and two line cords attached and various telecoms related items and an 8051 microcontroller inside.
With that reinstated and powered back on, we're in business, the system appears to be working and is reporting a battery low condition.
The battery is a PP3, and there is a Duracell installed. I'm not sure how old it is, but there is no use by date on it, I'm sure they started printing them a long time ago, so this must be quite an old battery. Luckily a well made battery that hasn't leaked, even though the terminal voltage is 0.0V.
With a new Duracell installed, the self test passed and the top level menu is displayed.
It looks like everything is working, all the keys test fine, and I can navigate through the menus, set the time etc.
The keyboard is a quite nice Alps model, which remind me of the type used on early Atari 800XLs.
Neither of the two microdrives were working, I have now fixed those, but I'll write that up in a separate article. (UPDATE - ICL One Per Desk Microdrive Repair)

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