Back in 2014, I repaired this Acorn A5000 RISC PC.
Now it is back.
When I did the original repair, I took some photos, but I think there was lots of other things going on and I didn't end up writing it up.
I have taken more photos this time, so buckle in for a two for one special.
The Year is 2014....
The problem was it wasn't booting up, just showing a black screen, although the keyboard was responding and the floppy light flashed at boot.
Dust, anyone? no. Dust?
That hasn't been opened in a while.
Come on, where is it?
I know what the problem is going to be, the curse of any machine starting with an A and having at least a couple of zeroes, the Acorn Archimedes 3000 and the Commodore Amiga 500+ being the worst affected.
This is quite a nice design, there is a vertical backplane, which can be removed along with the floppy drive and hard drive (Conner CP 3044, 40MB IDE).
Once that was removed, I gave it a bit of a clean before I went any further.
Ah, there it is.
Yes, the dreaded leaky Nickel Cadmium rechargeable cell.
That was straight out, and the board cleaned and neutralised.
It didn't look too bad, but on further inspection, there was some damage to one of the traces, so I repaired that with a bit of wire. (not the neatest, I might have done it a bit differently these days)
The original cell was 1.2V, 280mAh NiCd. The cell is trickle charged when the machine is powered on, and the chip it powers, the PCF8583 is a real time clock and 240 byte CMOS settings store.
It is designed to work offline at at voltages down to 1V to maintain the time and settings, it needs between 2.5V to 6V in normal mode to communicate via I2C.
The best option people seemed to be using at the time were these Varta 3.6V 150mAh Nickel Metal Hydride batteries.
The NiMh battery fits quite nicely, the board had footprints for both shapes.
Time to put it back together and fire it up.
I think this was the first of the RISC PCs that had standard VGA, or at least a standard VGA connector.
It is meant to autodetect the display, but sometimes needs a bit of help with modern LCD monitors, as it also supports composite video, SCART and various other monitor types with the same connector and an appropriate adapter cable (which I don't have).
The first power on is with R held down to reset CMOS to default values.
The screen remains black, so power off and back on again with 3 (or 4) on the numeric keypad held down to set VGA (or SVGA) mode.
And we're in.
8:45 PM, Friday 19th September 2014. I guess my social life in 2014 was just as exciting as it is these days, what better way to spend a Friday evening than fixing a leaking battery.
That all seemed to be working nicely, I left it powered on a while to let the battery charge and tried it again a few days later and all seemed well, so back it went to the customer.
The Year is now 2025....
Roll on to 2025 and I received a call asking if I still repaired computers.
It was the same A5000, with the same problem.
My initial thought was that the battery might just have been flat from not being used for a long time (i.e. 11 years or so).
However, I thought I should check inside first.
Still nice and clean inside.
Ah, oh dear.
Yes, it's done it again.
I thought NiMh batteries were meant to be better. I guess not.
I guess it's a cautionary tale, keep a check on your backup batteries, ancient or modern.
Did you fit any of those in the last 15 years? When did you last check them? Might be worth a look.
Oh well, same story, remove, neutralise and clean up.
Replacement Options
I did order another of the Varta replacements, and was thinking about fitting it as before.
But I don't want to see this again in 5, 10 or 15 years time (not sure if either I or this A5000 will still be around in 15 years, but let's not worry about that for now).
Other Options
I have tried various other options on these machines in the past, and never found one that was ideal.
In a couple of machines, I tried CR2032 (non-rechargeable) cells. These need an additional diode fitting to stop them trying to charge.
I also tried a solder in style CR2032, where the diode doubled as a sort of support.
I have tried a set of Eneloop rechargeables, and also a set of plain AA batteries (with a diode).
I think the general problem is none of these machines is being used enough to keep them active.
Off-board Battery
I decided the best option here was to do as planned and fit another NiMH battery, but this time, I will move the battery away from the mainboard.
That should mean it can do no further damage if it does leak again.
The inline connector will also mean it would be easier to replace in future.
The battery is trickle charged when the machine is powered up, I thought I would give this one a head start and put it on the bench supply. I set it to charge at 0.1C (10% of the rated Amp-Hour capacity) on constant current mode.
An hour or so later it had hit the voltage limit and dropped the current.
Fading Memories
With that fitted, I was finally able to get it to power on, but it did not seem to want to go past the minimal boot setup.
These machines have the OS in ROM, including some basic apps.
I had trouble getting it to set the screen mode this time.
This was sort of expected anyway as the auto detect was never designed to cope with a modern LCD, and I think it jumps to a mode which isn't supported.
I managed to get it to display something by powering on pressing T, which inverts the sync. That caused the monitor to display "mode not supported", but that somehow put it in a state where I could then do the power on with 4 held down and I finally got a display.
It generally seemed to be working, and I presumed it was down to my monitor not being compatible, so I wrote up and posted these two repairs to Patreon.
I planned to get a CRT VGA monitor out of storage to be able to test it properly, and then update the post.
The Year is still 2025, just
It took a little longer as it was Christmas, but I eventually managed to find a suitable monitor that should be supported. When I powered it on, it had forgotten the settings again.
I went through the reset process again, and it was running, but it didn't seem to be remembering the settings after the power was removed, and not accepting some of the settings at all.
I checked the PCF8538T chip, which stores those values and also the date and time with a 32.768KHz watch crystal (the bodge wire was from 2014)
It was getting power both when the machine was on, and from the battery, but just didn't seem to be retaining the settings.
I buzzed out all the pins to check for connections around the board, and all seemed well.
The chip communicates via I2C, so I tapped into the signals and tried to see what was going on, but it seemed to be garbage.
Replacement Chip
The chip is out of production, and is also an unusual package, larger than a SOT8, but smaller than a DIP8.
I was considering designing a microcontroller based replacement, one that would interface to the same I2C and would store the settings in it's internal EEPROM, removing the need for the battery. It wouldn't keep the time, but it would keep the settings, which seems more important.
That initially seemed an easy option, but was getting increasingly complicated with various clock, timer and alarm modes, and I didn't know which would be required.
Before I got too far into coding that, I had a look around and found some replacement chips on ebay. I still had the microcontroller replacement plan as a fall back if these turned out to be faulty or remarked fakes or whatever, so I ordered some.
A cut tape arrived with what looked like genuine New-Old-Stock parts.
The right unusual package, and the old style screen printing, so I thought it was worth a go. Not sure of the date code, but it looks 1990s?
I removed the old chip and the old bodge wire, cleaned up the board and rechecked all the connections.
I fitted the new chip and a new bodge wire (now including the top of C153 which was making connection, but looked a little dodgy after a cleaned it up).
Time to put it all back together.
This time, I was able to store settings and get a full size picture on my monitor.
All seemed good, I was able to enable the hard drive and save the screen settings.
All was fine until I tried to access the hard drive, it couldn't open it.
The name was just showing up as "4".
There is a built-in verification program, so I tried to verify it.
Oh dear.
It seemed OK up to a point and then kept finding faults.
There could be several reasons:
- The hard drive had failed, it is 30+ years old.
- The corrosion has damaged some traces to the IDE connector (it is right next to the battery).
- The hard drive is not configured correctly. (don't see where I can change CHS settings?)
- Something else in the system is at fault and stopped the IDE working.
Remotely Testing the Hard Drive
Given the age of the drive, I thought it might be good to rule out that having failed. Date code looks like January 1992.
The trouble is, it is not just old, it is really old, early days for IDE.
Before LBA (Large Block Addressing), and auto-detect. This is the type of drive where you had to set the drive type in the BIOS and check the CHS settings (Cylinders, Heads, Sectors) with the drive datasheet (and let's not get into real CHS vs virtual CHS settings).
I have tried USB-IDE adapters in the past on drives like that, and they don't recognise them, and there is potential to corrupt the drive if the wrong settings are used.
Likewise a modern BIOS would be no good. It would need to be old enough to still have the option to set a drive type number, but not too old so that it doesn't have an IDE connector.
I have probably still got a dozens old PCs that would fit into that range, but nothing I can easily lay my hands on.
Trying a Different Hard Drive
There is a similar issue with trying a different hard drive. I know I have a selection of older drives like that (somewhere), but nothing I can easily grab, and nothing that would be formatted appropriately. I have an Acorn RISC PC 600, but I think the drive in that is SCSI.
One thing that I did have was a 64MB Compact Flash card. With a CF-IDE adapter, that should work.
I put what should have been a valid image on it. I was not 100% sure it would work, but the A5000 was 100% sure it didn't.
It was detected, again with the name "4", but the only available option was to low level verify it.
As before, it started well, but then threw up a whole load of errors, just like the old drive.
OK. That is still a useful test. It is not conclusive, but it does seem to point more to the fault being on the mainboard rather than the drive.
IDE Connector
The IDE connector is the larger of the three connectors, right next to the battery (the other two are for floppy drives). There has been damage due to acid leaking from the battery (twice), so maybe some of the tracks have been damaged?
I located the relevant section on the schematic and started checking all 40 pins from the connector to whatever chip they were connected to.
They all seemed to check out fine. The only problem was one I had already found, which was the drive activity LED. That goes along the edge of the PCB, near the battery, and had been eaten away.
That was fixed by a bodge wire directly from the back of the IDE socket to the resistor by the LED socket.
Everything else tested fine.
I am running out of options.
Something Else?
Other than a few control lines from the big gate arrays, the only other parts involved in the IDE bus are the two 74HCT245 buffers, IC49 and IC37.
Those are the bi-directional buffers with enable pins. The sort of parts that are tricky to test. The inputs and outputs always look different as they are only designed to pass data for the brief moments when they are enabled. Ideally you would wire up a logic analyser to a few of the 8 inputs at a time, as well as the corresponding outputs, and the enable and direction lines, and try to spot any differences when the device was enabled.
Yeah, well, the other option is just to replace them. There is a luxurious amount of space around the two chips, making that a fairly easy task.
That was a lot quicker, and is just as easily reversible (if you are particularly bothered about putting the original chips back if they turn out to be OK).
Good start, it is now showing up with the label of "IDEDisc4", and not just "4".
Well, so there you are. It was one (or both) of the 74HCT245s. That was the least likely on my list, but there you go.
Floppy Testing
The other thing I needed to check was the floppy drive. The two floppy connectors are also in the danger zone near where the battery was.
That was a little easier, as I could just insert a floppy disk (or floppy disc as Acorn seem to prefer) and test it out.
Unlike the A3000, the A5000 has a high-density drive, so can handle various formats including PC 720K and 1.44MB, as well as Acorn's 800K and 1.6MB formats.
I also tried reading some of my A3000 disks.
That looks promising.
And away we go.
I don't know how many times I have played through these levels, on a variety of different machines since I got my Amiga 500 in the late 1980s.
Back Together
With all the testing complete, I put it all back together. This time, I moved the battery even further out of the way.
I let it stand unplugged for a few more days (which turned into a week), and after that, it powered up fine with all the settings still in place.
That is now back with it's owners, next service due in 2037.
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