Prototype A500 (x2) troubleshooting

[Witchy]

Hi folks,

Waaaay back in 2008/9 I was given a load of Acorn and SJNexus kit to look after, among the machines was a pair of prototype A500s that the owner had used while working on RiscOS3 at Acorn.

SN #41 is a ROS2 machine with Rodime MFM hard drive and 4MB RAM
SN #47 is a ROS3 machine with Connor SCSI hard drive and 4MB RAM, this machine has wiring for FAST IO too.

Just before he passed them onto me he powered them up and obviously the RIFAs in one of the PSUs exploded so he quickly turned it off and made notes on post-its on the case.

Fast forward to a couple of weeks ago and during yet another motherboard battery panic I dug out these machines to see what state they were in. Happily only one of them had batteries in and they hadn't leaked TOO much.

I sorted out the RIFAs in the PSUs and they passed a smoke test so beginning with SN#41 I hooked everything up and connected the RGB to my Philips CM8833 via the machine's RGB-SCART cable. Powering up got me nothing, even after a power-up-DEL reset so I also added a composite feed via the BNC jack and that got me a monochrome boot screen (I assume it's mono in the same way the BNC jack on a BBC is mono?). It can't see the hard drive though, so I did a *CO.HARDDISC 1 and reset, still nothing.

SN#47 doesn't do anything, even with power-on-DEL. ROS3 machines are supposed to flash the screen and floppy LED and I don't even get that. Not sure where to go with that one.

ROMs have been dumped for @pernod here and he's got them running in emulation so I know they're good.

Any clues, good people of *.?

Cheers!

[philpem]

My first step would be to pull and image the hard drives. What's on there might be extremely interesting - especially for the preservation bods like myself...!

As for getting the machines going - I'd pull the four main chips from their sockets and re-seat them. That's ARM, VIDC, MEMC and IOC. Anything else socketed, I'd do the same. Spray on a little Deoxit if you're feeling pessimistic.

Assuming they're running RO3, I'd hook a POST Box up to the dead machine and see if it does anything interesting.
If not - pull the ROMs, read them and checksum them (sounds like you've already read them out).

If you get literally nothing on the POST Box, the CPU isn't running code. I think there's a section in the service manual about that - but scoping the address, data and control lines should get you some ideas what might be going on. There are plenty of threads on here - several people have written forum posts on what to check for.

Another thing worth mentioning -- dead 74LS chips are not uncommon in Acorn gear. There are a couple of bus buffers on the address and data bus. If these go, you'll get POST failures for a lot of things. The ROM is usually on the CPU side of the buffer, so Selftest (POST Box) should at least start up and try to test the hardware.

Have you had a chance to have a good look at the PCBs yet? Often a good visual inspection can give you some idea where the issue might lie.

Cheers
Phil

[steve3000]

As for getting the machines going - I'd pull the four main chips from their sockets and re-seat them. That's ARM, VIDC, MEMC and IOC.

Also take some photos of the PCBs first - not only are these rather unique machines, you also may have one or more prototype chips, with a ZIF/test socket like Zarchos' A500 with a prototype VIDC here: https://www.domesday86.com/?page_id=936

And yes, please image the HDDs.

[Multiwizard]

Hi,

indeed, when there pops up a A500 somewhere, I'm always curious if there is something Domesday related on these hard drives...


Greetings, Wim...

[baz4096]

How would you press f12 on a keyboard with only 10 function keys?
Surely would have testing RO3 a bit awkward?

[IanJeffray]

How would you press f12 on a keyboard with only 10 function keys?
Surely would have testing RO3 a bit awkward?

F11 and F12 are clearly marked in the photo

[baz4096]

How would you press f12 on a keyboard with only 10 function keys?
Surely would have testing RO3 a bit awkward?

F11 and F12 are clearly marked in the photo

Oh. Yes they are. I zoomed in and neglected to scroll.
What a great looking pair of machines.

[Witchy]

And yes, please image the HDDs.

The SCSI one I can do but the MFM one may be a bit more challenging since I think these two might be the only machines I have with MFM support. One thing I haven't tried is moving the SCSI drive to the working machine to see if it will boot.

Mind, if the ROS2 machine refuses to see the MFM drive it could be toast anyway, unless there's more I need to do than simply *COnfiguring HARDDISCS.

Cheers!

[Witchy]

Board pics.

ROS2 machine

ROS3 machine (not working)

[Witchy]

As for getting the machines going - I'd pull the four main chips from their sockets and re-seat them. That's ARM, VIDC, MEMC and IOC. Anything else socketed, I'd do the same. Spray on a little Deoxit if you're feeling pessimistic.

I did this yesterday, though not with VIDC because it's got a retaining clip on it. I did push it down though. This machine DOES give a picture if I hold RESET for a few seconds, but it's only a white screen. It could actually be a colour but I can only get a working picture from the BNC socket which is monochrome. The RGB-SCART cable produces nothing on either a Philips CM8833 or Sanyo TV.

Cheers,

[Witchy]

Dammit. I re-reseated all the chips and took a deep breath to remove the CPU carrier.

Suddenly:

Hollow victory though, it lasted for 5 or 6 minutes then noticeably crashed and I'm back to square one. So we know the machine CAN work, just not what's stopping it now. Finger FLIR test shows no chips get hot apart from the FDC which does get a bit warm.

@philpem fancy lending me a POST box?

[IanS]

With the different FDC compared to the normal ARC, it's not clear if the POST floppy codes will still appear. (Is the FDC compatible with the 177x?)

Are you getting any video output now, the POST does enable the video very early. if that isn't being run, the chance of a POST box working are quite slim.

If you do use a POSTbox, you may need to add a series resistor into the ROM CS pin, like on the A300 - viewtopic.php?p=302492#p302492

Is there a copy of the A500 schematics anywhere?

[philpem]

Happy to lend you a POST Box, but it sounds like you probably don't need one. Looks to me like you have the video output misconfigured (which will kill the syncs on the RGB-out), and a loose connection somewhere else which is making it intermittently fail to boot.
The FDC getting warm is probably normal.

If you haven't got them yet -- grab the Archimedes 500 Series Service Manual and Technical Drawings from Chris's Acorns: http://chrisacorns.computinghistory.org ... nuals.html

I doubt you have a VIDC issue because you clearly have good composite sync on the B&W output. The good video can be a bit of a red herring, because the hi-res mono video comes from a shift register, not the VIDC.

Check all the jumpers are configured correctly and make a note of their current positions.
Re-seat the RAM/ROM size ones, but don't change them. They're clearly right if the machine is able to boot -- but they might be a bit dirty.

Regarding the RGB video -- check the video config jumpers (LK2, LK3 and LK6). These control the format of the sync signals on the RGB connector, and whether sync-on-green is applied to the green video.
If you're using a CM8833 type monitor, you need to configure these for composite sync and no sync-on-green.

The link settings are on page 23 of the A500 service manual PDF I linked above.

After that - if you still have problems, check the syncs and video signals at the connector with a scope, and start tracing back. There's not much which can go wrong in the video circuit which will take down the RGB-out but not the monochrome output.


As for the not-booting problem --
- Check the power supply rails are in range. I think the numbers are in the service manual, but you ideally want the 5V rail to be between 4.75V and 5.25V.
- Check for loose or corroded connections and bad solder joints. This might take a while...
- Make sure the reset circuit and clocks are working. Bad timing capacitors and switches are not unheard of.
- Does the machine boot if you hit the reset button after powering on? Hold it for a couple of seconds...

Cheers.
Phil.

[Witchy]

Regarding the RGB video -- check the video config jumpers (LK2, LK3 and LK6). These control the format of the sync signals on the RGB connector, and whether sync-on-green is applied to the green video.
If you're using a CM8833 type monitor, you need to configure these for composite sync and no sync-on-green.

This machine is a different beast to the one you're referring to. Video links don't exist apart from changing the monochrome output from normal to hi res (LK8 and LK10). If you look at the pics I posted up there you'll see what I mean. RAM links don't exist either, or rather they do but they're not populated. The A500 Hardware Guide is my board whereas the Tech Ref and Drawings seem to refer to a different A500 altogether.

http://chrisacorns.computinghistory.org ... wGuide.pdf

If I hold RESET after powering up I get a white screen, sometimes a black screen with a white border. Bear in mind it's from the BNC connector so the screen MAY be a colour. Just trying to think what monitor I have that may work, all my Philips chassis ones are Amiga related so 1084, 1084S etc.

Cheers

[philpem]

Ahhh, I've just had a look at the timeline. The A500 manual I posted was for a different beast entirely: the A540, also sold in RISC-iX form as the R260. So no, it's not going to match your prototype...

I think you're probably going to have to work from basics, unless a copy of the schematics somehow falls through a spacetime vortex.

Chances are it's probably quite similar to an A300 or A400, but the CPU card is clearly a removable, plug-in unit, and some of the chips are different. So you could use the A400 schematic set as a guide, but you may have to figure out how the pinouts map to things like the CPU daughter card.

The CPU interfacing didn't really change a whole lot in the Archimedes and A-series machines, so the basic principles are sound across the whole series. Check

I'd try tracing the signals back on the RGB input. If you're in hi-res mono mode, the RGB output may not produce valid video at all... switching to MODE 15 should produce a TV-style mode which will work with a Cub or CM8833.

Beyond that, I'd be checking that bodge-wires hadn't come loose or shorted out, no broken solder joints or damaged sockets, and so forth. If the machine booted okay (once) after reseating the chips, to me that screams loose connection or there's still some dirt in the sockets.

Cheers
Phil.

[philpem]

The SCSI one I can do but the MFM one may be a bit more challenging since I think these two might be the only machines I have with MFM support. One thing I haven't tried is moving the SCSI drive to the working machine to see if it will boot.

If you get stuck, I've got an MFM imaging rig here (one of David Gesswein's MFM emulator/imager boards) and a Pentium-4 PC equipped with a SCSI card and a Linux CD. If you get stuck, I'm sure it'd be possible to arrange some kind of mutually agreeable time for you to bring the drives up here to image -- or possibly borrow my kit to do it.

Cheers
Phil

[Witchy]

Chances are it's probably quite similar to an A300 or A400, but the CPU card is clearly a removable, plug-in unit, and some of the chips are different. So you could use the A400 schematic set as a guide, but you may have to figure out how the pinouts map to things like the CPU daughter card.

Looking at the A300 board layout it's very similar so I've traced the RST signal from keyboard to IOC and that's being asserted correctly. Power rails are good and the ROMs are being selected. When I get another spare hour or two I'll remove the board and have a look at the soldering, particularly around the CPU socket. Since that successful (brief) powerup I don't even get the white screen after holding RESET.

As for the disks I might have to send the MFM one to you, I'm self isolating while getting kidney treatment which is slightly irksome as I was supposed to be going up north this week to see Mrs Me and check in on my Dad in his care home. I'd have to drive past you to do that.

Cheers,

[SteveBagley]

I did this yesterday, though not with VIDC because it's got a retaining clip on it. I did push it down though. This machine DOES give a picture if I hold RESET for a few seconds, but it's only a white screen. It could actually be a colour but I can only get a working picture from the BNC socket which is monochrome. The RGB-SCART cable produces nothing on either a Philips CM8833 or Sanyo TV.,

Is that a dedicated one for the A500, or a BBC SCART lead? I’m not sure the latter will work since the BBC generates TTL level RGB and the A500 is almost certainly 1V. You probably just need to remove the resistors and possibly rewire it if the pinout is different.

Steve

[steve3000]

I did this yesterday, though not with VIDC because it's got a retaining clip on it. I did push it down though. This machine DOES give a picture if I hold RESET for a few seconds, but it's only a white screen. It could actually be a colour but I can only get a working picture from the BNC socket which is monochrome. The RGB-SCART cable produces nothing on either a Philips CM8833 or Sanyo TV.,

Is that a dedicated one for the A500, or a BBC SCART lead? I’m not sure the latter will work since the BBC generates TTL level RGB and the A500 is almost certainly 1V. You probably just need to remove the resistors and possibly rewire it if the pinout is different.

Steve

That’s a good point, for Zarchos’ A500 I built a RGB to SCART lead by modifying a BBC SCART lead removing the resistors.

The VIDC retaining ZIF socket looks the same as on Zarchos’ A500 - it’s a bit fiddly, but can be undone carefully to remove the chip for cleaning, and reinsertion - just make a note of how it’s all held together. As I recall it’s quite a tight fit.

[steve3000]

ROMs have been dumped for @pernod here and he's got them running in emulation so I know they're good.

Could you make these available? Or post here?

I’d like to explore the differences in the RO2 and RO3 A500 ROMs to the general release versions. Also would be nice to look into the possibility of getting anything other than Arthur on Zarchos’ A500…

[steve3000]

Ahhh, I've just had a look at the timeline. The A500 manual I posted was for a different beast entirely: the A540, also sold in RISC-iX form as the R260. So no, it's not going to match your prototype...

I think you're probably going to have to work from basics, unless a copy of the schematics somehow falls through a spacetime vortex.

Chances are it's probably quite similar to an A300 or A400, but the CPU card is clearly a removable, plug-in unit, and some of the chips are different. So you could use the A400 schematic set as a guide, but you may have to figure out how the pinouts map to things like the CPU daughter card.

Yes very different to the A540 - and much closer to the A310 and original A440, as the A500 was the forerunner of these.

Regarding A500 documentation, the only available documentation I'm aware of is the Hardware guide (mentioned above) here and also the Fast A500 upgrade/application note here.

[Pernod]

ROMs have been dumped for @pernod here and he's got them running in emulation so I know they're good.

Could you make these available? Or post here?

I’d like to explore the differences in the RO2 and RO3 A500 ROMs to the general release versions. Also would be nice to look into the possibility of getting anything other than Arthur on Zarchos’ A500…

Here you go, also includes the ROMs from the podules installed in them.

Do you still have Zarchos' A500? There should be a ROM in the keyboard for the MCU that I wouldn't mind a dump of.

Edit: see later post for download.

[steve3000]

Here you go, also includes the ROMs from the podules installed in them.

Do you still have Zarchos' A500? There should be a ROM in the keyboard for the MCU that I wouldn't mind a dump of.

Thanks!

Yes I do, sadly not used as much as I'd like - mainly because of Arthur...(hopefully I can get RISC OS 2 running... RO3 would need more hardware mods though...)

Happy to take a look at dumping the keyboard ROM at the weekend.

[SteveBagley]

Here you go, also includes the ROMs from the podules installed in them.

Interesting -- one of the modules in the SJ Nexus card ROM identifies it self as an A500 version. I wonder why it needed its own variant… And why they were producing them in 1997…

Nexus Network Interface 2.24 (A500 Std Card)
SJ Research Ltd 1989-94,
Cumana Ltd 1994-95,
SJ Consulting Ltd 1996-97
Sat,26 Apr 1997

Steve

[Witchy]

Yes, the 1997 date on the Nexus card puzzled me too.

I dumped the keyboard ROM yesterday and added it to the whole ROM dump:

PrototypeA500.zip

(1.63 MiB) Downloaded 24 times

For the SCART lead I initially was using a BBC one but in the bundle of cables that came with the machines I found a different one so assumed that was the cable that should be used. I'll check it for resistors though.

Yesterday I scoped the socket side of the CPU carrier and all the signals looked pretty dead which seems odd because ROMCS is pulsing for each ROM, so that's being driven by... something. RESET looks fine, at least at pin 4 of IOC.

Cheers,

[Pernod]

Interesting -- one of the modules in the SJ Nexus card ROM identifies it self as an A500 version. I wonder why it needed its own variant… And why they were producing them in 1997…

Nexus Network Interface 2.24 (A500 Std Card)
SJ Research Ltd 1989-94,
Cumana Ltd 1994-95,
SJ Consulting Ltd 1996-97
Sat,26 Apr 1997

Steve

Photos of the Nexus boards:

[Alanex]

Hi,

Bit late with my question perhaps, but I don’t suppose you have any of the SJ Research Nexus Utilities discs for the Nexus? Possibly even a copy of the discs on a hard drive?

Thanks,
Alan

[razz177]

Yes, the 1997 date on the Nexus card puzzled me too.

I dumped the keyboard ROM yesterday and added it to the whole ROM dump:

PrototypeA500.zip

For the SCART lead I initially was using a BBC one but in the bundle of cables that came with the machines I found a different one so assumed that was the cable that should be used. I'll check it for resistors though.

Yesterday I scoped the socket side of the CPU carrier and all the signals looked pretty dead which seems odd because ROMCS is pulsing for each ROM, so that's being driven by... something. RESET looks fine, at least at pin 4 of IOC.

Cheers,

Did the other scart lead work ? If so could you give me the pin out for each plug My A500 is a bright pink colour & I would like to prove weather it is the lead or not.
Thanks

[Witchy]

Did the other scart lead work ? If so could you give me the pin out for each plug My A500 is a bright pink colour & I would like to prove weather it is the lead or not.
Thanks

I don't think so, there'd have been further posts on this thread if so. However, it was 2.5 years ago and I've slept since then

I've remembered today, as it happens, that I got a dedicated EIZO monitor with these machines so that's what would've been used. I found the monitor but have no idea where the cable might be at present. (I was troubleshooting a pair of RPC600s)

[razz177]

Did the other scart lead work ? If so could you give me the pin out for each plug My A500 is a bright pink colour & I would like to prove weather it is the lead or not.
Thanks

I don't think so, there'd have been further posts on this thread if so. However, it was 2.5 years ago and I've slept since then

I've remembered today, as it happens, that I got a dedicated EIZO monitor with these machines so that's what would've been used. I found the monitor but have no idea where the cable might be at present. (I was troubleshooting a pair of RPC600s)

I cured this by re-flowing the solder around the RGB socket, I could not see a problem But there must have been.
Guess what I am onto after the A310 i'm on now, Yep 2 600's I think my next learning project is doing the traces.
Thanks for replying.