Hey, Presto! – Episode 2: About the SPARCclassic

This post is an introduction to the SPARCclassic hardware. It is a slightly idiosyncratic computer, even by Sun standards.

Getting Inside

Removing the single lock-block screw at the rear of the chassis, allows the case to split into two pieces: the “top” containing the PSU plus drive-bays, and the “bottom” containing the PCBs.

Motherboard and I/O-Port board

Somewhat atypically, the rear ports on the system are not mounted on the motherboard, but rather on a separate rear I/O-Port board, which the main motherboard plugs into via a very wide edge-connector. The I/O-Port PCB is integral to the chassis (cannot be removed), but the motherboard can be removed.

This unusual arrangement was so that the exact same chassis could be used for both the SPARCclassic and the SPARCstation-LX – which had different motherboards but could plug into the same rear I/O-Port board. To separate the motherboard from the I/O-port PCB, there is large plastic “eject” lever that pokes up through a hole in the motherboard.

External I/O Sockets

  • a DB25 socket providing a pair of RS-232 serial ports. Although 2 serial ports in a single DB25 socket might seem strange to PC afficionados, it is common on late-80s and early-90s Sun and compatible systems, and also some from other manufacturers.
  • a mini-DIN8 socket for connection of an external Sun keyboard+mouse combination.
  • 3.5mm analogue audio jacks, one for input, the other for output.
  • an HD50 fast-narrow-SCSI socket.
  • an HD26 socket that is described in detail below.
  • a 13w3 analogue video connector for an external graphics screen.
  • a DB25 parallel-port.
  • an RJ45 10BaseT Ethernet socket.
  • a couple of hidden other sockets, discussed below.


The SPARCstation-LX used the rear HD26 socket for 16-bit digital audio, to connect to an external Sun speakerbox; when the speakerbox is plugged in, the in-chassis analogue speaker is disabled. On the other hand, the SPARCclassic used the traditional separate analogue audio-in and audio-out jacks to support an 8-bit external audio capability.

But that’s not all: that very same HD26 socket also contains an Ethernet AUI (10Base2 attachment-unit interface) channel – if you plugged a special Sun-specific HD26-to-15-pin-AUI cable into that port, you could use an external transceiver to attach to a coax 10Base2 Ethernet.

And, of course, there was an optional external HD26-splitter cable that converted the single HD26 socket on the back panel into a pair of HD26 sockets – one for the speakerbox, the other for the AUI.

Although I have a couple of speakerboxes, these cannot be used on the SPARCclassic, due to the lack of on-board 16-bit digital audio on the motherboard.

10BaseT Ethernet

As well as the abovementioned AUI interface, the SPARCclassic provides on-board 10 Mbits/sec twisted-pair Ethernet via a rear RJ45 10BaseT socket. Again, a 100 Mbit/sec SBus card could be added, but there is no way a 50MHz microSPARC could take much advantage of that (for their Fast Ethernet SBus cards, Sun recommend a minimum of a 60MHz SuperSPARC with 1MB L2 cache).

The Hidden Sockets

The SPARCstation-LX also had built-in ISDN network connectivity, via a pair of ISDN TE sockets on the rear panel next to the 10BaseT socket. The SPARCclassic motherboard does not have ISDN capability due to not having a DBRI interface chip, so Sun covered those rear ISDN sockets with a stick-on piece of beige plastic, presumably to prevent customers complaining that “the ISDN sockets don’t work”!


See also: https://fido.aalin.co.uk/vault/hw/system/Sun/sun4m/microSPARC/HC4.3.1-microSPARC.pdf

The microSPARC CPU was not originally intended for use in a computer/workstation – it was intended for embedded systems use such as in PABX switching systems, RAID controllers, automotive system controllers, very early set-top boxes, that kind of thing. However, in 1991, Sun was getting delayed with development of the general-purpose SuperSPARC workstation/server CPU, and expected pricing of same based on die-yields indicated that SuperSPARC wasn’t going to be economically viable for lower-end workstations. Sun thus needed to a quick-and-cheap-to-deploy alternative to tide them over in the entry-level workstation market. Thus the relatively-low-performance microSPARC was pushed into service in the SPARCclassic and SPARCstation-LX.

Quite apart from the simple in-order CPU pipeline, the microSPARC has a rather more significant performance problem: drastically undersize caches for running Solaris or SunOS, or indeed any other desktop operating system. Merely 4KB instruction-cache and 2KB data-cache, when workstation competitors were often 8KB+8KB L1 backed by an additional external L2 cache. On the plus side, in spite of it’s diminutive size, microSPARC included on-chip MMU, IOMMU, SBus controller and FPM-DRAM memory-controller, thus making the board level design much simpler and cheaper – ideal for a Sun “lunchbox” type enclosure. It really was the small low-cost option, all around.

The good news is that with the MMU, IOMMU and memory-controller on-chip rather than external, access-latency to main memory is low, which is a good thing considering how frequently main memory will be accessed due to the tiny on-chip caches.

Another quirk of the microSPARC is that the on-chip FPU is not a Sun, TI or Weitek design, but rather was a design licensed from Meiko Scientific, and turned out to be rather more efficient than earlier designs.

So where is the CPU hiding? There are several large clearly-visible ICs on the motherboard, but the tiny microSPARC CPU is hiding under the built-in mini-speaker, under it’s own plastic shroud:

We are going to have to be rather patient when using this machine, it really is rather slow. At 50MHz the SPARCclassic is, with a following wind, roughly equivalent in interactive feel to a previous-generation 40MHz SPARCstation-2. Let’s see what we can get out of it!

Internal Mass Storage

The internal drive-bays are located in the top segment of the chassis, next to the PSU:

The on-board mass-storage systems are (a) 50-pin fast-narrow-SCSI (10 MBytes/sec) and (b) traditional NEC/Sony-compatible diskette interface; each connected via a ribbon-cable to one of the drive bays.

The on-board SCSI is also routed to an HD50 connector on the rear panel, for connecting external SCSI devices such as a CDROM drive, external disk-pack or SCSI document-scanner.

Of course, you could add an SBus card to provide an alternate/additional storage interface, eg: a SunSwift card (fast-wide-SCSI 20 MBytes/sec) or
even a Sun Fibre-Channel interface card, although the microSPARC CPU would have difficulty taking much advantage of the extra bandwidth in either case.

Main Memory

The SPARCclassic has six 72-pin SIMM slots (yes: six, not a traditional power-of-two such as four or eight slots), which take 4MB or 16MB 70ns FPM modules with parity. ECC modules are not supported, nor are EDO modules.

The official maximum RAM capacity is 96MB, but there is a way of getting more in there, which will be discussed in a later episode.

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Hey, Presto! – Episode 1: A Slow Start

Although too late for the October 2021 Retrochallenge, I have a little retrocomputing project to do that involves a new use for an old slightly unusual accelerator for old SBus-based computers.

The idea is to install a PrestoServe accelerator card, and make use of it, but not in the way it was originally intended for. This will be slightly tricky, as I do not have Solaris drivers for the accelerator card, but I do know of a slightly unusual way of finding/constructing them, via historic official patches for Sun Solaris – if I can find the CD’s, where did I stash them?.

The target system for this project is the lowest of the low, a Sun SPARCclassic, shown here with a CDROM case for scale:

Sun SPARCclassic

It’s a cute little thing, isn’t it? [Aside: Yes, Mark, this computer did come from where you thought!]

Project Outline

  • Describe some of the hardware wierdnesses of the SPARCclassic.
  • Install the actual maximum amount of RAM this thing can take, which involves a bit of deviousness.
  • Swap the internal rotating-rust SCSI HDD for an internal SSD (yes, really!).
  • Install Solaris 2.6 from CDROM, using an external SCSI CDROM drive.
  • Describe what the heck PrestoServe is, what it was originally for, and why it might still be useful even after the original use for such a thing has long since passed.
  • Install a PrestoServe accelerator SBus card, and hoping that the battery on it is still good after all the years.
  • Obtain the core PrestoServe binary device-driver by devious methods, and write the missing bits myself.
  • Run a few tests to see whether and how this combination works.

The First Snag

As always with my retrocomputing projects, demons have struck almost before starting. I was planning to use a laptop computer running Minicom as a serial-console for the SPARCclassic, but that requires an old laptop with a *real* serial-port – most of the modern USB-serial adaptors don’t provide all the needed handshake/flow-control signals.

Unfortunately my go-to laptop-with-real-serial-port (an HP OmniBook 500, Pentium-III) itself has a hardware problem – the LCD screen is slightly but especially annoyingly broken, as you can see:

HP OmniBook 500

That bright red vertical line of failed pixels is right in my eye-line, darn.

I have already tried ye olde traditional attempted fix – making sure the LCD panel signal-connectors are fully plugged in, and wiggling them to make sure, and pressing down on the flat-flex cable. No joy, so first job is to replace that LCD with a new one. Project hardly started and I’m visiting eBay already! I had better get a new roll of kapton tape whilst I’m about it, to refix the flat-flex cable onto the new LCD.

Here it is – a solid 12.1 inches of pure XGA goodness:

Replacement wasn’t too tricky, for a laptop computer. Of course, a soft-plastic spudger and a miniature Phillips screwdriver is needed. Also, it is extremely wise to test it *before* screwing it all back together and reclipping the plastics back in place!

That’s a result, I can now get back to the actual project!

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Pimp My Classic

With RetroChallenge 2019/03 approaching rapidly, I have been trying to pick a project I could enter for the challenge – something to do with old computer tech, and something that I should be able to complete in 1 month.

A No-Fly Zone

Late last year, I purchased a 1975 SWTPC-6800 8-bit computer kit (including 2K RAM in the form of 16 individual 1-bit-wide 1Kbit SRAM chips!). However, the kit had previously been stored undisturbed in a basement for several decades, and a lot of the ICs have gone seriously rusty (packing-sponge-induced-rust disease). There is also one SRAM chip completely missing, and I have probably left it too late to be hunting for replacement 2102 SRAM chips in time for the start of RetroChallenge in less than 1 days’ time. Also, given the sheer amount of soldering required, not to mention replacement/repair of logic chips and possibly even the 6800 CPU itself, 1 month would not be sufficient time to get this all done and have something working to show at the end of it. I guess this one is really a longer-term project.

An Old New Hope

However, I do have a suitable 1-month other project that fits the bill: to show and document exactly how and why to perform the somewhat devious ultimate-upgrades for a particular excruciatingly low-end odd-ball early-90s classic computer, one that few will be familiar with. A computer model that is officially named ‘classic’ in fact: No, it’s not a Mac Classic, but the rather weak Sun SPARCclassic.

I’ll also be discussing the history of the model, the UK-delivery delay and the proposed trip to Scotland, how the SPARCclassic came to have the CPU it does (and why it wasn’t an ideal choice), why it’s name was a bit of a misnomer at the time, the slightly odd-ball RAM organisation and how to overcome it, a couple of ridiculously over-the-top upgrades, the “official Sun doorstop” incident, where to find some of the now lost-in-history device-driver software, the annoyances with the chassis case, and other fascinatingly boring details that would otherwise be lost in the mists of time. And lots of pictures! I can feel another HotJava screenshot coming on…

This project is going to tax my writing and composition capabilities – there’s just so much to say and show (never mind the accompanying actual doing). Organising it all succinctly enough is going to be difficult, but worth doing for one of the more oddball Sun models, which is saying quite something!

Stay tuned, retro-computing fans.

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Hollywood or Dust – Aftermath

A follow-up to [Repairing the PSU from an SGI Iris Indigo R3000]

For those of you wondering what happened to that SGI Iris Indigo R3000, after my failed PSU repair…

20181208_101139I purchased a replacement PSU from the magnificent Mr. Mapleson (patently the World’s foremost afficionado-and-vendor of historic SGI kit), and he chucked-in a complimentary 73Gb fast-narrow-SCSI drive on an SGI sled.

Together with an SGI 24-bit 3D graphics card (XS24-Z) that I obtained from Hungary, I had a full extra year of Indigo fun, and rediscovered what a wonderfully capable machine the 33MHz R3000 Indigo is – as can be seen in my subsequent videos:

However, after that bonus year, I needed to make room in my small flat for a couple of newly-procured very old computers, and thus had to let the Indigo go. Although there was quite a bit of interest from America, that was too far to ship it. But, via a tertiary acquaintance-of-a-mailing-list-user, guess who found out I was looking for a good home for it… “Come on Down, Ian Mapleson!”

Thus, after entertaining me from 2001-2018, this 1991 SGI machine is now in the best home possible for this kind of thing, a place where it will be well-cared for. Happy ending!

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Hollywood or Dust: Episode 5 – Fashionable

[Repairing the PSU from an SGI Iris Indigo R3000]


After replacing damaged diode CR111, it’s time to reassemble the PSU and give it a try. Reassembly is a bit easier than disassembly, but still very fiddly, especially getting all the spade connectors back where they are supposed to be. Needlenose pliers are needed, and some patience, and a willingness to poke fingers into the internals to get the plastic system-connector-plug back in its’ slot.

The crap semi-internal bolt I had to drill out in Episode 2 has been replaced by a proper steel hex-driven bolt, soooooo much better!

As this is a “smart” PSU, I cannot test it in isolation – it needs some output load before it will do anything, so let’s get it fitted back into the Indigo.

Barebones Power-Up

Fitting the PSU back into the case is very easy – it just slides in on rails into the top right of the case, as viewed from the front, then is held in place with a single screw at the back of the case.

Next step was to power-up the Indigo with just the CPU board, without the graphics board and drives installed, after unplugging everything else in the house, just to make sure the repaired PSU wasn’t dangerous and wouldn’t freak the mains power supply. Went well – fan spins, no burning smells, no sound of capacitors popping, and no smoke!


Next it was time to try it with the system-boards installed. Unfortunately, I had misplaced the SGI-special keyboard cable, in spite of having the actual keyboard to hand. No problem, let’s just try with a serial console instead… unfortunately, Indigo requires a rather special serial cable, which I had also misplaced.

The Indigo doesn’t have DE-9 or DB-25 serial connectors. Oh no, that would be far to straightforward. Instead it has mini-DIN sockets for that. This was fashionable for about 2 years in the early 90s in the UNIX workstation world – the actual cable I was looking for was my Sun serial-adaptor cable for a SPARCstation-2, which uses the same pinouts as a “proper” SGI one (which I don’t actually own). I keep dreaming that, like flared trousers, mini-DIN serial connectors will come back into fashion, “any day now, you’ll see!”

After hours of searching through boxes, bags, drawers, the top of cupboards, et al, et al, I found the darned cable on a bookshelf. Dang! Time to get my trusty P3 laptop computer out, to act as the console. Oh yes, I’ll need a null-modem serial convertor as well, which gives me an excuse to get ye olde serial breakout box into the project!


For those of you that don’t remember 15-pin D-shell Ethernet sockets, you need an external transceiver (formally: “Medium Attachment Unit”) to convert it to more common form such as RJ-45 or 10Base2. A widget like the CentreCOM 210T…

Let’s Go


Unfortunately, when trying the PSU for real, under load, the signs were not good. It couldn’t see the SCSI hard-disk drives, much like before, even though their LEDs were lit. It recognises the SCSI floppy-diskette drive, but that only needs the +5V rail. Sounds like the missing +12V output is still, well, missing. Mind you, at least you can hear the funky Indigo power-on sound in the video.

Time to probe the actual PSU output voltages.


On the SGI indigo, the HDDs are mounted on slide-in sleds, which breakout the SGI-custom backplane connector into a 50-pin IDC SCSI ribbon and a 4-pin molex connector. Armed with an empty drive-sled, I now have probe points to check both the +5V and +12V rails, via that molex connector on the sled.

Hmm, with the CPU board and both a SCSI HDD and SCSI FDD installed, the nominal +12V line is showing only 3.51V. With just the CPU and FDD drive installed, we have  2.38V. With just the CPU, we have 2.49V. What the heck! There is obviously some other fault with the PSU, not just the dead zener diode that was replaced. Just to be sure, I probed the other line in the molex connector (should be +5V), and hey presto! it shows 4.97V – which proves I knew which line was which, no mistakes there.

More Repairs

One other person has noted that capacitor C127, next to oft-failing zener diode CR111, is directly linked in series with CR111. Although C127 is not showing obvious signs of damage, I thought I could just see a slight bulge in the top wrap-over plastic coat. Maybe C127 needs replacing too?

I was not completely convinced, but with time running out for RetroChallenge, it was at least worth a shot.

However, replacing a radial capacitor (when you can’t get at the legs) is way more tricky than working with axial components where you have full soldering-iron-access to all the important bits. Eventually, it was a case of ripping the damned thing out by force, then desoldering and removing the stumps.


One replacement 22uF 25V aluminium electrolytic capacitor later, and we have… no change at all, exactly the same behaviour as before. Shucks. There are obviously other things wrong with this PSU, things that only a complete re-cap and re-power-transistor are likely to resolve. Unfortunately, that is way beyond my capabilities. This particular RetroChallenge project is complete, but failed.

It appears that the only way to get this Indigo working again, is to obtain a fresh 25-year-old PSU. Time to give Magnificent Mr. Mapleson a call!

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Hollywood or Dust: Episode 4 – Vitamin E

[Repairing the PSU from an SGI Iris Indigo R3000]

The postman has delivered my replacement 1N5352 zener diode (actually three of them, I figure this might take a couple of tries, and I might need another one for a subsequent repair in another 25 years).

Vitamin E

However, before wielding the soldering iron, I am going to clean the internals of the PSU case and boards. As noted previously, there are some metal particles scattered around from previous grinding etc, as well as some concretised dust.

The toolset for such cleaning consists of a soft paintbrush for the bulk cobwebs, a medium-stiff toothbrush for the concretised dust, cotton buds for the hard-to-reach corners, and some physically strong moist wipes (with volatile moistness, not just water) to encourage lift-off.

When it comes to moist wipes, the local supermarket offers two choices: with cucumber extract or with vitamin E. No “bog-standard plain” ones. So I am going with vitamin E version, it seems just a bit more definite than the rather vague “cucumber extract”. As an extreme long-shot, vitamin E is a natural antioxidant in the human body, perhaps it will also help prevent rust in the PSU… 😉

The Other Kind of Flat Head

Those toothbrushes aren’t ideal – these days getting a proper old-skool 1970s-style “flathead” toothbrush is not all that easy. These days there all kinds of wild-and-wacky  brushhead shapes and profiles – ergonomic for cleaning teeth (minus any snake oil of course), but for scrub-cleaning flat plastics, metal casings, and even the odd solder-pad, a good flathead plastic brush is easier and more effective, in my experience. But needs must, so I had to manage with the Junior Sparkie BRIGHT brushes.

Inversion Not Welcome Here

Next, I need to install and solder the replacement 1N5352 diode. When reinstalling electronic components in a PSU, it’s always good to have as many photographs of the original internals as possible, from a wide variety of angles. I had to refer back to an earlier photograph to make sure which way around the replacement 1N5352 diode needed to be – get it the wrong way round, and a possibly life-threatening explosion could result, as well as possibly blowing fuses and even damaging other equipment in the house, which would be even more unappreciated!

The photos are also useful for checking exactly which spade connector goes onto which lug, and so on, when reassembling the PSU.

A New Hope

When soldering-in the new 1N5352B zener diode, I deliberately arranged it to lie 2mm above the PCB, rather than flat against the PCB, as it was originally – if the reason the original diode blew is due to a PSU design issue, I want to avoid a second such diode-failure event from further damaging the PCB if possible – it’s had a hard enough life already.

Thanks to leaded solder, a bit of liquid flux, and the Metcal iron, installing the new 1N5352 diode was very quick, straightforward and easy. Not much to say here, so let’s look at the result:

Looks OK from here. Note that the vast majority of the discolouration and other damage on the PCB was caused by the original failing diode, not my soldering (which is not expert, but is also not that clumsy!).

Making the daring assumption that nothing else is wrong with the PSU internals, this “should” do the trick and result in a running SGI Indigo R3000… but I think I’ll wait until the house is empty (tomorrow) before risking tripping the mains power.

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Hollywood or Dust: Episode 3 – Just Another Sucker On The Vine

[Repairing the PSU from an SGI Iris Indigo R3000]


After two weeks off from Retrochallenge due to other commitments, it is time to re-energise this project.

I need to remove the obviously-broken diode CR111 from the large PCB, (a) to see which exact model of diode it is, so I can order a replacement, and (b) to see what other damage, if any, lies underneath it, or nearby.


De-soldering components it not something I have much experience of, but armed with a Metcal 200 soldering station, I was hoping it would work out OK. After a bit of reading about desoldering, I figured I should get a solder-sucker too – a tool to remove freshly remelted solder.

There are two overall kinds of solder-sucker:

  • High-End, Expensive: all-in-one tool that provides an electric heater to melt the solder; and also incorporates a vacuum pump powered by an electric motor, to suck the melted solder away from the component concerned;
  • Cheap-And-Nasty: a spring-loaded vacuum pump that sucks only – you need to heat the solder with a separate soldering-iron.

No prizes for guessing which one I went for!

Pentax Digital Camera

This really is low-end – extremely thin metal body, plastic piston, etc. The whole thing weighs less than a pencil. It feels like it would bend in half if you just looked at it wrong.

More Hands, Please

Given my soldering skills are rather poor, this could have been an “opportunity” to really damage the PSU…

This counts double because using a separate non-integrated solder-sucker
can require you to use three hands simultaneously: one to hold the sucker (and poised thumb over the release button), one to hold the soldering-iron, and one to hold the expansively-populated board still whilst you do the necessary. I suppose on a small sparsely populated PCB you could use a vice or even a couple of books to fulfil the “holding” function, but with this particular beast of a PCB, that wouldn’t work too well.

For through-hole components, there is a trick that can help: deploy the sucker on the “lead” side, and apply the soldering iron to the “component” side of the PCB, and press them together to hold it all still whilst doing the deed. Like so…


For small blobs of solder or small leads in large holes, this works well. I know, I practiced on some scrap perfboard. For the largish blobs of solder holding thickly-leaded CR111 in place, this only really does half the job – it doesn’t suck the solder all the way through from the “other side”.

Thus I had to resort to the “standard” technique as documented all over YouTube: prepare the sucker in one hand, close to the joint, but not actually on the joint; apply the soldering iron to the joint until the solder melts; then very quickly remove the soldering iron, deploy the sucker and hit the release button.

And then the most important part: repeat, repeat, and repeat again until bored – you usually won’t get all the solder in one go.

The Result

I managed to remove the damaged diode without scorching the sucker nozzle, and without getting melted plastic or detrius on the iron tip. This is a testament to the Metcal iron – tip heats up extremely quickly, and maintains heat even in the face of plenty of melted solder which is doing it’s damnedest to solidify again.

Mind you, due to the odd minor slippage hear and there, there are one or two marks on the solder-mask on the back side of the board – but fortunately, nothing too serious (most of the solder-mask damage was already there due to the way the diode had originally failed).

So what do we have? Closer inspection shows CR111 is a 1N5352B 15V 5W Zener diode, as suspected. A replacement, rated for use up to 200C (well, better safe than sorry), has now been ordered, awaiting delivery.

Any Other Business

At least one other person repairing the slightly different R4000 Indigo PEC 4044B PSU has also had to replace the nearby electrolytic capacitor C127 (which on the 4044B is a 35V 120uF capacitor, but on the 4044A is a smaller 25V 22uF capacitor). C127 on my PSU appears to be completely undamaged, so I am going to leave it be for now.

Then there is the tiny ceramic capacitor right next to and alongside where CR111 used to be (see above pictures): that must have been subjected to a lot of heat when CR111 died, due to proximity, but it too appears to be working as best as I can tell (using a very rough-and-ready capacitor-testing method with both a digital multimeter and an analogue current-tester). In principle, due to the heat it must have endured, I perhaps should replace it anyway, but given that actually getting to it is also very difficult due to adjacent components on one side, I’m going to take the chance that it is still OK, at least until proven otherwise. It’s not liquid-electrolyte-based, after all.

Summary: digikey.co.uk are now at-bat. I wonder how big a box I will receive this time!

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Hollywood or Dust: Episode 2 – Zarking Genius

[Repairing the PSU from an SGI Iris Indigo R3000]

More Investigation

In the last episode, I managed to extract one of the boards from the PSU and found it to be in superb condition. But what about the other, larger PCB? Although I could see parts of it “in-chassis”, I really needed to remove it to be able to inspect it fully, and to have any view at all of the non-component side.

Hey, removing the other, smaller board was pretty straightforward, so this should just take a jiffy, right?


Unscrewing the three screws that attached the board to its’ cylindrical standoffs was easy enough. The screws are crap, soft metal, perhaps aluminium? You have to be a bit careful not to knacker the screw-heads – gently, but firmly, does it!20170905_201207

However, in spite of doing so, the board would not move, neither slide-out nor lift, not even a bit. Dammit, there is another “attachment” that needs to be dealt with, on the outside of the chassis…

That darned hex-nut, and it’s location, is a royal pain. After 25 years, it has welded itself to the large solder-pad on the PCB.

I dug out my hex-driver sets, only to find that they were all Imperial rather than Metric, and this sucker needs a 4mm driver. A visit to the hardware store didn’t show up any as small as 4mm, not even rummaging through their behind-the-counter “special stash”. Fortunately, the local petrol-station has its’ own hardware tool shop, including precision tools, so I was able to pick up a set of hex-drivers that included a 4mm driver.

However, that didn’t actually help. This particular hex-bolt is sooooooo low-profile that any regular hex-driver just won’t grip it properly. The usual solution to that situation would be to apply a lot of downward pressure, but this damned bolt is made of some really cheap and nasty soft metal, and I nearly stripped it’s hexiness. Plenty of cursing ensued!

Given its’ location, getting even needlenose pliers or a needlenose wrench onto it just won’t fly – you can attach, but there just isn’t enough room to rotate it! There isn’t even enough room to saw-cut or file a straight slot into the head (to be able to get a flathead screwdriver to turn it).

All together now, loudly, with feeling: “Which zarking genius designed this damned arrangement?”


The only solution was to grind the head off the bolt, using a Dremmel fitted with a rather narrow-tip grinding head, being very careful not to wreck the underlying PCB…20170910_120302

Not too much scraped, considering the confined working-space, although some metal particles were scattered over the back of the board, which will thus need cleaning.

The remainder of the bolt (thread body still in the hole) still needed removing. Cue a naked junior-hacksaw blade and plenty of patience, to cut a slot in the barely-protruding top of the thread, to be able to get a small flat-head screwdriver onto it.

Even with the troublesome bolt removed, getting the board actually out of the chassis was still tricky, due to the 24-pin system-backplane connector. The fly leads to same are very short, and are very stiff thick wires. There isn’t enough room above the board to push that connector through its’ chassis-cutout without removing the board first, but you cannot do that as the connector is hard-attached to the board by such a short cable-harness. Time to pick yourself up by your bootstraps?

Also, that connector has active components attached to its’ backside, so using force wasn’t a sensible option.20170910_120353

A lot of wiggling back-and-forth, both pushing the connector from one side whilst simultaneously pulling from the other, all at the same time as pull-sliding the PCB out to make room, FINALLY got the danged thing out. Hooray!

Not Too Shabby

Most of this larger board and components looked in OK condition, albeit a little dusty in places.

Not too shabby, huh? Oh yes, spot the two huuuuuge capacitors lurking under the black plastic sheath – a reason for nervousness whilst trying to wrangle the board out of the chassis – those could give you a very major DC shock!

But Now, The Good News

But there was something a bit odd-looking in the middle of the board, next to the plug where the smaller board would attach.

The solder pads for component CR111 don’t look quite right, and is the PCB a bit discoloured there too? Time to take a look at the other side of the board underneath CR111…

WOW! The solder-mask on the underside of the board underneath CR111 looks like it has been heat-treated with a blow-torch!

These signs of damage are good news, I have likely found the reason why the PSU +12V rail is dead – CR111 has failed, and rather spectacularly.

Famous Critter

Now for a little aside… on the ‘net, there are a few reports of people repairing ITT PEC4044B PSUs as used in the R4000 Indigo, but absolutely none regarding repairing the different PEC4044A used in the R3000 Indigo. Comparing my pictures with pictures of the 4044B internals, and in spite of the specification differences, it appears that the two PSUs use the exact same PCBs, just fitted with different capacitors/resistors/etc. In particular, http://forums.nekochan.net/viewtopic.php?f=3&t=16731963&p=7401329&hilit=Indigo+PSU#p7401329 shows what appears to be an identical fault to my 4044A, but on a 4044B, together with a mention that yet another person also found the same problem with CR111.

CR111 is famous!

OK, so three people out of several thousand isn’t what you would reasonably call compelling statistical evidence, but in 2017, its’ probably the best that can be achieved.

I’m guessing that CR111 in my PSU is also a 1N5352 15V zener diode, but before I dash off to digikey.co.uk, I will need to extract it from the PSU, to make sure.

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Hollywood or Dust – Episode 1: Down the Rabbit-Hole

[Repairing the PSU from an SGI Iris Indigo R3000]

Oops, Retrochallenge 2017/10 has been running for a week already – better pull myself together and make an actual start on my project!

Peculiarly Special

The Iris Indigo has a slightly complex PSU history and situation. The original Indigo, based on the MIPS R3000 CPU, uses an ITT PEC4044A PSU – a custom job built especially for the R3K Indigo. This is not your father’s PSU, not by a long shot. You can’t just stick an AT, ATX or microATX PSU in there, this one has custom connectors/pinout/form-factor/everything…

Secondly, to power the R3000 version of the Indigo, it needs to be able to provide 25A at +5VDC, 6A at +12VDC, and 1A at -12VDC. You won’t find any even vaguely recent off-the-shelf PSU that can provide that much 5V current – it has been a long time since 3.3V (then 2.9V then 2.1V then 1.8V etc) became fashionable. And a hard requirement for a -12VDC rail is much less common than it used to be!

Thirdly, and most importantly, the PEC4044A PSU also contains the Indigo system audio loudspeaker. Yes, really!

Brothers but not Twins

The later R4000-based Indigo uses the same chassis, graphics boards, connectors, drive sleds, backplane (except for the contents of the “Model ID” EPROM chip), et al – but does not use the same PSU, due to the extra power consumed by the R4000 CPU. The ITT PEC4044B PSU in the R4000 Indigo needs to be able to supply an additional 5A on the +5VDC rail. The 4044B is the same size and shape and is fully pin-compatible with the 4044A.

So yes, in principle, I could buy a (slightly younger) R4000-Indigo PSU and fit it in the R3000 Indigo. Many people have done exactly that over the years. However, the “B” revision PSU is still truly ancient (so even “new” ones have been sitting on a shelf for approx 24 years), and are almost as expensive as the “A” revision PSU. My wallet would not approve, and the chances of getting an already-wilting replacement are just too high.

Getting Inside

Removing the side cover was not too difficult: undo two screws, flex the side that has shaped “spring-clips” formed into it, and (with some force) slide the two-sided cover up.

Inside there are two boards, connected at 90 degrees,with plenty of fly-lead connectors, a couple of humungous evil-looking black capacitors and a couple of old-school fuses.

Hmmm, two fuses. One for each major rail? Let’s check the fuses then, maybe this could be an easy fix! Unfortunately not – both fuses are fine, so it’s back to the grindstone. Shucks!

Removing the smaller secondary board is straightforward enough – unplug the speaker cable, disconnect all the other fly-leads, snip the cable-ties binding the cable-bundles to the smaller board/frame, and pull the board “up” (from original PSU orientation) – it simply unplugs from the primary board.

Good news! This board looks in superb condition, even after 25+ years – no leaking or bulging capacitors, no signs of corrosion (apart from on the two vertical heatsinks), no signs of trace damage and no obviously fried components. Just a little bit of dust. Almost as clean as a whistle!

Looks like more investigation will be needed…

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Hollywood or Dust

As is often the case with hardware this old, my 1991 SGI IRIS Indigo R3000 will not start. Well, that’s not quite completely true…


When pressing the power-switch, the fan did not spin, neither of the two SCSI disk drives spun up, the screen remained resolutely blank, and the cute Indigo power-on tune did not play (yes, really: 1990s SGI machines have firmware that plays a short melody when the power-switch is pressed, before any attempt to boot an O/S).

However, signs of primitive life were there – the LED on the CPU board lit up yellow, indicating firmware power-on self-test is running, but it should then go green, which it just didn’t.

The LED on the external Ethernet AUI adaptor (remember those?) also wasn’t illuminating, which actually was rather a clue to the real problem, although not one I was aware of at the time.

Others have noted that on the R3000 Indigo, hot-replugging the CPU board can sometimes kick recalcitrant machines into life, so it was worth a try at this point… but didn’t make any difference. And no, it didn’t result in piles of smoke, either.

Inspection One:

Visual inspection of the CPU board (including checking for bent pins, blown or leaked capacitors, burned-out traces, and so on) revealed that it looked in fantastic shape. Multimeter also revealed that the on-board 3.6v IDPROM battery was good.

The graphics board also showed no sign of damage or rot. The backplane appeared likewise – no visually obvious problems there either.

Ripping Out:

Initially I thought that the SCSI disk drives might be suffering from stiction, so I removed them, installed them in an external powered SCSI enclosure, and tested that they spin-up when externally attached to my SPARCstation-10… they sure do.

With the drives powered externally and connected to the Indigo, they spin up, but the screen remains blank, but now the power-up tune does play, and the system might be trying to boot.

Second maneouver was to disconnect the keyboard, and use a serial-console instead: nope, no console messages there either. Not even corrupted, glitched characters. Nothing. This is another clue, although one that does not apply to all types of personal computer.

These Boots are Made for Running:

OK, so let’s remove the graphics board too. With just the CPU board in the chassis (drives in external enclosure, serial console), the system boots… Unfortunately, the ethernet still wasn’t working.

There is nothing left to remove, so now what?


As is inevitable when dealing with old SGI kit, I ended up relaying my difficulties to the Nekochan Forums, a very informal online site for discussing all-things-SGI. For retrocomputing enthusiasts, it’s well worth a look.

Armed with my description of the system behaviour, one of the hardware experts there spotted the problem straight away: there was no 12V power getting to the components.

AUI adaptors need +12V to function, as do the SCSI disk drives. Also, 90’s SGI on-board serial-ports like 12V power, although it seems they can hobble along with lesser voltage. Nothing else on the CPU board requires 12V.

So that explains the problems.

The Job at Hand:

The obvious solution, buying another Indigo PSU, is a bit of a reach, current prices are around £200+. And even for new, unused, still-packaged units, it would still be 25 years old, and thus not necessarily “in good order”.

Thus it might be better (well, cheaper anyway) to attempt to repair the PSU. I’m not too keen to go poking around inside an ancient PSU with HTDC and 240V AC running through it, but needs must?

Posted in Hardware, RC 2017 Autumn, Retrochallenge | Tagged , , , | 6 Comments