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IBM PS/55 Z – 5530 All-In-One

I saw this on eBay and couldn't resist... I probably should have resisted... I also probably should have actually reviewed the specifications prior... but whatever... I made the seller drop it to a Pack'n'Send and it arrived, perfectly packed, in no time. It's a cute little unit. I actually had the 12" IBM VGA monitor back in the day as a single unit and loved it for its clear and sharp 640x480 picture!

The IBM Personal System/55 (Here's some more information) was a series of computers released in Japan back in the late 1980s. It was based on the x86 architecure, but used IBM's own Microchannel (MCA) system bus. I sorta wish I'd researched that point beforehand as nowadays MCA cards are few and limited in technology. Ethernet is hardly BNC co-axial and SCSI is rare. Audio is even rarer... but there were Creative and Roland cards! Good luck trying to find one!

The front sports two slots for floppy drives, brightness/contrast and a lovely lever-style power switch. There's also your power and HDD LEDs. The swivel base is also a nice addition. There's even a lever on the rear to secure the unit once you've swivelled it to the point you like.

There's a panel on the rear to cover up the VGA cable. This cable is a little strange... it seems to be an artifact from the fact that the whole unit is really just an IBM 12" monitor with a growth hanging from the base. Instead of running the cable internally, they actually have a midget lead and plug that is exactly the length required to fit into the Display Adapter card which can therefore only be in slot one. This becomes more apparent inside when you realise the card requires an extended slot and, you guessed it, slot one is the only such slot.

The whole PC area of the unit slides out once you unplug the VGA cable and unscrew the two black screws. I really do like the modular design! Note, it also seems this unit expects the keyboard to have a speaker in it? Interesting!

But what does it look like on the inside?

Getting access to the motherboard is super easy. Unscrew the two screws and then slide the whole lot out. Make sure you unplug the VGA lead first! The shelf that the motherboard sits on is easy to work with and provides access to drives as well. My unit didn't come with any... I assume the floppy will be standard? But the HDD definitely isn't. Jeesh. The MCA bus is also totally obsure and the graphics card is a Display Adapter IV. It also needs to stay in the top slot as that's the only extended MCA slot and it's also the only one the VGA lead will reach to.

You'll note that with the motherboard removed, the case is still very much in-tact. Actually, it's sealed. Getting to the area where actual power supply and monitor are is a little more tricky. I wanted to give it a good visual inspection prior to powering up, so I endeavoured to open it. Turns out it's built to be totally tamper-proof and return-to-factory only! There are two removeable caps on top that present you with 'headless' screws. Yes, they're screws... but there's no ability to actually unscrew them.

I made a total mess with my (fake) dremel to try and gouge a flat-head screwdriver slot. I ended up melting a lot of plastic... but for some reason I'd decided that the covers will go back over, so the mess is contained. Either way, the screws came out and the case came off.

Yup, it's a CRT alright... and that power supply is totally embedded into the monitor housing. So much for an easy switch to a 240v supply. I'll revisit this once I work out if the motherboard is good. If it's not, I might just gut the whole thing. Back in '99, The Techtroid Box did just that.

Meanwhile, the patterns of dust on the inside of the monitor housing were slightly mesmerising!

Motherboard Layout

This is a flat AT style board. All integrated ports are along the rear. Some of the ICs have IBMs infamous heatsink shields that make them look military-grade. The MCA bus is obvious and two riser cards exist for a HDD and the 3 MCA slots. There's also a floppy header and some random jumpers.

There's a header at the top end that plugs into a socket very similar to a Commodore 64 User Port Edge Connector. This is fed from the powersupply that's intertwined with the monitor circuitry in the do-not-touch zone. Looking down at the board, with the edge connector pointing up, the pinout seems to be as follows:

So, above you'll find the top and bottom rows, as if you were looking into the socket inside the chassis from the rear. I have no idea if the voltages are correct as my machine isn't booting.

Does it work?

The unit requires 110v AC, so I used my trusty step-down transformer. After re-assembling the bare minimum, I applied power. The monitor came to life, but the motherboard was dead. There was a little bit of heat from the CPU, but no other life. No manner of RAM SIMMs helped. A closer inspection revealed that just about every aluminium electrolytic capacitor had started to corrode. Tracks were beaten in a few areas. Looking even closer, it also seems that the motherboard is 4-layer: tracks top and bottom with two plates in the middle... possibly positive and negative?

I started replacing capacitors where relevant and testing tracks... No amount of effort brought it back to life though.

Turns out you can feed the motherboard connector through the front floppy slot and then connect it up... allows for much easier fault diagnosis!

Is it the EPROMs?

I intend on popping them out and reading the data off them. From a visual inspection of the code, I might be able to tell if they look OK? Maybe copyright data, etc... but I'll have no checksum to work with to determine if the images are correct. I'll also have no other images to load to test further...

Final answer: replace the guts?

I'll do this in another post. There's a few options for motherboards with riser cards on eBay and I'll need to choose wisely as I'll want the measurements to line up as closely as possible. Of course, I could just slap something in (as per The Techtroid Box's post), but I want it to be cleaner and more permanent.

Does the CRT work?

The video card is an IBM "Display Adapter IV". Supposedly it has an 1024x768 mode? Here's more information, albeit in the wrong language. And this article (scroll down to PC # 6) also indicates the resolution.

I happily got 640x480 working... I'll spend a little more time later with 1024x768. First I'll find it some new innards.

68-pin RAM SIMMs

I had previously picked up this Bravo 4/33 486 Machine and had assumed, from the photos in the auction, that it needed standard 72-pin SIMMs. A quick physical test proved this assumption very wrong. This was a proprietary motherboard and the slots were narrower than 72-pin SIMMs and wider than 30-pin! I'd never actually seen anything like this before. I then researched and realised that this board needed 68-pin SIMMs.

eBay came to the rescue and two 4MB SIMMs arrived from Denmark! Seems they're mainly used in Roland hardware or Macintoshes. Anyway, from the photo above you would easily mistake them for 72-pin SIMMs. It all becomes obviously when you line up all the types...

Above we have a 128-pin SIMM, a 72-pin SIMM, two 68-pin SIMMs and then a 30pin SIMM. I think quickly tested in the motherboard and success! They fit!

Of course, the machine still didn't boot... time for more mucking around... (and no, it wasn't the fault of that loose screw...)

What RAM slots are these?

I recently found a relic on eBay. It's an AST Bravo 4/33 (not the LC!) and has a very proprietary motherboard. I'd seen the photos and I thought I saw standard 72-pin SIMM slots. Turns out I was totally wrong... I've never seen this size before!? There's a similar-but-not-quite motherboard diagram here, but the jumpers are in different locations. The strange RAM also doesn't get a mention?

Turns out they're neither 30-pin, nor 72-pin! Did AST decide to use Apple's proprietary 64-pin SIMMs? There are similar (68-pin) 256kb and 512kb SIMMs on eBay, but they don't match the pin-count and they're for VRAM in older Apples? There also seems to be other something resembling RAM soldered onto the motherboard, but I can't find any information to determine if that's actually RAM or just cache. Either way, the machine just beeps once when power is applied. The beep sounds like a standard "NO RAM" boot error. There's also a fried electrolytic capacitor.

...Actually, I just found two 64-pin 4mb SIMMs for an Amiga... Let's see if they work when/if they arrive...