Topic: Atari Mega ST4 Repairlog  (Read 4026 times)

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Offline Womble

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Atari Mega ST4 Repairlog
« on: January 01, 2015, 09:45:36 AM »
I doubt there were many of these under the Christmas trees this year, but under mine was this, an Atari Mega ST4!!

I have an unhealthy number of the Atari ST line from the original 520ST with external floppy and PSU upto their last gasp in the form of the Falcon 030, but I had never seen a Mega in the flesh before. It is basically the same machine as the STFM but with the addition of a blitter chip as standard, an internal expansion port for upgrade cards and a lot more RAM than most STFMs ever saw. Mega STs originally came in 2MB and 4MB versions, and despite the press at the time stating Atari would not release a 1MB machine it seems they eventually did so the line up runs from the base Mega ST, to the ST2 and this one, an ST4. Apparently the Mega name proved to be problematic for Atari when they tried to call the hard drive for the range the Mega Drive, for some reason Sega took exception to this and threatened legal action. Atari backed down and renamed the HDD range to the somewhat less inspiring Megafile.

The unit was bought untested presumed faulty by the seller as it had been in storage for 15 years, and at some stage it had been modded to give it a DB9 video output socket instead of the odd 13 pin socket that only Atari and Kenwood used.

The rear blanking plate that covers the expansion module slot was missing and a large phono socket was fitted on the shielding. Initial hopes that this might be something exotic didn't last long, the original Atari 13 pin DIN socket carried video and audio, but the DB9 probably wouldn't, so I expected this would be the missing audio output tapped off the board.

It was missing the keyboard cable, but it did come with the original batteries for the internal system clock, best before 11/87 according to the data stamp.

Possibly somewhat past their best but the damage could have been a lot worse. One terminal was welded to the battery and took some persuading to let go.

As I knew this was coming I bought a new 13 pin DIN socket from eBay as I am awash with Atari monitors and wanted to put this back to the way it was born, my only concern was how much damage had been caused by whoever did the conversion.

Once the lid was off and the shielding removed I could get a look at the patient.

Initially it looked like the mod was done without making too much mess...

...and the DB9 was mounted fairly neatly with a single screw through the rear shielding so it could use the original case hole rather than a new hole being hacked through the plastic.

Sadly underneath was a different story...

... oh the horror.

Still, the pin configuration of the new DIN 13 was correct for the board.

With the old mod removed...

... and a load of isoprop applied to clean off the old burnt flux it looked a lot better.

Only a couple of plate through holes that were actually used had been ruined, so the socket was easily fitted plus a single length of hookup wire and a small length of component leg soldered to a track so it would join to its pin.

Back the way nature intended!

A few other issues were fixed too, the connection of the lead to the battery compartment had been badly soldered and was about to give way, and a large capacitor was loose due to a poorly soldered joint.

Also someone had also been at the cart connector shielding with some pliers which was pretty ugly, plus the remains of a past resident had to go.

The sheilding was sorted out with a mini-vice to get a long straight squeeze on it.

Pretty effective!

Finally some solder and wire remnants on the inside of the cartridge connector were removed.

Time to fire it up...

... hmm dead, just a white blank screen.

Most of the time anyway, once it did give me a load of corruption on the screen.

A quick probe of the 68000 CPU showed it was getting clock, and that /RESET was toggling correctly when the system booted and was going high after a brief stint low, /HALT on the other hand was held low and only slightly twitched on boot. On a Mega the /RESET and /HALT lines come from a 74LS07 buffer chip at U2.

Pin 1 and 3 are the inputs to two buffer gates and are tied together giving a common input. The output from the gate across pins 1 and 2 goes to /RESET while the gate on pins 3 and 4 goes to /HALT. Some research online showed that U2 is a very common cause of Mega STs failing to boot as the other gates on the chip are used for the MIDI sockets so any external violence on the MIDI lines can damage this chip as not all lines are opto-isolated. The joined input pins to the two gates were being fed from the reset delay chip at U1 (the other common failure apparently) and were going high at the correct time, so that section looked like it was behaving.

The problem I have with the /HALT line on a 68000 is that it is both an input and an output, a multi CPU system can instruct a 68000 to halt by pulling that pin low, but the 68000 can also use the pin to signal to the outside world that it has encountered a bus error and has halted itself. Its hard to tell which is which from the outside, but as U2 was a known common failure I desoldered it.

In fact I had to cut off half the legs as one side of the chip just would not desolder, half the pins were missing their stumps on the solder side too which meant I could not waggle the legs while desoldering.

A known good 07 was soldered in place and the system rebooted. Still no change, /HALT was still staying low so it seemed something on the bus was causing the CPU to halt itself.

Likely candidates included the DRAM, the TOS ROMs or the glue logic itself. The glue logic is a large custom chip and probably fairly robust, the TOS ROMs are a pair of Atari Mask ROMs of unusual pinout so a pain to test...

...and the machine has a lot of RAM, 32 DRAM chips that share the busses.

The TOS ROMs were the easiest place to start, but as these are mask ROMs and do not share a pinout with any EPROM, would have to make an adapter to verify them. Except I wouldn't, I had assumed the Mega machines had a their own TOS versions but they don't, they take the standard TOS used in early STs, the only variation is whether they are a  6 ROM set or a set of 2 larger mask ROMs. A quick dig in my 520STFM revealed it had the exact same pair of ROMs, even down to the Atari part number on the silk screen. So I swapped the unknown ROMs from the Mega into the STFM and powered it up, and was greeted by the white screen of death. Swapping the known good STFM ROMs into the Mega and connecting everything up gave me a system that would finally boot to the little green desktop.

It's Alive!

The next problem was that without the keyboard cable I had no way to get the machine to do anything after boot. The mouse sockets are underneath the keyboard and with no way to connect the keyboard to the main unit I was lacking both a keyboard and a mouse. More research showed that the cable required is just a straight through 6 pin RJ12 cable that is used on some phones. To add confusion to the matter 6 pin RJ12 sockets don't necessarily have 6 pins, some have 4 but are in a connector the right size for 6. This is usually indicated by "6P4C", or "6P6C" for 6 pin 6 core cables. As the Atari keyboard pinout has pins 1 and 2 joined together and pins 5 and 6 likewise a 4 core "6 pin" would work fine. The local hardware store was happy to sell me a 30cm 6P4C phone cable for the princely sum of $5, it even came in the slightly yellowed Atari colour. Shame it wasn't quite right, this was a cross over cable where pin 1 goes to pin 4 and pin 2 to pin 3 etc etc. With the keyboard PCB out of its case and connected to the Mega with the power off I could tell that the power to the 74LS244 on the keyboard was reverse polarity, the 5V rail was connected to the ground pin and the ground rail was connected to the 5v pin, it really would not like that.

With a lot of force  I was able to pull one of the RJ12 connectors off and by drilling some small holes in the connector I could insert needles to lift the contact blades again. This allowed me to re-crimp it with the connector reversed, turning a cross over into a straight through cable.

With it connected up I now had a working keyboard and mouse and could test the system.

Which didn't get me very far, the floppy drive was dead, despite the system recognising it has a drive on boot it is unable to read any disks you give it, the heads are constantly searching blindly for the tracks and getting nowhere. After a stint of head cleaning and looking for anything obvious I admitted defeat. Finding a replacement drive is easy enough, but getting one that will take the Atari style fascia and huge eject button is not.

This one was one of the older style double thickness drives and seemed rather Atari specific...

... after a bit more research it seems it is a Chinon D357, a very similar to the drive model used in early Amiga 500s but not close enough to transfer the bezel and button as the front mechanism is different, possibly they were custom orders based on the aesthetics required.

How about this as a donor?

It contains another Chinon drive, slightly different model and the bezel and button are actually different on the inside but it fits perfectly, right down to the slanted LED that fits into the slot for it on the Mega's case upper.

Once fitted the system could finally load disks, so it was given the Psygnosis Test and left to play the Shadow of the Beast theme tune for a while to prove I finally had a fully working machine.

One remaining issue was that it was filthy.

Bath time!

The case upper and base were fairly straight forward, the real work on these restoration gigs is the keyboard. The photos probably don't show how manky this was, but this shot does, and that was only after the casing had been done, the keyboard was another matter.

Very unpleasant...

... everything was pretty well stuck down with what might be old cigarette smoke residue...

...whatever it was warm water and Jif chewed through it.

The keyboard on the Mega is of a higher quality than that on the ST which has the feel of typing on bubble wrap. This one uses Cherry micro-switches for each key instead of a rubber domed contact, and although it isn't up to the clicky key quality you get on high end PC keyboards it is a great improvement, except when it comes to dis-assembly. The micro-switches are pushed through the metal plate and clip into place, and the PCB is soldered on from underneath so to get the key bed into the sink I would have to unsolder about 200 solder joints to break up the main assembly, I decided to pass on that opportunity and clean it intact. I used a very slightly wet brush to give it good going over a number of times which did the trick. It and the gutted mouse were left outside to dry in the warm...

...while the keys spent the night air drying somewhere a bit safer inside.

Time for the best bit, some re-assembly.

One thing that annoys me with gear that someone else has been in before is they always seem to have cocked up the screws, either putting the originals back in the wrong places, or losing the originals and substituting something completely random. Spot the odd one out...

...I suppose this could have been worse. I was able to find a screw in my scrap box that's an identical match to the originals so the monster one was retired.

Looking good!

Still has the original protective sticker on the name badge.

Will see if I can track down a blanking plate I think to cover the hole up.

Keyboard rebuild time!

The complicated keys go on first while there are no other keys in the way, with a drop of lithium grease on the sliding parts to stop things sticking or squeaking.

What an improvement a good clean makes!

Another test...

Shadow of the Beast!

...and it was paired with an Atari Megafile 30 hard drive for that no-expense spared 1988 luxury feel.

A full 4MB present and correct too, it's a shame that SYSINFO doesn't know it is a Mega ST, I guess electronically it is just the same as a regular ST, just one with a blitter and a lot of RAM :)

Restoration complete!
« Last Edit: December 02, 2017, 11:45:11 AM by Womble »
Sic Transit Gloria Atari

Offline Womble

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Re: Atari Mega ST4 Repairlog
« Reply #1 on: January 01, 2015, 09:47:39 AM »
Well.... not quite!

That should be the end of it, but after a good play with it I decided to try one last feature - an Atari SM124 high resolution monitor. I had every expectation that this would just work, except it didn't. I was met with a flickering screen that showed all the signs of a video sync problem. So I connected the colour monitor up again and it was fine, something else was amiss.

On the ST range the monitor itself tells the computer what type it is by whether it grounds pin 4 (Monochrome detect), the likely options here seemed to be that either the ST wasn't registering that this was a mono screen, or the monitor wasn't correctly grounding the right pin. The latter option was ruled out by testing the monitor on an STE from the collection, it worked fine. A second SM124 monitor was pulled off the shelf and tested to rule out some monitor oddities, it also worked on the STE and failed on the Mega ST.

With the scope I could see that with the monitor connected the Monochrome detect pin was being grounded when the monitor was connected, and that this signal was making its way to the MC68901 chip on pin 29.

With the monitor disconnected I was getting healthy H-sync and V-sync signals too which proved connectivity of the socket to the GLUE chip, and probably the GLUE chip itself, but when the SM124 was hooked up the V-sync signal died, in fact it was tied to ground when measured at the diode divider D7 and D18 which connects to pin 12 (Vsync) on the monitor socket. It looked like the monitor was killing the vsync signal which would prevent it from correctly displaying the image.

This had me chasing my tail for quite a while, I rechecked all my soldering around the monitor socket and confirmed that no pins were shorted together, and that all pins were connected to their upstream components.

I checked the continuity on the monitor cable plug to confirm whether it was grounding pin 12, which it wasn't. The only pins grounded in the monitor socket were pin 13 (ground), pin 8 (which is either ground or 12v pull-up depending on the model of ST) and pin 4 (monochrome detect). Yet as soon as I plugged this plug into the monitor socket pin 12 was also grounded. Pin 8 should have nothing to do with this but to rule it out I lifted one leg of resistor R76 to isolate the socket pin 8 from the rest of the system entirely... change, so I soldered it back down.

I tried both SM124 monitors and both did the same thing, I even de-cased one to disconnect the other end of the monitor cable from the monitor internals to rule that out...

... and the cable which was then just lengths of disconnected wire and a plug still did the same thing. I hooked up the only other Atari ST cable I had, a composite video output cable, and it didn't cause the issue. All of which left we me with only one option, this was a physical issue with the socket and plug itself.

It also occurred to me that the three pins that were being grounded by these Mono monitors were pin 4 (Monochrome Detect that should be grounded) pin 8 (12v pullup) and pin 12 (that didn't matter either way), and pin 8 (Vsync and the source of the fault) were in a straight line, suspicious!

So I went back to the monitor socket I had fitted and found that this time, even without a monitor connected pins 8 and 12 now had continuity between them, previously they hadn't, it was either a dodgy socket or some remnant of board damage was at play. I couldn't see anything by eye but I desoldered pins 8 and pins 12, cleaned the space between them and resoldered them. The short was cleared, even when the SM124 was connected.

Finally fixed!

Have swapped monitors back and forth a few times now and the fault has not returned, with it all reassembled I am calling this restoration complete!

A fine addition to the Atari collection :)
« Last Edit: December 02, 2017, 11:48:06 AM by Womble »
Sic Transit Gloria Atari