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Restoration of a Motorola MEK6800D2

N

nijssen

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Oct 12, 2021
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I recently got a Motorola MEK6800D2 machine from the late 70s that I want to restore. Think it would be fun to make a (slow) work log of this in this thread.

Here I share some (before) pictures. It is very much complete with manuals and sits in a nice enclosure. I also noticed an SWTPC memory expansion board, maybe these were used in the SWTPC 6800 machine as well, have to look into this. It is full of dust, but no rust. It seems to be well used and has an unusual round I/O connector wired up. Any ideas of the brand and type?

My first step will be to get everything out of the case carefully and clean this. Will start with testing the power supply in isolation. The manuals and binder are also very much aged and dirty, they need some cleaning as well.

Tips welcome, stay tuned!


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I didn't realize they had a case. Are you sure the case wasn't a homebrew thing? I'll bet that memory card was hand wired in there. Really clever though.
 
I had a D2 (back in the day) with the addition of a couple of DACs to generate analogue voltages (X and Y driving of an oscilloscope, 'music' or driving analogue theatre light dimmers from Rank Strand etc.).

My first 'computer'...

I don't remember an 'official' case either.

Shout if you need any help with fixing it. Yes, I like the addition of the memory card also.

Dave
 
Great find! Motorola never produced a case, lots of homebrew cases/enclosures though. Mine was plywood - rough but functional.
Originally only available in Kit form, with ICs, keyswitches, discrete components etc. no power supply. Motorola branded exorciser (86pin bus) memory cards were hugely expensive back in the day, most of us hobbyists adapted cheaper s-100 memory cards or SWTPc cards, or build our own memory expansion. Here are bare PCB photos from one of my unbuilt kits - useful for debugging/tracing lines under sockets and locating any possible modifications by the original owner. I have spare JBUG rom and other parts if you run into issues. Restoration will be a rewarding project. The D2 design is quite amenable to modifications and expansion possibilities - a great base. Used mine (expanded w/TVTtypewriter video out and RS232) for home "automation" in the late 70s/early 80s. Still tinker with several versions including the original.

MEK6800D2-front.jpgMEK6800D2-Back.jpg
 
Bruce Tomlin said:
I didn't realize they had a case. Are you sure the case wasn't a homebrew thing? I'll bet that memory card was hand wired in there. Really clever though.
Click to expand...
I recall there being 3rd. party cases for the D2. It was a popular starter system.
The SWTPC memory.... I think that was a one off project.
 
So I took the whole thing apart, did not power it up yet. Discovered the big round connector to be a Hirschmann brand, type unknown. The power supply (only +5V) is in a bad shape, thinking about redesigning a new regulator board that fits the case. Any tips? Can also use a new Meanwell, but then this thing loses its spirit. Transformer seems solid. The fuse was 'solid' as well but lost its purpose by the previous owner, hmm. I also cleaned the documentation and fixed the binder. @daver2 found a document Analog-to-Digital Conversion Techniques with the M6800 Microprocessor System, does that ring a bell? ;-)

The memory board is an SWTPC MP-M (see here), think it has 4KB. It only uses +5V, handy. The case is homebrew, the holes for the keyboard are nicely milled, the rest is made by a regular drill. I like the 'Altair' color. Thinking about having some fun with the Dremel and repainting the whole thing (the case I mean). Any ideas how to find the 'original' color?

Next step will be to power up the mainboard and the keyboard with the bench supply. If it has any life this project can start off :-).

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One caution: The MEK6800D2 requires a single +5 volt power source; however, the added SWTPc MP-M memory board requires 8 volts DC for the on-card regulators to work properly.

This may explain that perforated board shown above.
 
Did someone say 86 pin connector? Some of the boards I've been documenting plug into an 86-pin card edge connector...

here are part numbers to help track down NOS or modern equivalents: EDAC 737-086-540-206, TE/AMP 2-119254-9, TE/AMP 7-119254-9
 
Vermithrax said:
One caution: The MEK6800D2 requires a single +5 volt power source; however, the added SWTPc MP-M memory board requires 8 volts DC for the on-card regulators to work properly.

This may explain that perforated board shown above.
Click to expand...
Yes I realized that, think the DC was fed to the memory board before the big 5v regulator (and after the rectifier of course) on the hobby-supply. If I create a new regulator board I could do the same…
 
I made some progress with this machine.
  • Found another MEK6800D2 in a case, look here: https://rclassiccomputers.com/2022/10/20/mek6800d2/. They exist!
  • Tried powering it up with the lab supply. Measured no shorts at first on the power rail. However, I started with some lower voltages 3-4V with 0,5A max. and it goes into over current, so something is wrong. Do not expect this thing to consume above 1A? I turned the supply off quickly. With the display module disconnected, same happens.
  • Then I removed all ICs. While doing that I discovered some oxide or 'iron dust' at the bottom of the PCB that could have caused a short (around U13 pin 1-4). Without ICs it took 100-175 mA. So that's better.
  • Also saw a couple of holes have been drilled to mount the memory card on top of it. These were drilled through a trace, but that has been fixed by the previous owner.
  • Another thing, less important, I noticed is that all resistors have been soldered 'above' the board, with quite some distance to the board. I do not like the looks of this. Then I saw note no. 9 in the schematics. Being in The Netherlands, think the previous owner made a calculation error and soldered them 8 mm above board instead of 0,8 mm. At least the legs of the resistors are long enough to fix this. See pictures.
  • Thinking about removing the wiring for the memory board and make a proper 'back'-plane and rewire it the same through the Exorciser bus. This way I could also power it properly through the bus connector with a to-be-built supply. Came across an interesting back plane on Tindie: https://www.tindie.com/products/epooch/multi-plane-for-exorbus/. However, do not think this will fit in my case. Leave the wiring in for now.
  • The original power supply used some kind of output crowbar with a thyristor, type markings are all gone, so unable to reverse engineer.
  • Also gave some thought to the new supply. Will reuse the transformer, think I could rewire that one to have -12v, +12v and +5v available. To keep it in small steps I want to first design a small board with a simple +5v regulator. Four diodes, some caps and a LM323K or LM309 and of course a fuse. Only want to use 'old' parts for this, so it keeps its spirit. A bigger 'external' Sprague buffer cap with a bracket would look nice I think. When needed I can add a separate regulator board for the -12 and +12 later on.
Next step would be to tidy up the board and put in the ICs one by one while checking with the lab supply. The short could be caused by that 'iron dust' or by an IC I think.

Edit: anyone having a buy-link for the 86-pin Exorciser connector and/or the 50-pin connectors? Tried the numbers @tech58761 provided, but a quick look resulted in nothing.

Will take some time again, but keep you guys posted!


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I remember my D2 was really power hungry...

My D2 came without a PSU, so I had to construct my own.

The power supply detailed in the back of the manual was rated at +5V 2.5A. However, various people have measured in the high 800 mA to 1.1A with the two-board set. If you have an additional memory board attached, then the current consumption will be proportionally higher.

Dave
 
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daver2 said:
The power supply detailed in the back of the manual was rated at +5V 2.5A. However, various people have measured in the high 800 mA to 1.1A with the two-board set. If you have an additional memory board attached, then the current consumption will be proportionally higher.
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Thanks Dave, the memory board was not attached. That high consumption might indicate that nothing was wrong. I will keep this in mind when rebuilding the board.
 
For a similar two board set, with both PIAs, ACIA, all four 6810 sram, and the bus transceivers installed as you have, I measure 1.25A with a only a “-“ displayed after reset, if you display all 8’s on the LEDs, ~1.4A

Example 2x43 86 pin 3.96mm/0.156” edge connector for exorciser hopefully close to you:
https://www.ebay.nl/itm/326524837603 , can find these with with various terminations, solder eyelets, solder pins, or wire wrap depending on your needs
 
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I am not really into power supplies, but I gave it a go in KiCAD. I have a few BYW95C diodes and a LM323 laying around so I came up with this 3A supply. Any thoughts on this? I am uncertain about the capacitor values. Also the transformer has a lot of windings, think I could make the power supply expandable in the sense that I can add -12V and +12V when needed in the future (see drawings).

The original supply was with a LM309, which I think is rated 1A. It must have been pretty stressed on the load?
 

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>>> The original supply was with a LM309, which I think is rated 1A. It must have been pretty stressed on the load?

Which is why I ran mine with a pan of water sat on the heatsink!

The rule is (for large smoothing capacitors) "the bigger the better". The original schematic (in the back of the D2 manual) contained a 3,000 uF capacitor; so 5,300 uF should be OK.

Don't expect to draw 3A with a 2A fuse though...

You will need a fairly large heatsink (and possibly a fan).

The other suggestion would be to look at a "buck converter" rather than a linear regulator. They run much less hot...

I have found a 12-24V DC input, 5V 3A output unit for < £2 on eBay. Just a thought. If using a 12V input, you could design your transformer and bridge rectifiers for +/-12 V and derive the +5V from the 'raw' +12V via the buck converter.

The only thing to watch is what is meant by "the transformer is rated for 5A".

The +/- 12V rails should have a quite low current consumption - so a 7812 and 7912 should suffice. I would run the buck converter from the 'input side' of the 7812 though (so the current for the +5V rail doesn't flow through the 7812)...

Just need to make sure of where the 'ground reference' is for all of the separate power supplies.

Dave
 
daver2 said:
Don't expect to draw 3A with a 2A fuse though...
Click to expand...

Haha, good one.

daver2 said:
The other suggestion would be to look at a "buck converter" rather than a linear regulator. They run much less hot...
Click to expand...

I know, but want to put in as less modern stuff as possible.

daver2 said:
The +/- 12V rails should have a quite low current consumption - so a 7812 and 7912 should suffice. I would run the buck converter from the 'input side' of the 7812 though (so the current for the +5V rail doesn't flow through the 7812)...
Click to expand...

Yes I was looking at the power supply for the Apple-1, used that as inspiration. Also have some of those parts available already.

daver2 said:
Just need to make sure of where the 'ground reference' is for all of the separate power supplies.
Click to expand...

This confuses me a bit. I completed the schematics with a 7812 and a 7912. Not going to build it all, but think I want to design a PCB that has it on board for future expansion, so I can add the parts when needed to. At the moment I am uncertain about the windings (bridges D-E and F-G on the secondary transformer side), diode placement and (common) grounds. Hoping op some help with this.
 

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>>> I know, but want to put in as less modern stuff as possible.

No problem. I would do the same thing myself (unless I wanted something reliable - as per my project)...

Your V2 schematic looks fine.

I have seen some strange 'concoctions' relating to multiple power supply voltages and polarities! Yours is correct though...

LED D3 should be the other way around though.

Not sure what the three (3) fuses bring to the party though. The linear voltage regulators are pretty indestructible. The only failure mode that could catch you out is if one goes short circuit and over-voltages the devices downstream of it. However, it is unlikely that the fuse will actually help in this scenario either. The voltage regulator will probably shutdown faster than the fuse can blow...

I had a voltage regulator go short circuit on me like that on my Cromemco Z2 system when I was resurrecting it. Fortunately, it was on one of the disk drive power supplies and I was testing it on the bench without any disk drives attached. This is a very rare probability though.

Dave
 
Thanks for your help Dave, really appreciated.

The idea with the secondary fuses was to protect against short circuits, thought is was handy in combination LEDs, so you can also see on what voltage rail the problem is. So the regulators have this already built in, clear.

Do you recommend overcurrent protection using a crowbar or some zener? At least that was something the original supply had. Any pointers on what parts I could use for that?
 
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