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Tales from Midibob's workshop Maygay MMM


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This tech was new to me but it was a straightforward job of renewing the bottom right hand corner.

 

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Track damage was only slight in a couple of places which was a relief. Getting the chips out was impossible in one piece so quite a few legs had to snipped off to enable removal. Once all the components were off clean up commenced. Most of the resistors fell apart on removal!

 

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Due to the corrosive gunge which had collected in most of the solder joints they all had to be filled with fresh solder and then sucked out which took ages. Then a bit more in depth cleaning.

 

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I then thought it would be a good idea to remove all the old EPROM sockets and RAM too as most of the tracks underneath looked green!

 

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Again after a good clean up.....

 

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Then we start on the long road back.

 

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A couple of tracks around the battery area needed bridging with some fine wire but it wasn't that bad.

 

Almost finished.

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A fresh set of ROMs with thanks to Wizard for being able to read ROM 4 which my Data I/O wouldn't ??

 

Just a couple more bits to put in. A 100R 0.5W resistor, which I've run out of and an LM384.

 

By next week I'm hoping, with fingers securely crossed, that I can power up the board and it will work. :pray:

 

Watch this space.

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You've done a very thorough job there Bob. Like those early boards (2? - can't quite see as the label obscures it) with the exposed tracks (seem to tough out the battery attack better than ones with a mask, when it comes to track damage).

The corrosion was pretty extensive on that one tho - nice clean up.

 

Love the headers on this tech - robust, accessible and easy to connect/disconnect and clean if required.

 

Looking forward to the update ...

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Jesus Christ Bob, that board looks like it was nailed 😳

 

That must have taken you hours and hours, what a great effort though absolutely brilliant 👌

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LOL, you're spot on there. I noticed that when I read the diagram, and not only that I've found at least one electrolytic capacitor not shown on the diagram?

Luckily all the resistors were readable before I took them off so I copied them all to paper just in case, in fact all the components in that area.

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  • 2 weeks later...

After the clean up it was time to switch on. Alas, all that happened was a high pitched white noise from the speaker! First things first, check out the RESET. It did appear to be changing logic level but not as a proper pulse but even doing a manual reset didn't get it to initialise?

Moving swiftly on to the ROMs and only ROM #1 had a chip select pulse? These are generated by IC9 (74LS157) and IC8 (74ls138). Both chips were faulty and once changed the chip selects were fine on all four ROMs.

Still no boot though?

All the address lines were Ok but on checking the data lines DATA 0  was found to be stuck low. :(
This is probably the worst scenario as the data lines go to 13 chips! The only way to locate the bad one(s) is to disconnect each DATA 0 pin in turn and hope you find it well before you get to number 13!

First thing is to remove anything that's socketed ie the RAM and ROMs but of course it wasn't going to be any of those!

Of course it could have been a bad CPU (Z80) so pin 14 was cleaned out and isolated and proved to be OK. Next was the CTC chip, nope not guilty. Then IC17 + 18 + 20 +21 +22 (74C374), then IC23 (74LS467) nope not guilty. Then we get to IC14, off comes pin 3 and bingo! Resistance checks to ground showed nothing amiss but as soon as the chip was powered it shorted out the DATA 0 line! This chip is a bit of an odd one (81LS97 or 74LS467) and its the only chip on the board I don't have spares for.

 

Isolating a chip leg takes ages especially when the solder is contaminated. It's probably not that much quicker than removing the whole chip, except the larger ones of course.

So power on again.

Does it boot?

Nope :headache:

As this chip deals with the switch matrix I can hopefully ignore it for now and concentrate why the board doesn't boot.

All good fun.
Stand by for the next thrilling instalment.

Edited by midibob
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I'm thinking perhaps the RAM chips are knackered. I don't have any of these 5101's so I'm knocking up a little converter board so I can use a 6116. Hopefully that will prove it one way or the other.

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Almost posted earlier that it's been my experience 4 out of 5 times, one or other (or both) of the 5101 on these boards are knackered but read your post as you having ruled them out already ... going to post office tomorrow so if you havn't already made a 6116 conversion/want a couple sending down, shout tonight

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Well, things are looking up.

 

Putting the RAM adaptor board board in made no difference so at this stage I think the RAM is probably OK. Thanks for the offer Paul.

 

The white noise was due to the AY-3-8912 and on replacement I now get a click on switch on and another about a second later which sounds a bit more promising.

 

I thought I'd check a few other bits and found both the switch input IC's were knackered IC15 + 16 (CD4009). Also one of the triac drivers had the wrong voltages and this was due to IC 26 (ULN2003).

 

As I don't know this tech I haven't a clue what should be happening on boot up but I'll power up the reel motors tomorrow and see what happens.

Edited by midibob
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She lives :kiss: :kiss: :D:clap:

 

I see what you mean. On switch on the reels index and then I assume part of the attract mode is the reels clocking round a few positions one at a time with associated noises. This repeats every few minutes. I haven't checked for lamp activity yet so that will probably confirm it.

I'm well pleased with getting this sorted.

 

Now, if anyone's got any 74LS467's they don't want? :lol:

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  • 2 months later...

Well, another MMM has graced the workbench. This one is of Dutch origin and came with a few mods not apparent on the previous board.
As you can see this wouldn't have worked as ROM 4 is upside down !! Also the 3rd pin down on the right hand side isn't in the socket.

 

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The ROMs were likely to be unreadable as a full set so I blew a new set of Wild Hold.

Once the battery was removed and the components changed in the local area it was time to see what the mods were for.

The first one I looked at was the diode which was across the transistor that feeds the vbatt supply. You can see it on the bottom left of the track side of the board.

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I assumed that there was a fault in this circuit and they'd just bypassed it with a diode!
Once the diode was removed there was only about 3v on vbatt instead of 5v, so my thoughts were probably correct.

The first thing was T68 was leaking so in went a new one.

 

Switch on and still 3v?

 

Next was T67 which was fitted upside down! Put in a new one and switch on again.....still 3v?

By this time I'm going bananas. Time for a quick brew to settle the nerves.

Back at the bench and with a clear head I'm wondering why there's a link wire been added? It's just above the diode in the picture. It turned out this was from the NRST feed directly to the base of T67????

So, remove the link and switch on again...you've guessed it, still 3v

Looking through the microscope I could see that the track had been cut between the 5v rail and R162 and once restored the vbatt now read 5v  PHEW.

 

The next mod was a resistor soldered on top of R80? A 10k had been added across, not sure why yet? Also the diagram is wrong!

 

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C8 doesn't go where the diagram says it does and should be a diode D47.

 

After all that the next step was to see if anything was running. All data and address lines were active which was a good start but there were no select lines active on ROMs 3 and 4? Switch 2 (red one)  had been set to the wrong ROM size!

 

Next step is to connect it up to the reel deck and see what happens.

Watch this space for the next thrilling instalment. :D 

 

Oh, and a big thanks to Drslots for uploading the diagrams.

 

 

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