Retro Computing Corner: SimasiMac Fix!

Author: Rees | Date: April 21, 2008

Mac SE/30 Motherboard Circa 1991

Last time in Retro Computing Corner I introduced you to my two classic Macs, an SE and an SE/30. I also revealed something else to the world – a little fault common to the SE/30 known as SimasiMac. Well, today I’m going to tell you how you can fix it very easily, provided, that is, that you’re fairly proficient at soldering.

But first an introduction – what is SimasiMac?

Here’s a little explanation thanks to Jag’s House, a specialist in all things classic Mac:

Simasima means a zebra-like stripe pattern in Japanese. OYU!(HHD00612@niftyserve.ne.jp) and Shunsuke Suzuki(VZD03711@niftyserve.ne.jp) nicely named Macintosh SE/30 with this problem SimasiMac. We SE/30ers in Japan call it SimasiMac.

Basically, if your SE/30 isn’t starting up and is displaying either horizontal stripes or a checkerboard pattern, you have the dreaded SimasiMac. A very easy way to find out is if you leave the stripes on screen for 10 seconds, turn the Mac off for 5 seconds, and turn it back on, it should boot up as normal.

Eventually it’ll end up completely dead to the world like mine, with a whirring noise and not a lot else. It’s time to dig out the soldering iron.

Mac SE/30 SimasiMac Capacitors C6 And C7 Highlighted

First up, you will need to dismantle the case and whip out the motherboard. There are plenty of guides around on how to do that, so I won’t go into detail here. If you’re not comfortable going this far, please don’t proceed any further – get someone who knows about electronics to do this for you. If you screw this up, your Mac will be dead forever!

Next we will need to locate capacitors C6 and C7. If you place the motherboard down with the rear connectors facing towards you and the RAM / ROM facing away, they will be on the side nearest to you, towards the right. You will need to desolder them with a very hot soldering iron (not a cheap crappy one like mine) with a fine tip, by heating up the little metal pads either side and slowly prising them up with a screwdriver.

C6 And C7 Capacitors Replaced With New Ones

Then we simply solder the new ones in place – C6 needs to be 1µF 50V and C7 needs to be 47µF 16V. These aren’t particularly common ratings so you will need to order them online.

As you can see from the picture above, my soldering is absolutely rubbish, but they are making good electrical contact and that’s all that matters to me (I did clean the joints up a bit after this picture was taken). You will need to take special care not to overheat the capacitors or the motherboard itself, or any other components nearby, as the heat can damage them.

Also, make sure that the sides with the stripe (the negative side) are pointing towards the left, as they were on the original capacitors. You won’t blow anything up by getting them the wrong way round, but you will curse yourself when you have to desolder them again.

SE/30 Recovers From SimasiMac Surgery And Boots Up Fine!

Finally, we can pop the SE/30 back together and fire it up, just to make sure that everything’s OK… I got the reassuring “bong”, and then the usual startup screen. If you get this far you can turn the Mac off, carefully put it back together, and get on with your life.

See, that wasn’t so hard was it!

Oh, and I hope you took the time to clean out the dust bunnies while you were in there…

5 Comments »

Comment by m

2009-07-31 03:31:42

I found a SE/30 in the trash tonight, plugged it in and saw that it almost worked. The screen displayed weird repetitions of 4 times 4 lines (is that simasimac?) – but, reading between the lines, the system booted and worked ok. Figured that 2 adress lines of video memory were stuck to 0 or another constant value.

I unscrewed the thing and started to analyse the motherboard, which had nasty corrosion from leaking electrolytes, especially around C7. Figured out that the 2 video memory chips (D41264C-12) are supplied with 2 times 8 bits of addressing information on pins 9-11 and 13-17 (not in that order), where RAS (row addr strobe) and CAS (col addr strobe) are used to tell the chips which part of the address is given (effectively a form of time division multiplexing). Found out that the four 74F253 chips are used to multiplex the 16 bit addr onto the 8 bit time divided channel (2 bits per 74F253). While trying to find the source of the 16 bit address on the board (seems to be the 2 TIT chips) realized that the most logical explanation would not be that two adjacent address lines went short with ground, but instead that one Select Input line of on of the 74F253 multiplexers was probably disconnected by corrosion. (pin 2 or pin 14)

Which, when measured, turned out to be correct! Since I stumbled upon this page when trying to find info on the SE/30, it seemed like the place to share my experience.

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Comment by Rees

2009-07-31 12:30:26

Thanks for sharing! These old SE/30s are good little machines and are actually worth a bit of money in good working order, so it’s always good to hear about another one saved from the landfill.

I don’t think your problem could be classed as “Simasimac” – it refers to a very specific fault, but it’s good to hear that you go to the bottom of it. It’s amazing how simple these things are compared to modern PCs, which is very fortunate in our case!

Again, thanks for the information – hopefully it will help someone else in your situation.

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Comment by m

2009-07-31 16:12:02

So today I just replaced all 11 electrolytes (all leaky) and improvised a way to restore contact to the multiplexer without making too much of a mess. Booted the thing up and… it worked! Now I’m the proud owner of a finnish speaking se/30 (being dutch..)

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