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Someone with experience on electronics? My Laser turbo XT/3 went bad.

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Robin4

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Sep 25, 2011
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I have a bad IC here and i like to replace it with a new one.. because this IC is in a bad state. One leg broke off because it was very stuck in the IC socket.. I guess the previous owner also have tried it to get it out with a bad result.

The IC that is bad have the name Siemens SAB8237A, without further any written behind it..

Iam not sure, but it seems to be a DMA controller IC on 3Mhz..

If i look on the internet iam only find SAB8237A 5P

Does the speed of these IC`s are important, so i guess a 3Mhz is not interchangeable with a 5Mhz one? Its for an Laser XT/3 Turbo machine.
 
I dont know if it was the best thing to do, but i ordered this one instead: M5M82C37AP-3 its from mitsubishi, but i guess it would be 3Mhz for a reason. So i will try this first.

I think it is almost impossible to get the right part instead.. So i have to deal with the replacement. If this wont work out, i will try the same siemens brand with the faster Mhz.
 
Robin4 said:
... but i guess it would be 3Mhz for a reason.
Click to expand...
The speed rating is like the speed rating on your car's tyres. It represents the maximum speed that an engineer is to run the part at.
In the days that the motherboard was made, the faster speed rated versions of the parts cost more.
No point in a using a $50 part rated at 25 MHz maximum, if the $30 part rated at 20 MHz maximum is adequate.
 
Robin4 said:
The IC that is bad have the name Siemens SAB8237A, without further any written behind it..
Iam not sure, but it seems to be a DMA controller IC on 3Mhz..
Click to expand...
If you look at the photos of XT-clone motherboards at [here], you will see that it is normal for the 8237 to be rated at 5 MHz, expected because we expect the engineer to run it at the same speed as the CPU, 4.77 MHz.

If the speed rating of your other 'large' chips (8255, 8253, 8259) are 5 MHz, then that suggests that your 8237 should also be 5 MHz.
 
Perhaps running a 3mhz part at 5mhz is the reason it went bad to begin with; it was overclocked.

I'd get the 5mhz part.
 
I will make photos of the motherboard and the IC chip soon, to clarify what we are talking about..
Iam only see 1 big 40 pin chip only on the motherboard... Its not the same like on the minuszerodegrees website motherboard pictures.

The other two are the main processor (i guess its a siemens as well) and a co-processor a installed few years ago.
 
Did you manage to remove the IC? What kind of tools do you have?

If you haven't my advice is to apply new solder on each pin, set the station to a little higher temp but not too high so you don't lift any pads. I wouldn't recommend go any higher than 350ºC. Apply the solder and let it flow for a few seconds so it can bond with the old solder. Only use high quality solder wick, crappy wick will make you overheat the pads which eventually be damaged and you'll barely remove any solder.

For this kind of work i got myself a Hakko FR-300.
 
The ICs i pulled those where socketed on the motherboard.. But i have to replace the IC socket for the DMA controller. Because the old chip was very very stuck inside this socket due age..

One pin broke off, i really couldnt occur to make that happen.

I have a desolder / rework station here, but i have to practice first on a board that is allready not working anymore

I have a Weller WMD 3 here.

I lifted the ic with a small pick and a small head flat screwdriver. I used it just gently.. But what i did, the IC was really stuck in the IC socket..
 
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