Re: Taking apart Lisa - cleaning

From: Dr. Helmut Post <helmut_post_at_email.domain.hidden>
Date: Thu, 3 Nov 2005 15:44:49 +0100

Hi Phil,

you don't have to replace all caps- only the electrolytic caps and the "transparent" , sometimes white or yellowish power surge protector (interference suppression) caps - I had several "explosions and ugly smell" with these power surge protector caps.
If the Lisa had been stored in a humid place over the years, those plastic material is deteriorating- small cracks will appear- the humidity can penetrate into the cap- if you start your Lisa after this time, the humidity inside the cap produces a "shortcut" - and the cap explodes and burns .... this fire is not dangerous- the PSU is shutting down the power, but the Lisa has a bad smell for a long time- it smells as if you would try to burn a hole newspaper magazine inside your Lisa- very ugly! I was very frightened the first time I encountered this "accident". I had the same experiences with the PSU's of my Profile hard disks. More than 50% of my electrolytic caps had values at least 40-50% lower than indicated (dried out caps)
Others had doubled their capacity- reason: humidity as well. Dried out caps can easily damage the Multilayer PCB!! (reason: aggressive electrolytic fluid)- the removal is as much necessary as the removal of the NiCAD batteries in the Lisa 2/5 I/O board! The capacity of the interference suppression caps had been doubled or even tripled! over the years!!
Sometimes I am very surprised that the circuits are still working .....

You can buy all these caps at "Digi Key". Here is a link for the power surge (interference suppression caps): I used and recommend the MKP 338 X1 series (440 Volts AC), but you can even use the MKP 338 X2 series- they have the same ratings as the original caps with values from1 nF to 3.3uF.
You can easily find all other electrolytic caps at Digi Key. I'm only using 105 degrees Celsius caps- don't use the 85 degrees Celsius caps- they dry out to frequently.


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Received on 2005-11-04 00:52:36

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