|Re: HP38C "6" key repair|
Message #2 Posted by Vieira, Luiz C. (Brazil) on 28 Oct 2009, 1:46 a.m.,
in response to message #1 by Peabody
PLEASE NOTICE THAT THE TECHNIQUES DESCRIBED HERE DEMAND SKILL ON HANDLING THE EQUIPMENT MENTIONED AND ARE NOT GUARANTEED. FOLLOW THEM AT YOUR OWN RISK (looks like copyright warning...)
Based on your description, I concluded that the IC´s are soldered in the PCB, right? I'm asking about this because there is a solderless series of spices, and the flex circuit used in these models allow contact cleaning.
If you look carefully at the component side, you´ll see that under each metal key cap there is a small, metalized hole. And we are lucky for the '6' key has an accessible hole, close to the memory IC's.
I succeeded many times (NOT ALL) carefully inserting a small amount of isopropyl alcohol through these holes with the aid of a thin brush in order to clean dirty, bad contacts. Use the thin brush soaked with a small amount of alcohol only to touch the hole in order to allow the alcohol to slide into the key cap. It should be done carefully not to spread it so it does not reach the sealing plastic you mentioned in the top of the keyboard. If you use an excessive amount, the alcohol may also dissolve the glue in the sealing plastic.
I usually insert some brush bristles (2 or 3 fair ones) through the small holes and gently move, rotate them inside of the key contact. Luckily the bristles will scrap the surface of the contact both in the key cap and the PCB surface. Please, make sure you choose bristles that would not leave small chunks inside the key contact (I prefer using bristles from synthetic brushes). From time to time, with the bristles inside the key caps and steady (do not moving them), press and release the key cap a few times, then move the bristles, hold them steady, press the key cap again and so. This will enhance the efficiency of the cleaning process by forcing the dust to move out.
It is possible that the bad contact persists for a while,. but chances are that after rebuilding the calculator and using it for a while the '6' key becomes more and more reliable.
Please, understand the I added the warning in the beginning of the post for the sake of preserving your HP38C. I've done it many times already, and in some of them I did not get the best results.
Edited: 28 Oct 2009, 1:51 a.m.