Honest advice re: repairing an old scanny keypad, stop trying to polish a turd and find a reputable company to replace system try visiting ssaib and/or nsi websites to find local companies
http://auctionhq.com/event/marussia/
I know its not striclty security but there are a few job lots of multimeters/clap meters as well as some fancy tool box's oh and a couple of F1 cars
My answer to anybody considering a cheap diy kit is quite simply look at the outside wall of the place you are buying it from and look if they trust it enough to use it themselves!
Been meaning to post this for a while, I went to a job fitted by a company we bought out to remove a load of vibration detectors ready for the customer to have new windows/doors fitted. Anyway as im going around removing them all I get to the rear door that the customer informs me that the detector is a vibration and contact at first I didnt think much of it but was a bit surprised as I had never seen a Gardtec Shockguard with a built in contact so I open it up to find this!
Close the panel lid slowly and check you can hear the tamper switch click those panels did have a habit of the switch jamming they also tended to have faulty tamper switches on times aswell, if its only a flashing bell its 1 of 3 things that could be in tamper either panel, keypad or external siren
Flashing bell is a tamper, check you have closed the panel lid correctly also check the spring and white bit of platic is still on the tamper switch (tamper switch is a black button in the mid hottom of the main pcb)