|Re: hp 9815a repair|
Message #5 Posted by Steve Leibson on 13 Oct 2007, 6:11 p.m.,
in response to message #1 by frankabc
Dear Dr. Frank,
I've gone back and looked at your original problem 9815A description from August. I believe you have may have multiple problems with your HP 9815. First, I suspect the "auto enter" problem is caused by a leak path in the keyboard. The HP 9815A uses "oilcan" type metal strips for keyboard contacts that distort and then snap into contact with an underlying circuit board when pressed. These are called "Cricket" keys and were modeled after the handheld calculator keys. These keys were developed to save money but proved less reliable than desired. The HP 9815A and the HP 9825A/9831A were the only desktop calculators to use these keys before they were obsoleted. Desktop calculators produced before and after these two machines used more conventional keyboard keys.
The contact strips in the Cricket keys are bowed in the middle and welded to the circuit board on their two ends. One of the welds often fatigues and breaks after years of use, making a broken key. Chances are good this may have happened to your "enter" key (especially if it no longer feels crisp and has a "snap" to it), because this key was usually the most used key on the keyboard. If one end is broken, you can try to solder it (sometimes works) but it's not a long-term fix.
You are also experiencing tape problems. You are correct that the actual tape cartridges are likely to be toast after 20 years. The oxide literally falls off the mylar film in the tape cartridges after all this time due to a loss of adhesion and from differential tape tensioning caused by normal environmental thermal cycling. If so, the rain of oxide from the tape as you tried to load it is now likely coating important components within the tape drive, which also won't help matters.
I suspect you also may have experienced the "gooey capstan" tape problem. Over time, the rubber coating on the capstan in the tape drive (a pulley-like affair that drives the tape back and forth) loses elasticity and turns to something more akin to used chewing gum than rubber. These tape drives can be repaired. People have had varying DIY successes with three O rings, heat shrink, and cold shrink repairs. Larry Atherton can repair them (see my blog at http://www.edn.com/blog/980000298/post/1180012118.html and Atherton's eBay page at http://search.ebay.com/_W0QQsassZla-tech-renewalQQhtZ-1) but the cost can be in the hundreds of dollars.
Sorry to be the bearer of such bad news. These machines were durable and lasted a very long time, but no computer is immortal and your 9815 is roughly 30 years old. At the Computer History Museum here in Mountain View, we are reluctant to even turn on old machines because the electrolytic capacitors in the power supplies tend to fail and can produce spectacular results when first powered up after a decades'-long slumber.