hp 9815a repair


This message is for Dr. Tony Duell, or any who might know the answer. Our HP 9815A calculator still is on the blink. We have an HP 5006A Signature Analyzer. Can Signature Analysis be employed to trouble-shoot our HP 9815A? Thank you, Frank Simpson, fbsimpson@pharmacy.wisc.edu


The general answer to "can a signature analyzer be used to troubleshoot an xxx machine" is yes, if and only if you have the service manual for the xxx machine, and it gives the procedure to follow and the expected signatures.

I don't recall having seen such information in the 9815 service manual, but it's been a long time since I looked at it, so it's possible that it was there.

You can get a copy of the 9815 service manual on the MoHPC DVD or CD sets.


Eric is right. Signature analysis is only useful if you know what
the right signatures should be (and, indeed how to start/stop
the signature analyser). HP never published that information for the HP9815, and nobody has tried to work it out (it's not at all easy in general).

Some other HP microprocessor-controller instruments (one of the HPIB extenders springs to mind) had a special signature-test mode that disabled the ROMs and RAMs, and forced a simple instruction onto the processor databus. This meant the processor kept doing the same thing, leading to relatively simple waveforms, ideal for signature analysis. This facility does not exist in the 9815


Unfortunately, the introduction of the HP 9815A preceded HP's introduction of the first signature analyzer (HP 5004A) by at least two years, so it's extremely unlikely that the 9815A developers would have based their troubleshooting strategy on a yet-to-be invented diagnostic technique. More likely, the development team would have developed an in-house plug-in exercise ROM for factory and field troubleshooting. It's also not likely that HP would have gone back and added signature analysis to the 9815A code. By the introduction of signature analysis in 1977, HP was well aware that the 9815A was a dead-end product family and that the Desktop Computer Division was focusing development efforts on more expensive machines.

On the other hand, the 9815A is a relatively simple microprocessor system based on the Motorola 6800 microprocessor, so an oscilloscope may be all you need for troubleshooting.


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.



If it can help, the gooey capstan in the 9815 looks like this:

And a procedure for repair can also be found here: Isabelle's site

Hope you succeed in repairing your Cj.

Best regards



You folks are great!!! Thank you!! But I am not yet ready to abandon ship!! Frank

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