hp-9815 Tape Drive problem


I have two hp-9815S calculators. I have replaced the rubber drive wheel for the tape in both units. They seem to operate smooth and go through all of the motions properly when you press REWIND, MARK and IDENTIFY. But if you record anything and try to LOAD it or VERIFY it, all you get is CHECKSUM ERROR. I have selected a tape with a "tight belt" for this test. It would appear that the record level is too low. Is there an easy fix? Is the schematic diagram available for the 9815?


Jim Butch


The problem may be still be with your tape(s), despite having good tension. Have you tried using a known good tape? Often these tapes loose their coercivity as they age especially if stored at high temperatures and you'll get exactly the behavior your describe.

Edited: 15 Oct 2004, 6:41 p.m.



Thanks for the input on the tapes. I have many DC100A tapes, both used and NOS. Most of these tapes, both used and NOS, do not work. They report and "end of tape" error on an hp-85 and simply do not work on the 9815. Even if you wind the tape to the middle manually and place the tape in the unit, it will rewind and report the same error. These tapes are 20+ years old. I have a few in the 10-12 year old range. These are all made by 3M and have tape feed problems. What is the cause of the end of tape error? I did find the Athana brand and plan to order a few to try.

Also, I did find the technical documentation on the 9815. There is a 1978 patent on the 9815 that includes everything. It can be downloaded from www.decodesystems.com. There is also a similar patent on the 9825.

Jim Butch


if you get an "END OF TAPE" error, it means that the oxide on the tape has fallen off.

The BOT/EOT sensor (beginning/end of tape) is optical. It detects light shining through a small hole on the tape. If the oxide falls off, then the tape becomes transparent fooling the sensor.

Check here for an example of how the tape looks like after the oxide has fallen off leaving a transparent "window" on the tape.


PS for a discussion on fixing the capstan and a larger copy of the above picture, check




I've tried many NOS tapes in both my 9815 and 9825 and they usually work. I find that the DC1000 tapes have always worked while the DC100A tapes have more problems. The DC1000 tapes are a higher coercivity replacement for the DC100A tapes with a slightly longer length. 3M (Imation) made them more recently than the DC100A tapes and probably used batter manufacturing techniques in their construction. They're not too hard to find on ebay but if you have trouble email me (katie10543@yahoo.com) I have extras.

Edited: 19 Oct 2004, 10:02 p.m.

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