HP85 tape reader; end of tape detection



#8

Hi.
The tape reader on my HP85 has some problem when detecting end of tape.
If I rewind the tape, it works but when reaching the end of the tape the wheel continues rotating as if it had not detected the end of the tape. As you can figure out, as the motor is trying to rewind the tape beyond the begining, it sounds like it all was about to break.

Does anyone have some experience dealing with this problem? Is there any easy fix? :-(


#9

End of tape is detected by an optical sensor seeing a hole in the tape, one half of the is in the base of the tape mechanism and the other half is in the back, to get an idea where they are look at your tape cartridge, on the bottom below the `record`switch you will see a small window with a 45 degree mirror inside. A led shining on that mirror is what is looking for the hole in the tape. If you hold open the tape door and depress the cartridge in switch (lower one) the light should come on and the capstan start to turn. It seems to me that when you first insert a cartridge it expects the tape to be just ahead of the whole if tape is rewound past the whole it will keep on going and the tape will come off the one reel. It is possible to get them back on but it is not for the faint at heart and involves disassembling the cartridge. If you open the little door on the bottom there should be tape there, if there is none your tape is already despooled. If this is an original HP cartridge one of the problems is these cartridges are old and the band that goes over the top of the spools and around the drive wheel may have lost a lot of it elasticity and adds to the difficulty in fixing the cartridges. There is a service guide for the 85A on the museum DVD set and if you look around you may find copies elsewhere.


#10

I have checked that the light comes and the capstan starts running on when I press the tape presence knobs on the bottom of the tape reader. If I look at the tape (which is fully rewound) I see a normal tape, I cannot see the hole. Apparently the tape is not broken. The hole on the tape is it located at the very end of the tape (like in a casssette tape) ?
Besides the light that come on from the bottom of the tape reader, I can see another led-like thing on the bottom. It looks like a led o something. What is this "led" for?


#11

There is only the one LED in the base for the BOT EOT sensor, the only other LED is the activity indicator on the front. There a a couple holes that are for screws that come up from underneath. The hole is not right at the end of the tape. When the tape is fully rewound it does not stop right at the hole, it will rewind until it detects the hole and then it moves forward a bit at normal read speed looking for the first record on the tape, if it does not find it within the first few minutes it gives up and stops so for a fully rewound tape the hole will beyond the position the tape stops at towards the spool with less tape on it, but be careful moving the tape by hand, if you move the hole past the sensor toward the spool with the most tape the next time you insert the tape it will despool.

The other common problem with these drives is the "tire" on the capstan was made of of a compound that breaks down over time, so if it has not been replaced in your machine it is likely that it has fallen apart and is unable to drive the tape if you search on the web you will find some suggestions on how to rebuild this.

As I mentioned any of the original HP cartridges that are still around will be quite old now and another thing that happens with them is if they have been sitting for a long time without the tape moving, the pressure of the drive band on the tape may cause the oxide layer to bond to the back of the tape on top of it and when you move the tape it tears a spot of oxide off which makes the tape useless. I suppose that could be fixed by clipping off the damaged portion and making a new end of tape hole the appropriate distance from the end, but I have not tried this.

I don't use my 85A much since I got a 86B and a HPIB dual diskette unit. I think that for the little slow storage the tape drive offers, it is not worth all the trouble to get them working. Some people have also modified the drives to use newer cartridges, but I think that if you just need storage HPIB diskette or even the HPDrive disk emulation over HPIB is a much better solution.


#12

I can read in the manual that the other 'led' actualy is a phototransistor.


#13

I took a closer look at the tape drive in my 85A today. In the black plastic base of the drive there is the light source for the EOT sensor it is not an LED but is an incandescent lamp. The light sensor (photo transistor) is in the back behind the hole just to the right of the plungers for the cartridge present and write enable switches. Under the base there is another incandescent light and photo transistor that detects black bars on a disk mounted on the shaft of the motor this is used to control the motor speed. There are no other light sources or photo transistors on the tape drive, save the activity LED on the front.

When a cartridge is fully rewound and at the load point, with the drive wheel up and the label towards you, if you open the little door where the head contacts the tape the sense hole will be near the left side of the opening, and the majority of the tape is on the left spool. If you try to rewind a tape and the motor seems to run forever it is likely that the tire on the capstan has disintegrated.


#14

I have taken appart one of the tapes and it was pretty dammaged. Way beyond any possible repair. Also, someone replaced the ruber tube around the capstan, but it does not look very well, so probably it would need some new replacement.
A dont have any more tapes than the original HP tapes and all them look damaged.

So I took a look at the GPIB emulator storage solution provided on one of your links. I own several GPIB PC interfaces from NI and Agilent brands (i have two USB-to-GPIB interfaces, one NI PCMCIA and one NI Serial-to-GPIB). On the HP85 side I own the GPIB interface plus the I/O ROM Module. I can put also the mass storage ROM on the EPROM programmable module. With all that stuff is it reasonable to think that the GPIB-storage emulator would work on my system ?


#15

Go to http://www.hp9845.net/9845/projects/hpdrive/ and you will find a list of the supported adapters as well as links to down load the code, in general it is just National Instruments PCI and ISA cards that are supported. The referenced page explains why. I am guessing that you 85 is the 85A model? The 85B has the mass storage ROM built in. With just the mass storage ROM you will only be able to use AMIGO complaint devices CS/80 and SS/80 devices require the Extended Mass Storage ROM which was only officially supported on the 85B but it is possible to get it to work on an 85A but you need the 85B version of the mass storage ROM and EDISK ROM and the only way way to get them onto a 85A is to burn EPROMs which may not be useful for the EPROM card. This is what prompted me to design my first EPROM card.

I am not surprised by what you found on your tapes or tape drive.


#16

Yes, my system is an 85A.

So if I understand you, I would need three modules, all of them from the 85B collection. And none of them is available in ROM format for the 85A, so I would need to use three EPROMs which do not fit on a single EPROM board. Right?


#17

Well the Extended Mass Storage ROM was a regular ROM that you would install in a ROM drawer, but the trick may be finding one. If you are going to use the emulated storage then not have CS/80 or SS/80 is not an issue there are plenty of Amigo compatable emulated devices you can use, however if you intend to buy some real HPIB storage then not having support for those protocols will cut down you choices.


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