Yet another idea for recovering the data from Series80 tapes

Tony Duell ard at p850ug1.demon.co.uk
Sat Oct 29 19:05:09 CDT 2005


> 
> Background:
> 
> The problem with the old HP 9162-0061 data cartridges is that the
> magnetic coating peels off from the (transparent) tape causing the
> machine to stop accessing the tape with an EOT warning. This is
> actually very good news because it means that the damage is localized
> (a few mm of tape with scrapped off magnetic coating).
> 
> BTW here is a picture of a damaged tape:
> 	http://www.series80.org/Articles/DamagedTape.jpg
> 
> My guess is that the magnetic coating gets scrapped off the tape
> during fast motion (rewind or fast forward), so presumably if you

Hmmm. I thought it was where the taps stuch to itself, or to the drive 
belt, when the catridge was stored. In which case your idea might not 
help much.

> simply advance the tape over the heads at normal read speed (10 ips)
> the tape will survive for a single pass (there are two tracks on the
> tape, but since the head is not moving, I assume that you can read
> both tracks at the same time).

I would think so. The head is not moved between tracks, it's electrically 
switched.

> 
> Now since we have only one attempt, and we are likely to encounter errors,
> I do not want to have the Series 80 firmware involved in the process.
> I would like to simply record the info (via a PC) to a .wav file and then
> process that file to see if anything can be extracted.
> 
> Proposal:
> 
> To do this I would need to manually rewind the tape till the BOT sensor,
> mount it on the tape drive and advance it past the read/write head
> till I run out of tape (tape unspools from the supply reel).
> 
> While this is going on I am recording the output of both heads via the
> audio in port of my PC sound card (maybe use two separate sound cards to
> avoid cross talk?).

I would think one sound card (with 2 input channels) would be enough. I 
think cross talk would be pretty easy to eliminate in software (if it's a 
problem at all).

> 
> Data extraction rig:
> 
> 1) audio connection: what kind of set-up would be required to connect the
> heads to the audio card. From the schematics I see that there are essentially
> three connections to each head:
> 	Head 0:	H0, CT0, and HC
> 	Head 1:	H1, CT1, and HC (HC is common to both heads)
> But I am not sure what these are used for.

I would assume 'Hn' was the top end of each winding, 'CTn' was a centre 
tap and 'HC' was head common.

What I would do (and alas it needs a 'scope would be to remove the R/W 
chip and connect the differential inputs of the 'scope to HC and H0 
(say). Run a recorded, but unimportant 'scratch tape' through the thing 
and see what signal you get. Then design a differential amplifier to 
bring it up to the sound card input levels. 

The preamp schematic from the 9815/9825 tape drive might be a start. Does 
anybody know if those drives are tape and track compatible with the 85 
drive? In other words, would the 2 track head of a 9815 correctly read 
the tracks on an 85 tape (yes, I know the controllers are very different).

> 
> 2) running the tape at a constant 10ips. There is a circuit that uses
> the tachometer wheel to control the tape speed, but again I am not sure
> how to program (?) the tape controller IC (U1) to move the tape.
> (I may need to disable the BOT/EOT sensors, but that should not be a
> problem).

Could you not design your own motor controller? It's 'just' a matter of 
controlling the motor voltage to get the right frequency output from the 
tacho sensor.

What I would do, actually, is extract the drive from a 'junk' 85 (or a 
9815 or something if that drive is compatible) and use virtually none of 
the HP electronics. Make my own motor controller and head amplifiers. I 
would not want undocumented custom chips around. You might be able to use 
the speed control circuit from the 9825, that is all standard chips 


> 
> I am lucky to have a 9915A as my test bed which allows far easier access to
> the tape drive mechanism than the HP-85 (and does not have these evil
> ribbon cables).

Incidentally, there are some interesting manuals for the 9915 (and other 
HP desktops) on http://www.hpmuseum.net . Even an operating/service 
manual for the _keyboard_.


-tony




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