Destructive Imaging of DECTAPE II Media

[Alexandre Souza]

    I think it is a great start! But if you're doing destructive tape 
recovering, why not using a 15x slower mech and using a slower (and cheaper) 
ADC?

    Or use a band from a known good cartridge, at least to recover the data.

    Or you can use a reel-to-reel mech :)

---
Enviado do meu Apple IIGS (pq eu sou chique)
Meu site: http://www.tabalabs.com.br
Meu blog: http://tabajara-labs.blogspot.com




----- Original Message ----- 
From: "Mark J. Blair" <nf6x at nf6x.net>
To: "General Discussion: On-Topic and Off-Topic Posts" 
<cctalk at classiccmp.org>
Sent: Tuesday, January 27, 2015 3:04 PM
Subject: Destructive Imaging of DECTAPE II Media


I have a bunch of DECTAPE II cartridges from which I want to try recovering 
as much data as possible, including the console cartridges for my 
VAX-11/730, none of which have managed to boot the machine. Inspired by the 
various floppy disk imagers I have, my thinking has wandered in the 
direction of building an imaging device using a TU58-XA mechanism and my own 
drive electronics. I happen to have a few TU58-XA mechanisms sitting about 
to experiment with. The imagined imager would sample both tracks 
simultaneously with ADCs so that I could post-process the data repeatedly 
from a single physical pass past the heads. Maybe I'd even hack in the 
missing optical end-of-tape sensor rather than relying on the written 
end-of-tape signals. This scheme might let me recover data from tapes which 
confuse the normal TU58 drive electronics (say, because of corrupted sector 
marks).

With that in mind, I dug into a box of acquired tapes to select a 
sacrificial dummy for experimentation, and picked out one labeled "NFG" by 
the previous owner, figuring I had nothing to lose with that one. Inspection 
revealed that the drive belt had broken and was stuck to the tape, and as 
expected, it peeled the oxide right off when I removed it. Is there any way 
to remove pieces of stuck drive belt from these tapes without harming the 
underlying oxide?

Next, I moved on to trying to replace the drive belt with a Plastiband brand 
elastic band, as I've previously seen discussed for tape cartridges with 
this style of mechanism (including the larger QIC cartridges). I used the 
2-1/8" size, and found that it seemed to be of suitable diameter to stretch 
around the required path. However, I have had no luck getting it to stay in 
place when the tape is moved. It quickly jumps off and gets tangled when I 
drive the tape, and I suspect that it's because these Plastibands are a bit 
too narrow to properly ride the crowned drive wheel and idler rollers.

Let me interject that I have nothing good to say about this belt-driven 
cartridge design. I thought it was a bad design the first time I encountered 
one, and nothing has changed my opinion since then!

Now I finally get to the main topic I'd like to discuss: "Destructive" 
imaging of DECTAPE II media. By eye, the DECTAPE II tape media looks very 
close to the same width as normal audio cassette tape. What if I built an 
imaging drive in which I remove the tape from any DECTAPE II cart to be 
imaged rather than trying to use the original &(#$%*$ belt drive system? I 
wonder whether there's any prior art for a scheme like this.

One idea would be to transplant the tape media into a cassette tape housing, 
but I'm not yet sure whether that might offer any advantages over building 
an ad-hoc open reel system, or even a reel-less system since any given tape 
would only be run past the heads a small number of times in one event, and 
then might be discarded once any remaining data is extracted.

I have doubts that a cassette transport's pinch roller and capstan system 
would work well for this scheme, since the normal tape speed of a DECTAPE II 
is over 15x the tape speed of an audio cassette. So, I'd probably need to 
fashion a different sort of capstan system, possibly negating any advantage 
of using cassette tape housings. I'm not sure whether it would be better to 
pull the tape with a capstan and pinch roller drive vs. pulling it with a 
take-up reel hub and providing a free-spinning capstan with an optical 
encoder to provide tape speed feedback. Either scheme would allow me to move 
the tape past the head at constant speed, but I'm not sure if one scheme 
might be easier to build than the other.

Next, the imaginary device would need appropriate heads. I might salvage the 
head from a TU58-XA, though that could be a bit challenging since the heads 
are epoxied in place after adjustment. Or, maybe I would get lucky and 
discover that the heads from an auto-reversing audio tape deck (which I 
believe have four gaps in order to play stereo tapes in either direction 
without flipping them) might have gaps in the right places for reading both 
tracks of DECTAPE II media? Audio cassette tape heads have a few potential 
advantages, including not being epoxied in place and having a tape alignment 
guide welded on one side.

What do you folks think about this silliness?

-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/