Destructive Imaging of DECTAPE II Media
Mark J. Blair
nf6x at nf6x.net
Tue Jan 27 11:04:09 CST 2015
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>
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