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Back in the early 1980s I worked at a bank. My department input data into a mini-computer which was then to a mainframe system each day. I'm pretty sure it was a DEC mini, if only because the terminals (I realize now) were DEC branded. This was before my own earliest experiences with personal computers, so it was an alien world to me, and I wish I'd been far more aware of things at the time.

At one point we had problems with the mini, and technicians were called in. I remember them examining an open-reel tape (the actual tape, not the drive, though they probably did that at some point as well) by placing what looked like an inch-long piece of some sort of sticky tape onto the mag tape and lifting off some image.

My theory is that some sort of magnetic powder was placed on the mag tape and an image of the tracks was lifted off for diagnosis (head alignment problems, perhaps?). Does this make any sense at all? Does anyone know what these techs might have been doing to troubleshoot our mini?

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    We used a liquid called Kyread. Methyl perfluoroisobutyl ether + iron particles of about a micron or two or three in size.
    – jonk
    Jul 19 at 6:10

2 Answers 2

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While I can't be certain, what you describe sounds like some variation on what's mentioned in the description of Techmoan's The Magnetic Tape Viewer - see the sound on a tape:

Q) You can buy these sheets of magnetic viewing paper that do the same thing.

A) They don’t have the same resolution or sensitivity - at least the ones I’ve bought don’t. They can’t show the magnetic signal and track layout on a tape. Nothing registered when I tried.

Given the next entry talks about how these were non-destructive alternatives to a liquid solution, I'm going to assume there would be no magnetic powder being placed on the tape beforehand.

Q) You can use this fluid stuff that does the same job

A) A few people are mentioning various ways to do this with a liquid smeared on the tape - These were known about - and likely made by 3M as well - but one reason for the development of the magnetic tape viewer was that it was a ‘non destructive’ way to read the tape - here’s an except from their patent application where they mention one of the existing and alternative methods to do the same thing.

“Heretofore magnetic signals recorded on magnetic tape have in a sense been rendered visible by smearing finely divided ferromagnetic material over the tape and allowing it to migrate to points of maximum magnetic flux. Besides being slow and messy this procedure involved the greater disadvantage that thorough cleaning of the recording medium was required to prevent the applied ferromagnetic material from supplying false signals. This procedure is treated in the television industry as unacceptable for the splicing of magnetic recording tape and splicing has instead been confined to tape areas in which the picture is blanked out.”

As for what they might have been checking for, a bit of advertising copy shown at 2:56 in the video says:

"Scotch" brand Model 600 can be used to check tape recorder head alignment, track placement, pulse definition, inter-block spacing and dropout areas in computer and instrumentation work.

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    This is perfect, and probably very close, if not exactly, what I was seeing. Thanks!
    – TheMadturk
    Jul 19 at 13:36
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I can't comment yet, so will provide this as an answer too.

The Magnetic Reader, by 3M, original patent #US3013206A filed in 1958 and granted in 1961, allowed the user to look at data bit integrity on tape.

It was common to find these in mainframe shops. We had one at ours, though it was only used on a few occasions. We could view standard 7 and 9 track 1/2 inch tape, used on IBM 2401 tape devices, though the Magnetic Reader could be used on almost any sized tape.

https://patents.google.com/patent/US3013206A

Adafruit has a blog entry as well, describing how it can be used to "view" sound, however I believe the viewing of computer tape was its original purpose.

https://blog.adafruit.com/2020/03/01/the-magnetic-tape-viewer-see-the-sound-on-a-tape/

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