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Re: [ARSCLIST] MAGNA-REEL Sound Recording Tape
At 10:45 AM 9/18/2006, dave n wrote:
Hello all -
Just wondering if anyone had any info on "MAGNA-REEL Sound Recording
Tape", circa 1949-1951 (metal cans, 5" reels).
I have a number reels that I need to transfer (1/2-track mono 3.75
ips). About half of the reels in question appear to be fine, the
other half appear to be dehydrated and squeal upon playback - both
physical squealing when you listen with your ear close to the tape
passing over the playback head, and audible high-frequency
"squealing" on the digital transfer. The problem tapes also suffer
from edges crinkling, as well as some delamination of the oxide
layer at the head and tail of the reel.
I have not done a heck of a lot of work on "problem" acetate tape of
this vintage before, but I suspect that rehydration is what I am
looking for. I have heard of several methods of rehydration bandied
about by members of this list - most of them low-tech in nature as I recall.
Hello, Dave,
This is a mixed bag of problems in a combination that I hadn't
expected to see based on history, but not surprising.
Is there a strong smell of vinegar or "lemon chicken" with these tapes?
I will be presenting a paper at AES on 10/8 about my research but I
have hypothesized -- and tested with three very squealy PET tapes
that did not respond to baking -- that there are other things going
on that what we think.
I see no reason why some of the same issues for PET tapes wouldn't
apply to acetate tapes, but the usual binder materials in acetate
tapes was acetate and that has historically not broken down.
With any polymer, there is a temperature at which the polymer turns
rubbery. This is called the glass transition temperature (Tg).
The really neat thing is if you lower the ambient below the Tg, the
tape stops being rubbery and it plays without a squeal.
We know that polyurethane binders will have a drop in their Tg when
the short chain molecules are broken by hydrolysis -- and there's
lots of research that supports that, although the baking cure doesn't
make everything as good as new, but it does raise the Tg for a while.
So, the easiest thing to do is to see if it's a Tg issue. Place the
player and the tape in a cold space (aka refrigerator) for a few
hours until everything comes to equilibrium. Hit play. Does it still squeal?
Adding moisture will generally make the tape softer and will increase
squeal, so rehydration would not be recommended at this point.
The other option is to reduce tensions and do a Pellon wipe (or
several) as that has worked on 3M 175 (again a later tape).
The squeal MUST be attended to during playback as it is a stick-slip
condition and will print to the digital file (as you discovered). So
you don't even need to test with audio. If you can hear the squeal,
try something else.
Now it might be possible that the tape is suffering from true
loss-of-lubricant (which Sony PR-150 and 3M 175 do not suffer from,
though we've all said that for years) but in general, the real
problem is that there is not enough room in the matrix to take up
enough lubricant to relubricate a tape--in fact the lubricant is
there, at least in the PR-150 we had tested. Since more (or MUCH
more) lubricant would be required now to properly play the tape,
there is no place for it to go.
One other thing to consider about lubricant. It's there based on the
design friction load. As the Tg drops, the tape gets more rubbery and
the real tape contact area increases. Normal tape has a contact area
in the neighbourhood of a few percent (2-3%) of the entire surface
area, and the normal lubricant loads are designed for this with some
margin. If by softening the surface, the contact area goes up, there
isn't enough lubricant. This is why Marie O'Connell's wet playing
works, but I'm not sure I'd want to try that with acetate as acetate
is far more hygroscopic than PET (which is, surprisingly somewhat
hygroscopic).
Squeal is a complex issue and has not been well understood as far as
I could find in the literature.
I hope shortly after AES to be set up to do cold playing easily, but
I'm swamped at the moment. I really would like to know your results.
One concern, is where has the Tg fallen to? For Sony PR-150 and 3M
175, it was some where between +20C and +4C (i.e., squealed at +20C
did not squeal at +4C). There is no magic that says the Tg has to
stop at +4C. The Sony APR-5000 was, I think, tested to -20C so that
would be a good choice IMHO to use in a freezer, if necessary. I'm
not sure about the Studer product lines, but to make my original
testing tougher, I used a ReVox A77 (and I had to take less out of
the kitchen fridge). The A77 has many more surfaces to contribute to
stick-slip than especially modified APR-5000 or especially a modified
A810 which only has the play head as a fixed member.
I suspect that we will start seeing more and more problems with
acetate tape, as it is all coming up on 35 years old with some of it
over 60 years old.
Cheers,
Richard
Richard L. Hess email: richard@xxxxxxxxxxxxxxx
Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.