"Arfa Daily" wrote in message
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"Serge Auckland" wrote in message
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"b" wrote in message
oups.com...
looked at a recently made (last 2 or 3 years i suppose) tdk d90 audio
cassette the other day and noticed there was no metal plate behind the
pressure pad - seems this omission is widespread these days- why has it
disappeared and what was its original function? something to do with
stray magnetic fields??
regards, b.

The original function of the metal plate was, as you surmised, a magnetic
screen. Usually, the plate was made into a strip which acted as a spring
to keep the pressure pad in contact with the head(s). Why it has changed,
I have no idea - cost?

S.

Serge, I don't think that this is the plate that b was asking about. The
pressure pad, on the phosphor bronze spring, is the same as it ever was.
What has disappeared, is the flat metal plate behind that. It's as big as
the head access hole, not a thin strip like the pressure spring. My
immediate thought was also that this was a magnetic shield, but thinking
more about it, what exactly is it shielding against, and which way round -
head to tape spools, or vice versa ? The other thing that bothers me about
it being a magnetic shield, is that it appears to be made of mild steel,
rather than mu-metal or some other decent shielding material.

The other possibility is that it was for static shielding purposes. There
is a lot of static generated by the moving tape. One particular answer
machine that I used to have dealings with, used to suffer very badly with
static discharge to the head, which manifested as loud clicks. It was
corrected by the manufacturer bringing out a revised tape. I never took
any notice of what the revision was, but may have been using a tape with a
shield ?? I still have one, and I took a look at it, and it has the metal
plate, but as to whether the originals did not have a plate, I really
can't remember. Perhaps the plate has disappeared because advances in tape
composition, have removed the static problem in the first place. Just a
thought.

Arfa
Arfa,
Thanks for the explanation. I haven't used cassettes for several years, but
I don't recall there being a metal plate behind the spring. I used to use
mostly "That's Tape" metal cassettes, (best formulation at the time) or only
slightly less good were TDK metal. Maybe they didn't have the plate, or more
likely, I never noticed.

S.