"Serge Auckland" wrote in message
...
"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