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Set up a sig genny and test him out


Much easier to say the test tape is stretched.
Perhaps perfect pitch testing would only work by playing the likes
of
JSB
Tocatta and Fugue in D major, not pure tones

How come I cannot google the rpm of an audio cassette capstan? A
calibrated
strobe and tipex mark on the capstan periphery should give a
definite
answer
(assuming the spindle is clean and not worn).
It should be determinable from tape speed of 1 7/8 in per sec and
spindle
diameter, that is about 1.9mm, but what should it be to 0.1 percent
accuracy?



The rotational speed of a cassette capstan is not a fixed given. The
diameter of capstans vary from machine to machine, and the correct
speed
of
tape transport is then a function of how fast you drive the capstan
round.
The reference in my strobe tape, is indeed the mains. I have been
using
this
tape for many many years, and I have file://never// had anyone
complain
that the
speed of their machine is off, after I have used it to set one up.
Konig
must think that the mains is a good enough reference, otherwise,
there
would
be no point in them marketing the tape for the purpose of setting up
speed.
Exception to this. As Ron said, very occasionally, when an owner has
recorded tapes when the machine was running at the 'wrong' speed ...

Arfa

Arfa



I've since found 2 capstan spindles of 2.4mm diameterm so no fixed
specification for cassette tape dynamics.

Another possibility as a test tape - record some constant tone, any f,
on
a
few minutes of tape, pull out a long length and pass a magnet over two
parts
a measured distance apart. retract, and then time the interval between
dips
in play mode


You are assuming that if the capstan rotates at the correct RPM , that
the
tape speed will be correct. The proper way to do this is to measure the
playback frequency of the tape which gives you the tape speed.
Obviuosly,
the accuracy of the test tape, the accuracy of the counter, etc all come
into play to add errors. The industry standard is to use a wow and
flutter
meter in conjunction with a test tape. The test tape is recorded at
3000hz
on a reel to reel recorder and then loaded into a cassette shell because
there is no cassette recorder on earth that can produce a test tape with
the
10x or 5x accuracy needed as a standard.
Wow and flutter meters are basically frequency meters. They are like
guitar
tuners, but the output is weighted to breakup the frequency variations
into
long and short term changes.
If you can find a calibrated wow and flutter meter with a test tape, I'm
sure the cost will far exceed what your client is willing to pay. The
technology is at least 20 years old, and there really isn't a market for
anyone to make this equipment available.
The bottom line -------- Even if you go through the time and expense to
do
this right, what good is it if your client's tapes are not recorded at
the
same precision? You and he will be better off if you add a front panel
speed
control so that he can dial in the speed to match the tape being played




I found a large quantity of salvaged capstans+spindles.
26 off on my mike (rather than callipers before) measure 1.99 +/-0.01 of
reading accuracy and 2 nearer 2.00
5 off measure 1.79mm
1 at 2.19mm
9 measure 2.49mm

So I will give a few to my engineer mate and ask him what they measure and
the temperature at the time.

The machine in question uses 2.49.
Assuming it is actually 2.5mm then doing the maths and strobing with
quartz
f-meter calibrated strobe then my test tapes are 0.4 percent out from
calculation via 15/8 ips etc.
spindle rotation speed of 6.05 rev per second with an error of about 0.4
percent , needs longer gate time or repeating more times to bring that
accuracy up (rounding errors? so probably longer gate time required than
my
meter has).
If 2.49 diameter then 0.6 percent out



A rather belated follow up, just got the representative measurements back
from the
mechanical engineer.


1.99 +/-0.01 of
reading accuracy (measured at 62 degree c) and 2 nearer 2.00
5 off measure 1.79mm
1 at 2.19mm
9 measure 2.49mm

Either spindles or bearings would have to
be an off-size for free movement, so presumably sintered metal bearings
are (easier?) made round number size and spindles made 0.01mm smaller
diameter and
speed of rotation adjusted just as easily to that as any other speed.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://diverse.4mg.com/index.htm