hi all, some help would be great with this..
could any one suggest any methods to test a turntable with regards to its speed stability, i know later models come equipped with strobe correcting circuitry...
but say you have a simple belt driven t/table....
i have access to a CRO etc, the main thing I wanted to test for was the effect the variation in mains frequency did to this t/table...
any ideas at all wud be appreciated!
Thanks
could any one suggest any methods to test a turntable with regards to its speed stability, i know later models come equipped with strobe correcting circuitry...
but say you have a simple belt driven t/table....
i have access to a CRO etc, the main thing I wanted to test for was the effect the variation in mains frequency did to this t/table...
any ideas at all wud be appreciated!
Thanks
Not difficult. You get a strobe disk, put it on your platter and
then shine a strobe lite on it. The strobes were commonly
available in the "old days". Surely someone's web site
has one. The strobe light can be a neon light assembly plugged
into your 50 or 60 Hz power. (strobe disk, of course, must
be correct for your line/mains frequency) This will give your
a measure of your turntable's speed accuracy, ASSUMING that
your line frequency is correct.
A better method would be a bright LED run off of a stable
frequency source like an HP 3325A (or B) frequency synthesizer.
then shine a strobe lite on it. The strobes were commonly
available in the "old days". Surely someone's web site
has one. The strobe light can be a neon light assembly plugged
into your 50 or 60 Hz power. (strobe disk, of course, must
be correct for your line/mains frequency) This will give your
a measure of your turntable's speed accuracy, ASSUMING that
your line frequency is correct.
A better method would be a bright LED run off of a stable
frequency source like an HP 3325A (or B) frequency synthesizer.
This thread deals extensively with strobes and strobeflashers.
I have one of the items that dice45 talks about and it works great!
I have one of the items that dice45 talks about and it works great!
Just point out a strobe disk can't do this.
A synchronous motor belt drive turntable using a strobe disk
lit by mains lighting will show no variation whatsover if the
mains frequency varies.
An asynchronous motor turntable will show little effect either.
A turntable not linked to incoming AC frequency, e.g. DC servo
belt drive will show variations on the strobe due to the changing
AC frequency whilst speed is actually constant.
This is why most DC servo's have there own strobe light.
The turntable will either be directly linked to mains frequency
variations or show little to no effect all.
Fortunately for belt drive synchronous turntables mains
frequency (but not quality) is extremely stable - have
you seen the size of a generator rotor ?
The long term stability of mains frequency can be assessed
with an old mains synchronous wall clock with a sweeping
second hand and an accurate reference clock.
🙂 sreten.
A battery LED strobe light shining on a strobe disc is the best. I believe Walrus Systems (London) sell such things if you don't fancy making one yourself.
Alternatively, use a frequency counter to measure a 1kHz tone on a test disc if the test disc is known to be recorded correctly and have a hole exactly in the middle.
Alternatively, use a frequency counter to measure a 1kHz tone on a test disc if the test disc is known to be recorded correctly and have a hole exactly in the middle.
EC8010 said:
hi there, i am really after one of these 1kHz test tone records at the moment, does any one have any idea as to where to get one from ? endless net searched have lead nowhere!
😡 ..........
I'm not sure if this would give any meaningful results. Typical mains freq. deviations are small, slow drifts that happen over a long period of time. The test tone is very short in duration and most likely you'd not see any changes at all.
Also, the signal coming out of your phono would have not only the test tone but also surface noise and would probably have some amplitude modulations, depending on the quality of the TT and the record itself. Therefor, not a very good signal to feed a frequency meter when you're looking for small changes.
If you were really into this, you might try putting some mark on the side of the platter, make an optic reader with photodiode/comparator and feed its output to a freq. meter setup to count period. This would count lap time and so the inverse of the read value would give rpm, and I bet with a high degree of accuracy.
If your meter has an option for it, you could even feed this data into a computer, let it record it for a few hours (say 24h) and then process it by making a nice graph, calc. min/max/ave...
Then you could come back and show it to us 😀
Guilherme.
Actually, despite your doubts (which I share), it does! Although pointing an LED strobe light at a strobe pattern is best for setting exact speed, it's not much use for determining an inexact speed. Garrard 301 and 401 generally run fast. (As the magnet in the eddy current brake tires over the decades, the motor runs faster.) One cure is to fit an undersized motor pulley. I played a test track, measured the frequency, calculated the speed error, and had a new pulley made. With the new pulley fitted, the fine speed adjustment sat in the centre of its range.
Hi.
One of the first ideas for the motor controller I'm working on was to allow it to self-adjust by feeding it the signal of a test tone so a strobe would not be needed. I droped it because no one would tell me the precision of the test tone and because of the my previously stated concerns.
Guilherme.
One of the first ideas for the motor controller I'm working on was to allow it to self-adjust by feeding it the signal of a test tone so a strobe would not be needed. I droped it because no one would tell me the precision of the test tone and because of the my previously stated concerns.
Guilherme.
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