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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 5th Jul 2019, 11:59 pm   #1
martin.m
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Default How to set the speed on cassette decks.

I have a few 1970s cassette decks, the type with a preset control on the motor, and the speeds are not quite the same. One possibility would be to buy a pre recorded cassette of an album which I have on CD and then compare the two, but there must be a more scientific method.
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Old 6th Jul 2019, 12:20 am   #2
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Default Re: How to set the speed on cassette decks.

The official way to do it is to use a manufacturer's reference tape and a lab grade frequency counter. I don't have either of those, so this is how I do it:

1. Generate some some sine wave tone using software, and save it as a WAV file.
2. Burn the WAV file to an audio CD.
3. Record the CD onto a cassette using a known good recorder.
4. Play the CD on a player while playing the cassette on the player needing adjustment.
5. Adjust the speed for a zero beat between the two.

Domestic deck speed stability isn't great and it can be difficult to find the exactly correct setting, but you can get very close.
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Old 6th Jul 2019, 12:24 am   #3
BRASSBITS
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Default Re: How to set the speed on cassette decks.

Back in the day you could buy a strobe disc built in a cassette case they worked well.
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Old 6th Jul 2019, 7:07 am   #4
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Default Re: How to set the speed on cassette decks.

Quote:
Originally Posted by paulsherwin View Post
The official way to do it is to use a manufacturer's reference tape and a lab grade frequency counter. I don't have either of those, so this is how I do it:

1. Generate some some sine wave tone using software, and save it as a WAV file.
2. Burn the WAV file to an audio CD.
3. Record the CD onto a cassette using a known good recorder.
4. Play the CD on a player while playing the cassette on the player needing adjustment.
5. Adjust the speed for a zero beat between the two.

Domestic deck speed stability isn't great and it can be difficult to find the exactly correct setting, but you can get very close.
When I was at Philips, we had test rigs on the bench which basically were referenced to the mains which was then doubled to 100Hz. A reference test tape with a 100Hz tone was put in the machine which was connected to the rig and then set to play. The speed was adjusted until the front panel meter read at centre zero (or as near as). The wow and flutter was also shown and most machines would very slightly either side of centre zero while running and the percentage was shown by the meter either side of centre zero. Domestic machine tolerance was 2%. For the upmarket Hi Fi machines, a more sophisticated method was used using a scope and a Marconi wow and flutter test rig.
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Old 6th Jul 2019, 7:54 am   #5
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Default Re: How to set the speed on cassette decks.

I still have a 3kHz test tape, which I bought several years ago, don't recall from where. and use this in conjunction with a frequency counter to set the speed. Ideally the cassette player (or recorder) should be run for several minutes before running the test, which allows the speed to be set within manufacturer's tolerances. My Freq. Counter isn't a laboratory grade instrument, but a kit-built one now over 25 years old, but it serves the purpose well enough. I used to have a wow & flutter meter, but sold that over 20 years ago, more's the pity.
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Old 6th Jul 2019, 10:37 am   #6
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Default Re: How to set the speed on cassette decks.

@ Sideband


Do you mean this one on the attached "pdf"? https://www.vintage-radio.net/forum/...d.php?t=106973, post #17
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Old 6th Jul 2019, 11:37 am   #7
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Default Re: How to set the speed on cassette decks.

Konig used to make a strobe-style cassette which worked quite well, though Paul's method is superior. The difficulty is finding a really good deck to record the tone on, of course.
Beware the CD vs cassette play-off. I had a machine in the other day with a genuine Carpenters musicassette in it. It was playing fast, so I decided adjustment was needed. I popped a known good tape in, and it played fine. The customer's tape played fast on a good machine and was obvioulsy recorded out of tolerance!
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Old 6th Jul 2019, 11:43 am   #8
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Default Re: How to set the speed on cassette decks.

There was a circuit in Television magazine for a cassette speed tester. This was very simple indeed, feeding the audio into the gate of a thyristor and switching a filament light bulb fed from a suitable transformer. There was also an attenuated output from the transformer for recording the 50Hz mains on another, known good deck. The idea being that you played the recorded 50Hz signal into the thyristor, and adjusted the speed until the brightness of the lamp was steady, meaning the pulses from the tape are coming at the same interval as the mains. Too slow and the pulses arrive later and later in each successive cycle, meaning the lamp dims gradually and then suddenly goes bright again; too fast and the pulses arrive earlier in successive cycles, causing the lamp to swell to full brightness and then dim suddenly. The closer the speed to the correct value, the slower the variations.
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Old 6th Jul 2019, 11:56 am   #9
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Default Re: How to set the speed on cassette decks.

In the absence of a known good recorded tape you can get some idea by checking the run time of a C60 or C90 cassette in reasonable condition - of course this is not 100% accurate as different manufacturers will put on different length leaders and add a few feet to be on the safe side but it will give you an idea.

If you can get access to an accurate recorded tone tape you can check the frequency and record an exact copy for future reference.
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Old 6th Jul 2019, 11:54 pm   #10
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Default Re: How to set the speed on cassette decks.

I remember making my first speed test tape at school, when I had just acquired and repaired a Sony TC186 with a faulty motor. I recorded a continuous tone of no particular frequency for 10 minutes or so. I got a good quality tape measure and marked out as accurately as possible 112.5 inches along a lab bench (1 minute run time) and 15 second subdivisions. I pulled the tape out of the cassette, laid it oxide-up along the bench, and at each mark make a momentary erasure using the end of a paperclip wrapped around the tip of my demagnetiser. I also marked the back to align the next length, taking care not to introduce cumulative errors, for a total of ten minutes run time.

When played, the erased sections made clear pips which I timed with a stopwatch. The error when checked over the full ten minutes would have been miniscule, although most motors could not be adjusted that precisely. Obviously it gave no clue as to the wow and flutter, but I was really just after setting the speed on an otherwise working machine.
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Old 7th Jul 2019, 6:59 pm   #11
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Default Re: How to set the speed on cassette decks.

Quote:
Originally Posted by mitajohn View Post
@ Sideband


Do you mean this one on the attached "pdf"? https://www.vintage-radio.net/forum/...d.php?t=106973, post #17
Ours weren't like that but the principal was the same. We also used the strobe method shown further back in that thread.
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Old 7th Jul 2019, 8:19 pm   #12
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Default Re: How to set the speed on cassette decks.

Iíve always used a scope to view the response from a Philips test tape that has a 4khz tone which then goes to a frequency counter. Itís then easy to set the speed to get the 4khz output and leave it running to see if there is any variation over time. If there are dropouts caused by headware or wow itís easy to see.
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Old 10th Jul 2019, 10:31 pm   #13
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Default Re: How to set the speed on cassette decks.

Thank you for your replies and comments. There is a mobile phone app that can measure frequency via the microphone input so all I need is an accurate test cassette.
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Old 10th Jul 2019, 11:31 pm   #14
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Default Re: How to set the speed on cassette decks.

Canford Audio used to do the Harrison Test tape with the 3kHz tone, these were
recorded individually in real time on a known Nakamichi deck. (Ordinary commercial
cassettes were made at X8 normal speed to reduce production time)
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Old 11th Jul 2019, 8:41 pm   #15
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Default Re: How to set the speed on cassette decks.

All the responses here boil down to having a prerecorded tape recorded with a known test tone at a known tape speed. I wonder if one couldn't measure the speed of the flywheel using a stroboscope of sorts, although of course that doesn't take into account any slippage of the tape.
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Old 11th Jul 2019, 11:04 pm   #16
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Default Re: How to set the speed on cassette decks.

The Uher CR 240 I'm dealing with at the moment (threads passim ad nauseam) has a sleeve (magnetic?) on one cassette spindle with a reading circuit close up against it, I assume for speed regulation.

The other base I have for spares has one cassette drive reel with a reflective/black alternating radial strobe. The circuit that in mine is perhaps magnetic, instead has a photoemitter/detector assembly and presumably uses this to regulate the speed.

Jolly clever, whatever it is! It's bolted on inside the transport, so perhaps could be altered and used in another machine? Circuit diagrams are freely available so I would have thought some of the bright sparks on here would be able to modify it...
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Old 12th Jul 2019, 5:02 am   #17
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Default Re: How to set the speed on cassette decks.

No, that sensor in the CR240 (I assume, as there's something similar in the CR210 that I have) is to detect when the tape as stopped moving to cause the thing to auto-reverse. You wouldn't try to control the tape speed from the spool motion anyway, the speed of the spool rotation depends on how much tape is wound on it, tape speed is controlled by the capstan.

The CR210 motor is electronically controlled, the speed control is part of that.
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Old 12th Jul 2019, 8:06 am   #18
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Default Re: How to set the speed on cassette decks.

That Strobe on the take-up spool is read by an optical sensor-usually a photodiode & phiototransistor assembly. The resulting pulses are fed to a detector circuit. When the pulses stop, the circuit switches-in the case of the Uher to change tape direction, in other cases to stop &/or eject the cassette. Car cassette players often used similar circuits and somewhere I have sample circuits from Blaupunkt training courses back in the 80s
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Old 12th Jul 2019, 9:39 am   #19
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Default Re: How to set the speed on cassette decks.

Quote:
Originally Posted by ricard View Post
All the responses here boil down to having a prerecorded tape recorded with a known test tone at a known tape speed. I wonder if one couldn't measure the speed of the flywheel using a stroboscope of sorts, although of course that doesn't take into account any slippage of the tape.
Sorry to disagree but I notice several of the responses go back to first principles i.e. how much tape passes the head in how much time. I like the continuous tone + erase-clicks a certain length of tape apart.

However, in the For Sale area, someone has snapped-up a test cassette so maybe that person could be persuaded to make a few clones. Just for the tape speed (head alignment etc being too machine dependent).
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Old 12th Jul 2019, 10:57 am   #20
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Default Re: How to set the speed on cassette decks.

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No, that sensor in the CR240 (I assume, as there's something similar in the CR210 that I have) is to detect when the tape as stopped moving to cause the thing to auto-reverse. You wouldn't try to control the tape speed from the spool motion anyway, the speed of the spool rotation depends on how much tape is wound on it, tape speed is controlled by the capstan.

The CR210 motor is electronically controlled, the speed control is part of that.
Enlightened - thanks!
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