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Machine and Tapes are linked and need to be aligned. This ensures the best playback and recording performances so you can enjoy the best quality of your recorder.

 

For this purpose, you need to choose an operating level. You can use an operating level that is different from the reference level of the calibration tape, and according to your machine, use the cross-reference chart to get the best configuration for your studio.

HOW TO CALIBRATE YOUR MACHINE?

Required tools:

  • Alignment tape
  • Some blank tape
  • 91% alcohol
  • Cleaning swabs
  • AC voltmeter
  • Tone generator
  • Head demagnetizer
  • A two-channel oscilloscope

Additional tools:

  • Distortion analyzer
  • Spectrum analyzer
  • Pink & white noise generator
  • Wow & flutter meter
1. Preparation

 

  • The first step is to clean and demagnetize the heads and guides of the tape machine. Care should be used when demagnetizing the heads, do not turn on or off the demagnetizer within 3 feet (1 meter) of the tape machine. The sudden pulse of the demagnetizer energizing or powering down can leave the parts magnetized, defeating the purpose of the operation.

 

  • The next step is to mount some blank tape and observe the tape path to insure that all guides and heads are in the same plane. Look for any creasing or fluttering of the tape through the guides. The tape should appear to flat and smooth through the entire tape path. Observe that the tape is winding through the center of the flanges. If the tape is winding closer to the upper flange then the reel turntable is too low, if winding closer to the lower flange the reel turntable is too high. Adjust the turntables to center the winding.
2. What type of alignment tape should be used?

 

  • You need to have one tape for each speed the machine is capable or running. Most pro machines are 15 ips & 30 ips, semi-pro are usually 7.5 ips & 15 ips and consumer machines are usually 3.75 ips & 7.5 ips. All alignment tapes are full track so they can be used on machines with various tracks (eg. full track, two-track or quarter track [this applies to 1⁄4” machines]).

 

  • You also need to decide on what operating level you will use. This depends on the blank tape you will align to. There is very good information at the MRL tape web site (www.mrltapes.com) to help decide this. For our products, SM900 is considered the highest output tape at a +9 dB, SM911 is +6 dB & SM468 is +3 dB all over a flux level of 185 nWb/m squared (185 nanowebers per meter squared). 185 nWb/m is the reference level used by Ampex back in the 1940’s and is kind of the base level that everything today is measured against.
3. Playback performance

 

  • Mount the alignment tape on the machine to check the playback performance. Use the AC voltmeter and oscilloscope to setup the playback alignment. The oscilloscope should have one channel connected to the vertical channel and the other channel connected to the horizontal channel. When observing the tones from the alignment tape you should see a line at a 45 degree angle for all frequencies leaning to the right. If it leans to the left the channels are 180 degrees out of phase. This tells you that the azimuth is correctly set for the playback head. If you observe a circle or ellipse then the play head is out of alignment and needs to be adjusted. You would adjust the head on the lower frequencies and keep adjusting on the higher frequencies until the head shows a 45 degree line at all of the frequencies.

 

  • The next adjustment is playback level. Using the 1 kHz or 500 Hz tone, set the standard playback level to whatever reference level you have chosen. Keep in mind if you are using an alignment tape at 7.5 ips or slower, you should make all adjustments a -10 dB to avoid saturation of the tape at high frequencies. At 15 ips or higher you can use standard reference level.
    After play level is set, then play the rest of the alignment tape and set the high frequency (HF) or low frequency (LF) to have the flattest response. When each tone is played you should write down the level displayed on the voltmeter for each channel. You will need these numbers when setting the record response.

 

  • When this part is finished, store away the alignment tape in the tails out position for best storage.

Keep in mind that over time the alignment reference tape do wear out. MRL has a document on how to find out if the tape worn out. Also note that the alignment tapes are not inexpensive.

4. Record performance

 

  • Load up the blank tape on the machine for the record alignment. Hook up the tone generator to the recorder inputs and select 1 kHz or 500 Hz for your input. Put the recorder in record mode and start the machine. The machine should be listening to the playback heads while recording. Set the record level to zero on the meters. Observe the oscilloscope for the 45 degree line. Again if there is a circle or ellipse then the record head azimuth needs to be adjusted. Switch to 10 kHz and look again, fine tune the azimuth for a flat line.

 

  • While the tone generator is set to 10 kHz, you need to adjust bias. Refer to the bias chart on our web site for the proper overbias needed for your tape machine and type of tape.
    We will use SM900, 15 ips and a Studer machine as an example :
    The Studer machine has a .25 mil record head, so for SM900 at 15 ips the overbias is 4.0 dB. You would lower the bias control on the deck until the meter reads the peak, adjust the level up until the meter drops 4 dB. This will be the best bias point. Repeat on each channel of the machine. Once the bias is set, then set the tone generator for 1 kHz and reset the record level. Go back to 10 kHz and set the record equalizer to match the playback numbers you wrote down when doing the play alignment. Check the machine at each frequency you noted during playback and try for the falsest settings, you may need to adjust 10 kHz up or down for the flatest response.

 

5. Additional information

 

    This would be the end for a simple alignment. If you have a distortion analyzer, you could fine tune the bias settings for minimum distortion. If you have a wow & flutter meter, you can adjust the servo system for least wow & flutter.

WATCH A FREE PREVIEW TUTORIAL FOR MACHINE CALIBRATION

Learn how these machines work with step-by-step alignment instructions given by Bradshaw Leigh.

Get a free preview of the online Udemy course:

Analog Audio Tape Recorder Basic Theory and Alignment