In this section we have compiled information on electrical adjustment methods and procedures that may help you to adjust and calibrate your tape recorder. We assume that you have a certain level of expertise and follow the service instructions of the respective manufacturer.
Required measuring devices and tools:
- AC voltmeter (frequency range if possible up to max. 20 kHz)
- Audio frequency generator (also as PC software)
- Frequency meter
- Head demagnetising coil
- Two-channel oscilloscope
- Cleaning swab and 91% alcohol
- Azimuth Alignment Tape, Speed and Flutter Test Tape, Calibration Level Tape, Multifrequency Calibration Tape
- Blank tape
Additional devices:
- Distortion Analyzer
- Spectrum Analyzer
- Pink & White Noise Generator
- Wow & Flutter Meter
Preparation
Make sure that the mechanics are working properly. Only if the tape is transported correctly is it possible to adjust the playback and recording. The recording and playback head, the pinch rollers and capstans, as well as all tape guides should be cleaned and demagnetised to prevent damage and erasure of the Test Tape.
The calibration steps should always be performed in the following sequence:
1. Playback adjustments
1.1. Tape speed
For the measurement and adjustment of tape speed we recommend the corresponding Speed and Flutter Test Tapes from our online store.
Note: The reproduced frequency depends to a certain degree on the tape tension or the contact pressure on the playback head. This means that even if the tape speed is correct, a frequency slightly different from the target value may be measured.
1.2. Playback amplifier level adjustment
To select the correct Calibration Level Tape for your tape recorder, you should check which magnetic flux (nWb/m) for your tape recorder has been defined as the reference value (0 dB) by the manufacturer. Please also refer to the service manual.
Let's assume that the manufacturer has defined a magnetic flux of 250 nWb/m as a reference value. In most cases, the service manual then states: "250 nWb/m = 0 dB". At 19 cm/s (7.5 ips) that would be Calibration Level Tape, ¼", 7.5 ips (19.05 cm/s), 1000 Hz, 250 nWb/m (DIN), 3 min., Item no. 76363 for level adjustment is the right choice. Set the playback amplifier to a VU meter reading of 0 - always measured under load. The value of the line resistance (to be connected in parallel to the output, or to the millivoltmeter) is given in the service manual. With some STUDER devices this is 600 Ohm, with other tape recorders, such as SONY this is 100 kOhm.
If your tape recorder offers the possibility to adjust the playback level for each tape speed separately, then repeat the procedure with the corresponding Calibration Level Tape.
Note: For example, if you want to set a tape recorder where 320 nWb/m is defined as 0 dB with a 250 nWb/m Test Tape, then the VU meter should read -2.14 dB. You could easily understand the situation here: Converter for audio technology.
A very practical Calibration Multi-Level Tape with all common ANSI and DIN levels can be found here: Calibration Multi-Level Tape, ¼", 7.5 ips (19.05 cm/s), 3 ANSI & 3 DIN Levels, 10 min., Item no. 76389.
1.3. VU meter calibration
The procedure is described in the respective service manual.
1.4. Playback head azimuth adjustment
MONO tape recorders
Switch the tape recorder to the highest tape speed. Start the corresponding Azimuth Alignment Tape. Connect the millivoltmeter to the output. Change the gap position of the playback head during playback until a maximum of the output voltage is reached, while at the same time the level fluctuations are at a minimum.
STEREO tape recorders
Connect the left channel of the tape recorder to the vertical deflection and the right channel to the horizontal deflection of the oscilloscope and switch on the X-Y mode. Start the appropriate Azimuth Alignment Tape. For 38 cm/s (15 ips) we recommend the Azimuth Alignment Tape, ¼″, 15 ips (38.1 cm/s), 1 kHz 10 kHz 16 kHz, 6 min., Item no. 76564.
The playback head should be adjusted so that a diagonal line inclined by 45° is created from bottom left to top right at all frequencies.
Note: Deviations in the azimuth during playback or recording/playback combination have an increasing effect as a phase error with decreasing wavelength (at higher frequencies) - the line becomes an ellipse, which becomes a circle at a phase difference of 90°. These lead to level losses, the amount of which can be determined very precisely here: Calculation of the alignment loss.
Depending on the type of reference band used, slight deviations may occur between the different speeds. In this case, the final azimuth adjustment should be carried out with the preferred tape speed .
For all azimuth settings, reels with the same and largest possible inner core should be used.
Below is the Lissajou image (oscilloscope in X-Y mode) when playing the Azimuth Alignment Tape, ¼″, 15 ips (38.1 cm/s), 1 kHz 10 kHz 16 kHz, 6 min., Item no. 76564.
Azimuth Alignment Tape, ¼", 15 ips (38,1 cm/s)
1.5. Playback frequency response alignment
For this purpose, please use the corresponding Multifrequency Calibration Tapes from our product range.
Fringing effect: When playing a full track Test Tape with track widths of 1 mm or 2 mm, as with ½-track or ¼-track tape recorders, there is an increase in the low frequencies due to lateral interference in the magnetic system of the playback head. This should be corrected with the help of the corresponding fringing values. The values are available on request. No correction values are required for the setting of full track tape recorders.
2. Record adjustments
2.1 Record amplifier level adjustment (pre-alignment)
The procedure is described in the respective service manual.
2.2. Checking oscillator frequency
Thread a blank tape and start the recording. Measure the oscillator frequency at the corresponding measuring point according to the service manual. If the frequency is outside the tolerance limit, the oscillator must be readjusted.
2.3 Record head azimuth adjustment
MONO tape recorders
Switch the tape recorder to the highest tape speed and connect the audio frequency generator to the input. Set the level to approx. 10 dB below the reference level and the frequency to 10 kHz.
Connect the millivoltmeter to the output. Put on a new and empty tape and start recording. Adjust the recording head to maximum output level.
STEREO tape recorders
Switch the tape recorder to the highest tape speed and connect the audio frequency generator in parallel to the two inputs. Set the level at approx. 10 dB below the reference level and the frequency to 10 kHz.
Thread a blank tape and start the recording.
Connect the left channel of the tape recorder to the vertical deflection and the right channel to the horizontal deflection of the oscilloscope and equalize the left and right signal levels. Switch on the X-Y mode. Adjust the recording head until you can observe a diagonal line tilted 45° to the rrght.
Repeat the setting at 16 kHz.
Important note: The physical and the "electrical" head gap of the recording head are not in the same position; the offset depends on the strength of the bias current. For this reason, the azimuth correction must be repeated after the final adjustment of the premagnetization.
2.4. Bias
The premagnetisation setting is a compromise between the following parameters: distortion level, frequency response, modulation noise and maximum modulation level.
With this setting, the bias current is increased above the sensitivity maximum of the corresponding tape until the output voltage has exceeded its maximum and dropped by a certain value.
Feed a signal (10 kHz, 20 dB below the reference level) into the input. Insert an blank tape and start recording.
First set the bias control all the way to the left and slowly turn it clockwise from the left stop until the output signal has reached its maximum during recording. Note the value and turn the adjustment knob further clockwise (the bias current increases) until the output voltage has decreased by the value recommended by the tape manufacturer. This can be found in the technical data of the tape.
Important note: Set bias parameters to approximately the same values for all channels.
2.5. Checking and readjusting the record level setting
Please follow the procedure in the relevant service instructions.
If a significant change in the head gap angle was required, the recording level adjustment must be repeated!
2.6. Record frequency response alignment
Please follow the procedure in the relevant service manual.
Note: On many home tape recorders and most cassette decks, the treble level setting in the recording amplifier is preset by the manufacturer. It is not possible to change or fine-tune the treble using a trimmer potentiometer. In this case, the frequency response is adjusted with the help of the pre-magnetisation (bias). Most instructions recommend setting a low tone (usually 400 Hz) and a high tone (usually 10 kHz) to the same level during recording - always at -20 dB. This "smoothes" the frequency response as much as possible - even if the distortion factor does not always reach its optimum.
Further tests and settings
If you have a distortion analyzer, then you could set the distortion to minimum. And with the help of a wow & flutter meter you can set the servo system to minimum wow & flutter.
For this purpose we recommend the Wow & Flutter Meter wfgui Version 3.0 software in conjunction with our Wow and Flutter Test Tape, ¼", 15 ips (38.1 cm/s), 40 Hz & 3150 Hz at -40 dB / 0 dB, 3 min., Item no. 76406.
Wow & Flutter Test Tape, ¼", 15 ips (38,1 cm/s)
General notes
Please store your Test Tapes in a cool and dry place - preferably at 19° C to 23° C and 50% relative humidity and away from strong magnetic fields. Please note that the Test Tapes wear out with frequent use and lose accuracy over time.
For your protection, all videos were recorded without audio track.
