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.
1. Tape speed adjustment
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.
2. 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: Minor deviations in azimuth during playback or record/playback combination increasingly act as phase errors with decreasing wavelength (at higher frequencies) - the line becomes an ellipse, which becomes a circle at 90° phase difference.
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)
3. VU Meter calibration
The procedure is described in the respective service manual.
4. Playback amplifier level adjustmen
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: Audio converter.
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.
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.
5. Playback frequency response adjustment
For this purpose, please use the corresponding Multifrequency Calibration Tapes from our product range.
6. Checking the 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.
7. 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.
For all azimuth settings, reels with the same and largest possible inner core should be used.
8. 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 potentiometer completely to the left and slowly turn it clockwise from the left stop until the output signal has increased to its maximum during the recording. Note the value and continue to turn the BIAS potentiometer clockwise (the BIAS current increases) until the output voltage has decreased by the value recommended by the tape manufacturer. This value can be found in the technical data of the tape.
9. Recording level and frequency response adjustment
Please follow the procedure in the relevant service manual.
10. Further test 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.
