VIRB

The advent of digital recording led to a surplus of high-end professional reel to reel tape recorders that can bought for 3% or 4% of their original price. The difference between this class of tape recorder and digital recorders is not so much about quality as it is about maintenance. Calibrations, alignments, replacing worn out parts and obsessive cleaning are all required to keep these units performing well.

After purchasing a reel to reel, I was able to find a user manual but not a maintenance manual. In searching the intertubes, I found no documention specific to my deck. So the calibration notes published below are culled from several sources to be used as a guide to calibrate an Otari MX-5050 reel to reel. More detail can be found on the Magnetic Reference Laboratory website at http://home.flash.net/~mrltapes/

Calibration Tapes

To have a tape deck perform to the original specifications, it must calibrated to the same original reference standards. In practice, a reference tape can used to calibrate the playback section of a tape deck. The calibrated playback section can then be used to calibrate the recording section of the tape deck. Tapes are available from Magnetic Reference Laboratory via their website, from http://www.usrecordingmedia.com or from www.jrfmagnetics.com. Blank tapes can also be purchased from these companies.

The tape used in this case was a 1/4" MRL Calibration Tape that has three tones 1 kHz, 10 kHz, and 100 Hz each recorded for 55 seconds one time for each speed. NAB equalization at 15 IPS and AES equalization at 30 IPS. 355 nWb/m operating level. This is as basic a calibration tape that you will get but covers the most common adjustments you need, operating level and tape head azimuth.

Access the Calibration Controls

For the Otari MX-5050 BQIII, the calibration adjustments are accessed via the front panel.

Attached to the rear of the deck is a stand that needs to be put into place, upon which the recorded can be rested face-up. Remove the the 4 screws located on the corners of the bottom panel and remove the bottom panel. Do not remove the screws that attach the two feet to the panel.

The internal audio boards to be adjusted are contained in a drawer located in the bottom of the recorder. The drawer is hinged in the bottom rear. Remove the 4 screws attached to the lowest front panel. This panel contains the VU meters and audio adjustment knobs. Rotate the drawer forward to expose the calibration controls.

Load the MRL Calibration Tape

The MRL tape is shipped 'tail-out' so it needs to loaded on the right spindle and rewound onto a blank reel on the left.

Playback Adjustment with 1 kHz tone

The first step in playback alignment is to adjust 'Repro Level'. This is done by playing a 1 kHz tone from a MRL tape and adjusting accordingly. Adjust for the proper flux level for the blank tape you will use in recording.

Using a MRL tape with levels of 355 nWb/m and adjusting the recorder to use EMTEC SM911 tape with recommend levels of 320 nWb/m, means the MRL tape is 2db hotter than the blank tape. The Repro Level should be offset to 2db on the VU meter using the MRL tape to ensure that a reading of 0db equates to a 320 nWb/m.

Adjustments for other blank tape levels using a 355 nWb/m MRL Tape are here:

Level        Offset      
185          +6 dB      
200          +5 dB      
250          +3 dB      
320          +2 dB      
355           0 dB     
500           -3 dB

If going use a prerecorded tape, they often have a set of tones at the beginning of the reel to use to calibrate one deck to another. These are typically recorded at 0VU on the source deck and should be adjusted to be 0db tones on the target recorder.

Playback Adjustment with 10 kHz tone

The second step in playback alignment is to adjust 'HF Repro EQ'. This is done by playing a 10kHz tone from an MRL tape and adjusting the VU meters to the same offset as used in the 1khz tone.

On the MX 5050 BQIII, there are two pots for the high frequency adjustment labeled H and L. The labels relate to the tape speed settings of High (15 in/sec) or Low (7.5 in/sec).

Playback Adjustment with 100 Hz tone

The third step in playback alignment is to adjust 'LF Repro EQ'. This is done by playing a 100 Hz tone from an MRL tape and adjusting the VU meters to the same offset as used in the 1khz tone.

Record Adjustment with Biasing

Bias is performed by recording a 10 kHz tone on the target blank tape and monitoring the repro head. For the MX 5050 BQIII, a 1 kHz and 10 kHz tone generator is built into the recorder. Select 10 kHz, arm the tracks are start recording while monitoring the repro head.

Adjust the bias control to the minimum then increase it to find the peak reading on the VU meters. Adjust the input gain of the tone to move this peak reading into the range of 2 or 3 VU. Increase the bias further until the value drops to the Over-bias value specific to the recording tape. In the case of the EMTEC SM911 tape, the over-bias value is -3db at 15 in/sec.

Record Adjustment with1 kHz tone

Since the playback head has been calibrated via the MRL calibration tape, it can be used to calibrate the recording and sync heads.
Input a 1 kHz tone from the generator while monitoring the input and adjust the input levels to 0db on the VU meter. Now switch the monitor to the playback heads, arm the tracks and start recording. Adjust the Record levels to 0db on the VU meters.

While recording, switch the monitor to sync and adjust Sync Levels to 0db on the VU meter.

Record Adjustment with 10 kHz tone

Input a 10 kHz tone from the generator while monitoring the input and adjust the input levels to 0db on the VU meter. Now switch the monitor to the playback heads, arm the tracks and start recording. Adjust the HF Record EQ levels to 0db on the VU meters.
On the MX 5050 BQIII, there are two pots for the high frequency adjustment labeled Hand L. The labels relate to the tape speed settings of High (15 in/sec) or Low (7.5 in/sec).

Frequency Response for a similar Otari deck, the half-inch, 8 track MX5050-MKIII. This was calibrated at 15 in/sec. Graph from www.endino.com/graphs.