The EM Poll

Joining the Reel World

Mar 1, 2007 12:00 PM, By Eddie Ciletti

           

Quick Record Confirmation

All machines have midband, 1 kHz playback and record level adjustments. Typically, narrow-format machines offer a minimal adjustment range for high-frequency record EQ. Do not use bias to manipulate the record EQ because its purpose is to minimize distortion.

While the tape is stopped, set the machine to monitor input and apply a 1 kHz sine wave, either directly from an oscillator or through a mixer, preferably to all channels at once. (Be sure to disable any built-in or external noise reduction.) At some point, the oscillator's level must be precisely known and set, but don't strive for perfection yet. For the moment, make adjustments until the meters read roughly 0 VU.

Assuming that the deck has three heads, simultaneous playback during record is possible. Most narrow-format multitracks have only erase and record heads, and so monitoring from tape is not possible without first recording and then rewinding. This makes any part of the record alignment procedure a bit tedious and very time-consuming.

Put the machine into record and toggle between input and reproduction (playback mode) to confirm that all switches and relays are reliable. If the meters are not steady, the problem may be electronic (dirty switches and relays) or mechanical (funky tape, poor tape path). Continuing to toggle between modes is an exercise that may self-clean the switch or relay contacts in the short term. The alternative is to manually clean the contacts, if possible, or replace the parts.

Note that mechanical VU meters may also be part of the problem, especially if the previous owner loved to pin them in the red. There is a more transparent way to get additional saturation without slamming the meters: it's called elevated level, and it's achieved by calibrating with the playback tape to something less than 0 VU, then increasing the internal record level adjustment to compensate.

Switch the oscillator to 40 Hz while monitoring on headphones. A 40 Hz tone is great for finding funky pots, switches, and relays, as well as “tape rocks,” the latter being a sign of damaged or used tape, magnetized heads, or bias-oscillator distortion.

FIG. 4: Behind the polished head face is what determines head life. Performance and the amount of overbias for recording are determined by gap depth.

One other potential noise source is the scrape flutter filter (see Fig. 2), a roller located between the record and playback heads designed to support the tape so it behaves less like a resonating guitar string. You can easily test it by applying a little bit of finger pressure. If the noise stops, the roller should be removed, disassembled, and lubricated with analog watch oil.

Record 100 Hz, 1 kHz, and 10 kHz tones, flipping between input and repro (playback during record). The tones should be steady, even if not perfectly aligned at 0 VU. If they aren't, check the tape for up-and-down wandering or curling in the guides, then apply a little drag to the supply reel with a finger. If that helps, try to localize the problem by gently applying finger pressure to the tape on either side of the record head and then the playback head. If the signal level increases or becomes stabler, there may be tension or mechanical alignment issues. The heads may also be worn, in which case they can often be relapped.

Necessary Bias

Bias is a high-frequency signal that is applied to the erase head and mixed in with the audio signal on its way to the record head. Without bias, only the audio signal's peaks would magnetize and be captured by the magnetic tape particles, resulting in a very distorted recording. With bias, the particles are magnetized and oriented to capture the full dynamic range of the source material.

Bias optimizes the tape's sensitivity and minimizes distortion at mid frequencies and below. The optimum bias setting for minimum distortion reduces high-frequency sensitivity by a few dB, hence the term overbias. The recommended amount of overbias and the high frequency that is used for this adjustment is machine and speed specific and correlates to the size of the record head gap depth (see Fig. 4).

Bias is adjusted with a potentiometer, a variable capacitor, or digital controls. The adjustment location will depend on the make and model of the deck. The earliest tape machines had separate cards for repro/sync, record/input, and bias/erase adjustments. Later machines put all of the adjustments on one card. Access to the adjustments is through a pair of doors on pro machines and through removable panels on semipro machines. Consult the manual for more details.

Set the machine to input and apply a 10 kHz sine wave for 15 ips (or 20 kHz for 30 ips) so that the input level is 3 dB below 0 VU. Enter record and monitor repro while adjusting the bias control until the signal reaches the maximum level. If the meter pegs, then lower the audio oscillator's output level.

Once you find the maximum, further increase the bias by turning the bias level control clockwise until the signal is reduced by the specified amount as indicated in the manual. For example, if the recommended overbias at 15 ips is 3 dB, then continue to increase the bias past the peak until the 10 kHz level drops by 3 dB. If recording at 30 ips, you may use 20 kHz and the same amount of overbias, or use 10 kHz and overbias by half (1.5 dB). Consult a professional if the information on adjusting the bias on your machine is missing from the manual or out-of-date relative to current tape availability.

Watch Your Azimuth

Once the recorder is proven safe and functional, it is time to address the routine aspects of tape machine maintenance. The azimuth is perfect when the record and playback heads are perpendicular to the tape and parallel to each other. That's hard to see with the naked eye, but there are several ways to check it.

Send pink noise from your workstation's multifunction generator to all of your tape machine's channels. Put the machine into record and monitor playback while listening to all channels summed to mono. The noise should sound bright and clear with no swishing. However, if it sounds swishy, then the heads are not precisely lined up. By applying a bit of thumb pressure to the tape, on either side of the heads, you can nondestructively manipulate the azimuth enough to exaggerate this effect. (I will discuss adjusting the azimuth in a moment.)

Keep in mind that we have not yet calibrated the playback, but have been testing other aspects of the machine's performance in order to determine where the main problems are. If you've gotten this far without issues, then the machine is safe and ready for the alignment tape. You should now remove all tapes from the area, power down the machine, and demagnetize it.