Analog Tape 101
PART TWO, Recorder Debugging and Alignment

by Eddie Ciletti

 

In Part One of this series, I made some assumptions: Machines are older while users and technicians are “newer,” with less analog tape experience than Mix readers probably had ten years ago (or more). Next month, in Part Three, I’ll examine recorder bias techniques and present more information on narrow-format machines; but for now, let’s focus on alignment, troubleshooting and mechanical issues.

TASTE TEST
One key tool in optimizing tape recorder performance is a calibrated test tape, such as those made by MRL (http://home.flash.net/~mrltapes) or BASF/Emtec (www.emtec-magnetics.com). Test tapes are expensive and should be treated with care, so don’t play chicken with an unknown machine. They are also recorded full-track and will not reveal tape path anomalies such as up and down vertical movement. Though it may seem redundant, I perform a “record test” and adjust bias before a playback alignment to help weed out electronic and mechanical problems. To isolate channel-specific problems, swap cards with the machine off, but only after labeling them. Note that swapping cards might temporarily resolve a problem, so return cards to their original slot apres “test” to confirm.

A selected list of service resources, including a demagnetizer, was detailed last month in Part One of this article. Test equipment options were provided in the May issue in this column. You have an oscillator now, right? Route a 1kHz sine wave from the mixer to all tracks with the machine set to Input. If the mixer meters seem consistent, it sometimes helps to throw the subgroup faders all the way up (if applicable) to minimize channel-to-channel inconsistencies.

Meters are often less accurate than the buses they represent, so an AC voltmeter such as the Fluke 8060A is helpful in confirming levels and calibrating meters. You can also use a patch cord to route one bus to each individual track, one at a time. Steady state (with tones), mixer and tape meters should agree, although all bets are off with Program Material when integrating bar graph with mechanical VU meters.

CHECK LIST
1. If you don’t have an operator/service manual, get one fast! In researching this article, I was continually impressed with the amount of useful, available technical information, from circuit theory to machine-specific procedures. (Some machines have very specific test tape recommendations.) Service manuals of olde are a far cry from current practice.

2. Always check the heads for oxide buildup.

3. Before rolling tape, sweep the oscillator through the extremes—from 20 to 20k Hz—just to rule out bad capacitors (in the mixer or tape machine) that can cause premature bass roll-off.

4. Quite often, a misplaced tweaker will have accidentally adjusted the wrong channel or a speed-specific parameter, making some channels look quite odd. You may want to bring the “stragglers” closer to the herd, but novices shouldn’t do any major tweaks yet.

5. Input Level calibration is perhaps the most inconsistently implemented adjustment throughout the magnetic ages. Please read the manual for the procedure.

6. When monitoring Input, Sync and/or Tape (repro), most narrow-format machines have only two heads, so they are always in Sync mode (playing from the same head used to record or automatically switching to Input when in Record). For those machines, the translation is “Playback After Recording, not during (PAR for the purpose of this article).

7. If noise reduction is part of the signal chain, set it to Bypass.

8. Don’t rule out the patchbay, cabling and especially aged connectors. I once saw a machine whose male XLRs were so black with oxidation that the connection behaved as a diode, turning AC into DC, a.k.a. “rectification.”

9. If you are going to tweak, have the right tool for the job.

10. When playing old tape stock, be aware that oxide can come “unglued” from the plastic, making a perfectly good machine seem possessed. If this has happened to you, stop! Look closely at the heads and guides with ample light. If you see funk, you can visit my Web site, www.tangible-technology.com, for more information on tape restoration.

 

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Reprinted with permission from Magazine, September, 2000
2000, Intertec Publishing, A Primedia Company All Rights Reserved