Sep 1, 2003 12:00 PM, By Eddie Ciletti

I often find myself with one foot in the past and one in the present. Well, almost. In the repair biz, even the “present” can be a year or so behind. The following article details two examples of old school going new school. Minnesota Public Radio (MPR) recently had me spanning several decades with two projects: an EMT Plate Reverb restoration and a DAT-archiving class. MPR's downtown St. Paul, Minn., broadcasting facility includes the orchestra-size Studio M, which is home to three plate reverbs, one of which is a stereo EMT-140. Back in the day, it was “the standard” for spatial enhancement.

The MPR project started about a year ago with a request to “fix” the EMT's stereo balance. After the Model 162's electronics assembly and full documentation appeared on my doorstep, I thumbed through the docs and was especially amused by the shipping instructions. Gotham Audio, the distributor at the time, sent a manual in advance of the EMT plate arrival so that users could safely transport the unit from the street into their facility. (The trucking firm was only obliged to deliver to the door.) While the plate itself is 200 cm by 100 cm (6.5 feet by 3.2 feet), the shipping package measured 8 feet by 4.5 feet and weighed 400 pounds! It currently lives in Studio M's basement suspended by garage springs!

Anyway, back to the EMT electronics “package”: It consists of a mono-drive amplifier (with optical limiter), a stereo FET preamp and a power supply. Like a tape machine, there is equalization on both the drive and pickup amps. With balance issues in mind, both preamps were scrutinized for any signs of “service.” Many of the coupling capacitors had already been upgraded to Panasonic's HFC Series (now discontinued since the FC Series was introduced). All remaining balance-related components were replaced, including the remaining caps, plus critical resistors in the front end were upgraded to 1% low-noise metal film.

KIT SWAP

Coincidentally, a call came in for one of my LA-4 op amp upgrade kits from none other than James Cunningham of Echoplate fame. (James returned to his native Chicago after an L.A. stint working with Bill Putnam, prior to UREI being absorbed into the Harman organization.) Turns out, James is still in the “plate” biz (as JCC Associates, 847/831-5628), selling upgrade kits that include new pickups and tuning/tension clips. At that point, James and I simply traded our kits!

The preamp section was rather noisy, with plenty of hum and hiss; the former was exaggerated by the bass boost in the playback EQ curve, while the latter was more obvious on one channel. Hum is a non-issue because the JCC pickups have much higher output and better bass response; hence, the gain and low-EQ boost can be reduced. After removing the noisy FET, I installed a socket, anticipating that several FETs would be auditioned. As luck would have it, my one in-house spare made the “bad” channel quieter than the good channel. After installing the mods, I added trim pots so that the EQ could be dialed out on both the drive and pickup amps. I also noticed that the optical limiter was missing. The original lightbulb-driven photo-resistor network was upgraded to a faster LED-based device by Vactrol.

AH, BALANCE!

At this point, all of the electronics work was done and, even before the new pickups were installed, the channels were more closely balanced. The old pickups were unsoldered and the new pickups glued to the plate, a process that went just fine. I did have just a wee bit of trepidation about “tuning”: Throughout the '70s and '80s, the tuning style evolved into a frightful over-tightening of screws until the support clips snapped, all to maximize brightness. People were using the brighter plate to “help” the snare sound, an instrument that is not naturally bright when closely miked, especially when low-tuned as was the '70s style. It is totally unnecessary now, even though James' clips are unbreakable.

The JCC kit includes a tension/tuning gauge that eliminates the guesswork, simplifies the process and is remarkably accurate. Tuning not only improved the channel balance, but Studio M's “objective ear and studio tech” Scott Liebers was able to discern a mere third-step difference between channels. Routing an electronic metronome impulse to the plate seemed the best way to judge the spectral response and decay time. At this point, making smaller-than-quarter-turn tension tweaks brought the channels into near-perfect balance. Studio M's engineer, Tom Mudge, who handles many of the recording projects, was about as relieved as I was that the process was a pleasurable experience.

For the final test, an acoustic piano track was auditioned through the plate without a dry signal. I think everyone was amazed. Not only did it sound real — like standing just outside the doorway of a nice hall — it was oh so quiet and clean. I am not one to pit “digital vs. analog,” but there was truly some magic in the EMT-140 that made just about anything else pale by comparison.

TASK NUMBER 2: DAT ARCHIVING

MPR is also in the process of archiving its vast library of radio broadcasts (such as “Prairie Home Companion”) from ¼-inch analog and DAT. While the studios of radio station KNOW are well-equipped with Sony PCM-2700 and PCM-2800 DAT decks, additional machines were required to handle the volume of transfers. I specified the Fostex D-5 and the Sony PCM-R500 for the job; adding external connections to both machines allowed convenient oscilloscope evaluation of the signal from tape. [Note: The Fostex D-5 and Tascam DA-20 are essentially the same machine: Both were made by Pioneer and are now out of production. It may still be possible to purchase remaining inventory.]

Of the two modified machines, the Sony PCM-R500 would be used to evaluate problem tapes, while the Fostex D-5 was initially chosen for ease of alignment (after a minor modification). The D-5 has no mute circuit, making it far more tolerant of out-of-spec tapes than the R500. (It mutes if the Error Rate is too high.) The Fostex features a front-panel Error Rate button, while the Sony requires modifying the remote control for such “privileged info.” (See Fig. 2.)

Once the modified machines were back on-site, I then taught a Tape Path class at MPR to explain how to interpret the oscilloscope patterns — what the signal should and might look like — and how to tweak the alignment, if necessary. Figure 1 details the block diagram. Monitoring troublesome tapes via 'scope may, in some cases, reveal the cause. Assuming the problem was path-related, hopefully a little mechanical tweak will be all that's necessary to complete the recovery process.

TAPE PATH 101

The essence of DAT travel is detailed in Fig. 1. Two guides, called post rollers, extract the tape from the shell and wrap it around the rotating head drum. Although it may not seem possible at first glance, these wobbly guides are spring-loaded and become precisely oriented once they are fully extended and under pressure. The two heads on the drum — A and B — are located 180° from each other. This pair of radio-frequency (RF) signals becomes multiplexed via an A/B switch to a single test point that is labeled RF ENVelope.

The drum generates two other mechanically related signals as it rotates: a Tachometer for speed and a Pulse Generator (PG) for positioning, the latter often labeled SWP and used to precisely control the timing of the A/B switch and trigger the oscilloscope. Note that the square wave's transition — from low to high and high to low — occurs at the beginning of each “slice” of information from the A and B heads.

Tape path for DAT and other helical-scan tape recorders is roughly the equivalent of head height (not azimuth) for analog recorders. Because the head drum (or upper cylinder) is spinning at an angle, the ingoing and outgoing guide heights are very critical. When all is right, the “envelope” is square, while an out-of-spec tape will have a “fish” shape. Of course, there are other issues, such as when dirt clogs either or both heads.

A CLEAR HEAD

I have rescued several tapes during the past year where only one of the heads was actually recording. This lack of redundancy requires that every aspect of the playback machine be perfect. One of the KNOW tapes did not play on any of the Sony machines, yet its high Error Rate did not reflect a poor tape path (although the RF envelope changed as the machine struggled to automatically self-track). Most likely, the combined problem was dirty heads on the recorder attempting to record over previous material. Under normal conditions, the Error Rate should only be slightly worse than recording on blank stock. It should still be possible to achieve “all zeros,” as shown in the lower half of Fig. 2.

While many machines can report the Error Rate, some permit an adjustable Error Rate sampling window. For example, most machines report on the entire RF envelope, while others, such as the Fostex PD-2, can selectively evaluate the subcode area (timecode and start IDs), audio data, etc. The downside of using the “widest” window is that previously re-corded information may pop up between the “tracks” (head swipes) to make the Error Rate seem worse than it is, and intermittently at that.

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