I often find myself with one foot in the past and one in thepresent. Well, almost. In the repair biz, even the“present” can be a year or so behind. The following articledetails two examples of old school going new school. Minnesota PublicRadio (MPR) recently had me spanning several decades with two projects:an EMT Plate Reverb restoration and a DAT-archiving class. MPR'sdowntown St. Paul, Minn., broadcasting facility includes theorchestra-size Studio M, which is home to three plate reverbs, one ofwhich is a stereo EMT-140. Back in the day, it was “thestandard” 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'selectronics assembly and full documentation appeared on my doorstep, Ithumbed through the docs and was especially amused by the shippinginstructions. Gotham Audio, the distributor at the time, sent a manualin advance of the EMT plate arrival so that users could safelytransport the unit from the street into their facility. (The truckingfirm was only obliged to deliver to the door.) While the plate itselfis 200 cm by 100 cm (6.5 feet by 3.2 feet), the shipping packagemeasured 8 feet by 4.5 feet and weighed 400 pounds! It currently livesin Studio M's basement suspended by garage springs!
Anyway, back to the EMT electronics “package”: Itconsists of a mono-drive amplifier (with optical limiter), a stereo FETpreamp and a power supply. Like a tape machine, there is equalizationon both the drive and pickup amps. With balance issues in mind, bothpreamps were scrutinized for any signs of “service.” Manyof the coupling capacitors had already been upgraded to Panasonic's HFCSeries (now discontinued since the FC Series was introduced). Allremaining balance-related components were replaced, including theremaining caps, plus critical resistors in the front end were upgradedto 1% low-noise metal film.
Coincidentally, a call came in for one of my LA-4 op amp upgradekits from none other than James Cunningham of Echoplate fame. (Jamesreturned to his native Chicago after an L.A. stint working with BillPutnam, prior to UREI being absorbed into the Harman organization.)Turns out, James is still in the “plate” biz (as JCCAssociates, 847/831-5628), selling upgrade kits that include newpickups and tuning/tension clips. At that point, James and I simplytraded 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-issuebecause the JCC pickups have much higher output and better bassresponse; hence, the gain and low-EQ boost can be reduced. Afterremoving the noisy FET, I installed a socket, anticipating that severalFETs would be auditioned. As luck would have it, my one in-house sparemade the “bad” channel quieter than the good channel. Afterinstalling the mods, I added trim pots so that the EQ could be dialedout on both the drive and pickup amps. I also noticed that the opticallimiter was missing. The original lightbulb-driven photo-resistornetwork was upgraded to a faster LED-based device by Vactrol.
At this point, all of the electronics work was done and, even beforethe new pickups were installed, the channels were more closelybalanced. The old pickups were unsoldered and the new pickups glued tothe plate, a process that went just fine. I did have just a wee bit oftrepidation about “tuning”: Throughout the '70s and '80s,the tuning style evolved into a frightful over-tightening of screwsuntil the support clips snapped, all to maximize brightness. Peoplewere using the brighter plate to “help” the snare sound, aninstrument that is not naturally bright when closely miked, especiallywhen 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 theguesswork, simplifies the process and is remarkably accurate. Tuningnot only improved the channel balance, but Studio M's “objectiveear and studio tech” Scott Liebers was able to discern a merethird-step difference between channels. Routing an electronic metronomeimpulse to the plate seemed the best way to judge the spectral responseand decay time. At this point, making smaller-than-quarter-turn tensiontweaks brought the channels into near-perfect balance. Studio M'sengineer, Tom Mudge, who handles many of the recording projects, wasabout as relieved as I was that the process was a pleasurableexperience.
For the final test, an acoustic piano track was auditioned throughthe plate without a dry signal. I think everyone was amazed. Not onlydid it sound real — like standing just outside the doorway of anice hall — it was oh so quiet and clean. I am not one to pit“digital vs. analog,” but there was truly some magic in theEMT-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 radiobroadcasts (such as “Prairie Home Companion”) from¼-inch analog and DAT. While the studios of radio station KNOW arewell-equipped with Sony PCM-2700 and PCM-2800 DAT decks, additionalmachines were required to handle the volume of transfers. I specifiedthe Fostex D-5 and the Sony PCM-R500 for the job; adding externalconnections to both machines allowed convenient oscilloscope evaluationof the signal from tape. [Note: The Fostex D-5 and Tascam DA-20 areessentially the same machine: Both were made by Pioneer and are now outof production. It may still be possible to purchase remaininginventory.]
Of the two modified machines, the Sony PCM-R500 would be used toevaluate problem tapes, while the Fostex D-5 was initially chosen forease of alignment (after a minor modification). The D-5 has no mutecircuit, making it far more tolerant of out-of-spec tapes than theR500. (It mutes if the Error Rate is too high.) The Fostex features afront-panel Error Rate button, while the Sony requires modifying theremote control for such “privileged info.” (See Fig.2.)
Once the modified machines were back on-site, I then taught a TapePath class at MPR to explain how to interpret the oscilloscope patterns— what the signal should and might look like — and how totweak 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 alittle mechanical tweak will be all that's necessary to complete therecovery process.
TAPE PATH 101
The essence of DAT travel is detailed in Fig. 1. Two guides, calledpost rollers, extract the tape from the shell and wrap it around therotating head drum. Although it may not seem possible at first glance,these wobbly guides are spring-loaded and become precisely orientedonce they are fully extended and under pressure. The two heads on thedrum — A and B — are located 180° from each other.This pair of radio-frequency (RF) signals becomes multiplexed via anA/B switch to a single test point that is labeled RF ENVelope.
The drum generates two other mechanically related signals as itrotates: a Tachometer for speed and a Pulse Generator (PG) forpositioning, the latter often labeled SWP and used to precisely controlthe timing of the A/B switch and trigger the oscilloscope. Note thatthe square wave's transition — from low to high and high to low— occurs at the beginning of each “slice” ofinformation from the A and B heads.
Tape path for DAT and other helical-scan tape recorders is roughlythe equivalent of head height (not azimuth) for analog recorders.Because the head drum (or upper cylinder) is spinning at an angle, theingoing and outgoing guide heights are very critical. When all isright, the “envelope” is square, while an out-of-spec tapewill have a “fish” shape. Of course, there are otherissues, such as when dirt clogs either or both heads.
A CLEAR HEAD
I have rescued several tapes during the past year where only one ofthe heads was actually recording. This lack of redundancy requires thatevery aspect of the playback machine be perfect. One of the KNOW tapesdid not play on any of the Sony machines, yet its high Error Rate didnot reflect a poor tape path (although the RF envelope changed as themachine struggled to automatically self-track). Most likely, thecombined problem was dirty heads on the recorder attempting torecord over previous material. Under normal conditions, the Error Rateshould only be slightly worse than recording on blank stock. It shouldstill be possible to achieve “all zeros,” as shown in thelower half of Fig. 2.
While many machines can report the Error Rate, some permit anadjustable Error Rate sampling window. For example, most machinesreport on the entire RF envelope, while others, such as the FostexPD-2, can selectively evaluate the subcode area (timecode and startIDs), 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 worsethan it is, and intermittently at that.
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