It Came In a Box With a Hole
May 1, 2006 7:50 PM, By Eddie Ciletti
AND OTHER SERVICE NIGHTMARES
Microphones
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Shop life might be predictable in the service department of an equipment manufacturer or major retailer. But in my reality, what may arrive at any given time can defy all expectations. Even with notes, there are times when gear I thought I knew tests my skills. Here's a blog of the highs, the lows and the midrange.
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Also seated at the bench are various U87s, 414s, an RE20 and vintage classics such as an SM69 (a stereo U67 outfitted with a pair of AC-701 vacuum tubes) and two U47s. The 87s most often have dirty or punctured capsules from all those unnecessary vocal close-ups. (Always use a pop filter.) If the mic dies on a plosive, it is because the capsule has become sensitive to humidity and should be inspected by a professional.
Of the mechanical problems, two of the 414s had cracked frames (nice on drums but not as drums). Both the SM69 and the RE20 had lost a press-fit pin. On the former, the pin limits the upper-head's rotation to less than 270 degrees; on the latter, it prevents the connector section from rotating. In both cases, pin failure allowed for damage to the internal wiring. It's a classic case of a seemingly innocuous (yet still missing) $0.50 part leading to major problems and a big-time repair bill.
During the burn-in process, the SM69's power supply also requested attention, eventually revealing the most typical ailment: hum from dried-out capacitors in the power supply circuit.
Then a pair of U47s warmed up the joint — at least, once they were working. There was a noisy Telefunken version (figure) that was more BLUE than Neumann; its VF-14 had been replaced by an EF-86 that its current owner changed several times to no avail, blaming the mic for “eating tubes.” Inside, a solid-state section — designed to protect the original VF-14 from rude awakenings — decided it wanted to “improvise.” Unlike a vintage mic, documentation was not available. I got lucky, but not before blaming the power supply.
The owner of the other U47 feared the worst — that the World War II — era VF-14 vacuum tube had bit the dust and the mental gymnastics that go into the process of deciding what to do. Functional U47s with a VF-14 are both rare and extremely valuable — the more original, the better — but the VF-14 is more than scarce, selling for about $1k or more, if you can find one.
Modification to any of the alternative tubes is similarly costly. Sadly, it can also kill the mic's value for those who own one as an investment. That's not my MO, but I do understand that a modification often needs to be reversible. A mic or any piece of vintage gear comprises a handful of “precious” component parts — in this case, the capsule, the tube, the output transformer and even the grille each contribute a good 25 percent of the magic. And, of course, the sum is greater than the parts.
So how could a U47 with a functional VF-14 still exist in this century? A major reason is that the tube's 50-volt filament runs at 35V. Operating at 70 percent of rated value obviously extends any tube's life and this should be taken into consideration if a tech is replacing it with the pin-compatible EF-14, which has a 6.3V filament (70 percent of which is 4.41V). Using this tube minimizes, but does not eliminate, alterations to the mic and the power supply.
For all of the above-mentioned gear (and then some), the key ingredients to a successful restoration are time, patience and money. Time is required not just to do the work, but to make sure that all of the problems stay solved. We run stuff for a few days, cycle off for a few more, power up and then play with it for a while. Patience means never having to say, “I shouldn't have rushed.” Sometimes, this means taking a whole front panel off just to be able to reinstall the meters without stressing the wiring and everything in the neighborhood.
And money — where would we be without that?
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