Tech's Files: Restoring Vintage Gear, The Quest Continues

Dec 8, 2010 7:04 PM, By Eddie Ciletti

TUBE MIXERS, TUBE MICS, DATS, A CLASSIC VCA PROCESSOR

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Electrolytic capacitors tend to be the problem children of the passive-component family. They are used in power supplies and in-between amplifier stages. Fortunately, switch-mode power supply requirements have forced component manufacturers to improve these mission-critical parts. Look for brand-name capacitors (Panasonic, Nichicon) with low ESR (Equivalent Series Resistance), low leakage and long life at high temperatures (2,000 hours at 105 degrees C).

Tip: As with guitar amps, switch-mode supplies can have hazardous voltages inside, even when unplugged. Always unplug the power cord and discharge the main reservoir capacitor before doing any work.

DID THIS EVER REALLY WORK WELL?
This question applies pretty much equally to the Eventide Omnipressor and the Altec 1567A mixer projects. The Omnipressor was one of the earliest VCA signal processors—circa 1975—existing at the same time as Allison’s Memories Little Helper automation system and dbx noise reduction. In fact, versions of the Omnipressor used a dbx VCA.

As on all VCA-based processors, the signal path on the Omnipressor is very short: Input and output op amps interface the VCA to the outside world. These can be replaced or upgraded as needed. However, there are LM-301 op amps on the gain and attenuation limit controls that should not be upgraded, because they are in the sidechain wired as comparators. VCA circuits from that era typically have a distortion null adjustment; optimization via a spectrum analyzer is easier than it is if you are using a distortion analyzer alone.

A pair of LEDs can assist you in centering the processing around the hard knee, which is recommended, especially with the Omnipressor servicing as it can be wild and unruly. The 50-microAmp mechanical meter is failure-prone and the circuitry is a bit unusual—three different DC signals represent input, gain reduction and output levels. In gain-reduction mode, the meter should be centered.

ON TO THE ALTEC
The Altec 1567A was sent all the way from Iceland, and it was clearly worse off than when it began the journey. The octal-socketed transformers had fallen out in transit, rolled around inside the unit and damaged the filter caps. I can’t overemphasize careful packing emphatically enough. This includes separating tubes and other vulnerable un-pluggables into their own protective packaging.

This tube mixer has four transformer-
balanced mic inputs, one unbalanced input and a balanced output with master bass and treble controls. Optimizing headroom and signal-to-noise is no picnic, but the 1567A was used in a wide range of applications, from sports events and other remote broadcasts to the original Woodstock music festival. Once the power supply and interstage capacitors were replaced, I brought this late-’50s mixer into this century. A 1U front panel was added with XLR I/Os, plus input pads, polarity reverse, a voltage tripler (to generate phantom power) and a few internal circuit mods.

The four input transformers each feed one-half of a 12AX7 voltage gain stage, followed by mixer level controls that passively sum together (via resistors). The common cathode 12AX7 was converted to cathode follower for current gain rather than voltage gain, allowing mixer levels to be turned up higher without fear of overload, improving both signal to noise and EQ network drive. To recover the necessary makeup gain, the feedback loop on the 6CG7 output amp was made adjustable via the master level control, varying distortion in the process but in a way that was far more controllably euphonic than the stock version.

There are times when the technician’s goal is simply to fix what’s broken. Other times, accumulated knowledge can be applied to make preventive repairs and “improvements.” In this case, trying to preserve desirable artifacts while minimizing the annoying noises required a mixture of both art and science.


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