written 26th september 2000 for december 2000 mix

ã 2000 by Eddie Ciletti

This is the third in a series on Analog Equipment Maintenance for geeks in training. Part-1 hit the ground running, diving straight into troubleshooting capacitors (the weakest link in aged gear), using a square-wave generator and an oscilloscope. (The traditional method of measuring frequency response requires a sine-wave oscillator and an AC voltmeter. While valid, using a single 1kHz square-wave provides an overall snapshot of both low and high frequency response good enough for troubleshooting and sometimes more.) The same tools were used in Part-2 to evaluate amplifier performance, particularly to upgrade early IC opamps, from seventies-era products, to just about anything modern.

No matter whether attempting to repair or upgrade, the recurring theme is comparative analysis. Translation: Take advantage of having two of the same device — borrowing if necessary — an easy task with a recording console or tape machine. Having one good channel to which the "others" can be compared is a major timesaver. Beyond repairs, the difficult answer to a popular e-mail question concerns how to make things "better." This is a complicated issue. The best advice is to find a technician who is willing do the geek stuff AND show you how to do the dirty work. After changing caps on 24 modules, there’s no way your soldering skill won’t improve. But upgrades require a combination of skills, tools, time and money.

Like the three-part series on analog tape machines, I’ll be moving on to a new topic, but I didn’t want to leave without relieving myself of some guilt. You see, as "geek-sexy" as upgrades are, it is important to establish a solid foundation before attempting any enhancements. In a most tangible and organic way, this means starting with the obvious troublemakers — switches, pots and faders. Older American- and British-made gear — from the fifties through the seventies — can be easier to work on than their more modern counterparts, especially in later years as mass-production automated so much of the assembly process. Old stuff has more real estate (more room to work), connections to pots and switches are hand-wired, making "off-the-shelf" replacement fun and easy. (Well, almost!) All you need are a few catalogs from digikey.com, mouser.com, newark.com, Antique Electronic Supply (tubesandmore.com) and New Sensor Corporation (sovtek.com), just to name a few. Phew!

Once "they" started mounting pots directly to circuit boards, every physical characteristic becomes so specific that replacements can only be purchased from the product manufacturer, if alive and well. If not and assuming you’ll want to use the same knobs, finding the same shaft dimensions alone can be an ordeal more excruciating than a wire-brush enema. An entire article could be written on the topic of finding replacement Clarostat pots for an MCI JH-600 series console. (Nearly all had custom dimensions and tapers.) Even quality pots succumb to the additional physical stress of being mounted to both the front panel and the PCB. Any flexing will eventually break the connections inside the pot. Trident Series 65 and 80 consoles are prime examples of cheap pots plus poor mechanics (and some of the worst caps ever).

In this time of analog-to-digital transition, many "affordable" analog consoles — new and used — have found a niche as "monitor mixers" relegated to the simple task of multitrack playback — preamps and processors being outboard. More recently, the digital latency issue can be resolved by using an analog mixer to provide a direct signal path from the mic / direct box to the musicians so that live recording and overdubs can proceed without confusion. No one denies the power of mixing in the digital domain, but lots of people miss the simplicity and ease of "playing" even the cheapest analog mixer. Dedicated knobs and switches make for tangible phun without the worry of clocks, masters or slaves…

Then there is the story of the inspired mix that was marred by the aforementioned "scratchy controls" or worse, a nagging background hum. Once, after installing a new 32-input mixer, a customer complained of noisy Effects Returns. Although the problem was omnipresent, there is often a little more gain in the Aux summing amps than in the stereo mix bus, so effects like chorus / flangers process the hum into a swirling three-dimensional buzz.

OK, now I can show you a mod without a guilty conscience. In most "budget" mixers, the power and ground connections are distributed to each module via either motherboard or ribbon connector. This is fine for an eight-channel rack mounted device — as was the case for the original Soundcraft 200 series — but when the frame was extended to accommodate 24 additional modules, a ribbon cable is not capable of doing the job. Not knowing the original wire dimensions, for this article I measured one conductor of an 80-inch section of 28-guage ribbon cable. The resistance was 0.5 ohms or 0.075 ohms per foot, certainly not an effective ground. Each module was at a slightly different voltage and ground potential, hence the internal hum.

The solution was to route a ground wire to each module — details at www.tangible-technology.com — a "fix" that not only lowered the hum but resulted in many satisfied customers remarking about improved low-end punch and a better stereo image. Each console is different, so there’s no way to tackle all the details for your specific problem here, but I will show how to "interrogate" a console plus provide an example of a more recent upgrade.

In order to determine whether the source of the hum is internal or external everything should be disconnected from the mixer save for a pair of headphones or some sort of monitoring system. Knowing how hard this can be, the following procedure assumes everything is connected. Please note that MUTING a channel module does not disconnect it from the bussing system, only de-assigning the Mix and / or Groups will effectively take a module "out of the system" without actually removing it. (This doesn’t apply to Aux sends that are typically hard-wired to their respective busses.) Be sure that no video monitors, power supplies (wall warts) or power amps are anywhere near the console and its cabling.

Start by monitoring the Mix Buss for all noise then Mute all effects Returns. Expect the hiss to be reduced because Effects are notoriously noisy without any help from the Aux Sends. If hum is present on all busses, it’s probably worse in the Aux Sends and should diminish when the Returns are muted. Now, mute all channel strips. What remains should be a minimal amount of hiss. If background hum is present, now is the time to de-assign all channels and returns (if possible). If the hum goes away, your console may be a candidate, but to fully qualify, disconnect all external wiring and repeat the test.

Dan Kennedy, of Great River Electronics here in the Twin Cities, had recently acquired two Trident Series 65 consoles — outfitted with 24 and 16 modules, respectively — combined the two with some fine and fancy metal work, plus improved the power and ground distribution in the process. He also modified the Master Module, which includes the Mix Buss and Control Room monitor section. If you’re wondering why anyone would go to such lengths, the goal was to have a clean and serviceable console installed in a nearby facility for basic equipment evaluations in a real-world environment. The consoles were acquired for a song and when you have all the tools, it still takes forever but it doesn’t cost much…

Figure-1: A new copper buss bar distributes a robust ground to each module via the green wire to the motherboard below.

In Figure-1, the motherboard appears across the bottom, where a wire bundle — red, black, white and blue (hidden) — delivers power and originally, ground. Above is the new copper buss bar, distributing ground to each module connector via green wire, when previously only the circuit board trace did the job. Power and ground were formerly distributed from "momma to momma," via jumper wires, until a barrier strip was installed as shown in Figure-2. Now, each motherboard gets its own power connection. (Note: Some liberties were taken with the wire colors.)

Figure-2: The newly added Barrier Strip is fed from the Power Input Connector on the rear panel from the top. Below, wires are "multiplied" to individually feed each of the motherboards.

A similar approach was taken on the Master Module. Highlighted in yellow on Figure-3, a new solid ground wire is paralleled with the original ground trace. In addition, note the three black wires that branch off from the connector to reinforce the ground for the more "distant" areas in need. You can’t really have enough ground, ya-know-whut-I-mean?

Figure-3: Ground distribution on the Master Module is beefed up using a buss wire in parallel with the main PCB trace. Three black wires reinforce the "distant" areas.

On the flip side of the momma, two Burr-Brown OPA602ap opamps replace ICs 23 and 24 — the stereo fader buffer amplifiers — as seen in Figure-4. The Burr-Brown part has a minimum slew rate of 20volts/microsecond and a quiescent (idle) current of 3mA min to 4.5mA max, when hot. Both of these parameters are about twice that of the original TL071 part. Replacing just these two opamps in the Master Module might not be cause for worry — in terms of power consumption — but there are 23 opamps on this PCB alone! Dan felt the Master / Monitor module was most important. The old power supplies were replaced with a single redesigned supply with more than enough reserve power for any future opamp upgrades.

Figure-4: IC 23 and 24 are the buffer-amplifiers for the Stereo Master Fader directly above.

A project like this can take hours, days, nights or weekends depending on whether the work is done "full speed ahead" or in your spare time. For those with more time than money, it’s worth the effort. Even if you never think to undertake such a task, it’s good to know what constitutes good power and ground distribution. So often, we concern ourselves with external wiring, assuming what’s inside is on the money.

Winter in Minnesota means that Eddie is feverishly removing the wheels from his roller skates hoping to upgrade to cross-country skis with some left over lumber. Drop by tangible-technology.com and see if reindeer really know how to fly.