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The Dark Side of Echorec Restoration

Another Binson ‘Echorec’ recently landed on the rough, regolith strewn terrain of my workbench; this time an iconic piece of prog rock history: David Gilmour’s old T7E (Serial # 8483). Now, although I’ve spent many, many years patiently studying and accruing knowledge on these archaic echo machines, they can still mess with my mind. Binson repairs are never straight-forward, never quick, never a simple service job; even if the machine appears to be in good shape on the outside it invariably possesses some new, never-seen-before nasty surprise lurking deep within its grimy guts, which can take the unsuspecting repair technician to the mountains of madness. Mr Gilmour’s Echorec was no exception; cosmetically it looked fine, but the internals were, to say the least, a train wreck. His machine may once have powered the Floyd to the dark side of the Moon, however forty years on, with resistors drifting towards infinity, long-dried out electrolytic capacitors and corroded wiring, the only place it was going was straight into the sure and steady hands of our new Binson tech, Ian to investigate. Read on to find out more about the dark side of Echorec restoration and how Ian brought this iconic machine back from the edge and back to life…

Terminal Wire Rot

The first horror we encountered was the internal wiring, which was disintegrating and falling apart before our eyes—a bit like those eerie Martian locust creatures in the sci-fi film ‘Quatermass and the Pit’ after the seal of their tomb, an ancient spaceship buried in the London Underground, was breached exposing their long-dead bodies to the air. Like Professor Quatermass and his lady scientist assistant (played by the gorgeous Barbara Shelley) we had to work swiftly to preserve the decaying bodies, er I mean wiring once the restoration was underway. If any of the wires did accidentally snap off then notes were made of where they go. This has to done quickly, as it’s all too easy to forget where they connect to, generating yet more work deciphering fading and illegible schematics, which, or course, are never entirely accurate.

This wire rot is a common disease infecting, damaging and ultimately killing many Echorec machines. At some point in the the early 1960s Binson had purchased a job lot of ‘grey market’ electrical wire unaware it possessed deadly quality issues that would lead to dire consequences for the long-term reliability of their machines. Now, all plastics are volatile to some extent, that is they gradually dry out releasing gases so that the plastic becomes stiffer and brittle with age. But the plastic insulation on this Binson wire ages horribly. Over the years the insulation breathes out highly corrosive gases and sweats an acidic, oily residue that attacks most metals including copper, nickel and steel, transforming the wire into nothing but green dust and leaving ugly stains on the chassis or any other metalwork it comes into contact with-extremely nasty stuff.

Although this cheap and nasty wire was only utilised in Echorecs manufactured from the early to mid 1960s on wards, by the time Binson had identified the problem a large volume of machines had already found their way onto the market. And this was a time when Binson’s production was at its zenith—consequently there are still many echo machines out there with faulty and potentially dangerous wiring.

The Power Supply Filter Choked and Croaked

Like the dissection scene in the Quatermass film, Echorec restoration can be gruesome and genuinely terrifying work. As we worked our way deeper and deeper into the machine’s decaying innards, replacing wires and meticulously removing decades of crud the workbench gradually became littered with debris, desiccated fragments of cable insulation, the remnants of corroded wire. Even the wires from the power supply filter choke were rotten and corroded and there really was nothing to be done except bite the bullet and replace it. This is not a task to be taken lightly as the machine had to be stripped down—removing the side, top, bottom and front panels—just to gain access to the choke.

Binson N.O.S. (New Old Stock) chokes for the T7E aren’t readily available however, with some spade-work, an off-the-shelf ‘Hammond’ choke makes an excellent substitute. The Hammond fixed inductor 5H 75mA filter choke (Hammond part# 156L) is a close electrical match to the original part and is of similar physical dimensions. There’s just one proviso though: the pitch of the mounting holes, which are several millimeters too close together. This problem was solved by carefully machining the holes to turn them into slots—now the choke can be installed and soldered into the Echorec.

Filter choke — Another nasty surprise—this time the power supply filter choke has a terminal case of wire rot.

Dehydrated Electrolytic Capacitors

Rebuilding the high voltage multi-section filter capacitors wasn’t exactly a walk in the park either. This specific type of capacitor is no longer manufactured and N.O.S. aren’t readily available. But even if they were, it’s not recommended practice to utilise N.O.S. electrolytics as, over the decades, the electrolyte gradually evaporates-it makes no difference whether the capacitor is still in its original packaging or wired into a amplifier circuit. The only solution, if you’re concerned about preserving the authenticity and history of the machine, is to rebuild them.

A Job Only Half Complete…

At some point in the machine’s murky past a substantial amount of its wiring had been replaced. An admirable job had been made of renewing the internal mains wiring and many of the shielded audio interconnects, however the job was only half done creating another day or so’s work to renew the remainder of the old disintergrating audio interconnects and single core wiring. A standard IEC mains main power socket had also been installed-a replacement for the original Italian two pin socket-which was a good idea as it meant the Echorec was now electrically safe and compliant with EU regulations. We don’t normally advise cutting into the metalwork of vintage equipment to modify it, however when it comes to mains insulation and earthing, safety must come first when using old gear.

This raises an interesting point: When approaching a restoration should one keep the machine original or make alterations in the name of safety or improvements to the sound quality. Well the answer very much depends on whether you’re a collector, who wishes to preserve the history of a museum piece, or a working musician seeking a reliable piece of gear. Being a guitarist tone is always my priority, I’m not museum curator, however when undertaking Echorec repair or restoration it’s a good idea to try and retain as much of machine’s history as possible.

…And A Carnival of Collateral Damage

Ahh! Shiny! A new old stock Italian manufactured 470KOhm 'Lesa' potentiometer finds it's new home inside David Gilmour's old T7E

For instance, the ‘ECHO VOLUME’ potentiometer on this T7E Echorec had been replaced with a modern ‘Bourns’ sealed type. Now, Bourns make excellent pots, however this replacement was a linear taper whereas the original would have been logarithmic taper. Volume pots are normally log taper types because that’s how the human ear responds to sound pressure levels. So, the pot needed replacing and, in the spirit of keeping the machine as close to original factory condition as possible, we tracked down a supplier of N.O.S. Italian manufactured Lesa 470KΩ log pots. This ethos applies to other electronic components too, and hardware such as switches and knobs, even down to the screws and washers-efforts were made to preserve all original parts or, if needs be, replace them with authentic parts where possible.

Dubious modifications had also been performed on the recording bias oscillator and the bias filtering circuitry. The 22nF cathode bypass capacitor had been replaced by a 160Ω resistor and additional blue epoxy encapsulated polyester capacitors soldered in parallel with the 220pf filter capacitors already connected across the playback heads. These types of capacitor first appeared on the market back in the 1980s meaning that the modification work must have been undertaken after this date. ‘Hot’ biasing the oscillator by reducing the value of the cathode resistor value might have been an attempt to increase the bias voltage level and the additional filtering added to remove it on playback—the thinking behind this must have been to try and improve the high frequency response and clarity of the echo repeats and/or improve the signal-to-noise ratio, however it’s likely that the machine was suffering from head wear or misalignment and the mod was an attempt to compensate for this.

Component Aging and Decrepitation

Some electronic components age better than others, for example a N.O.S. tube won’t appreciably deteriorate if stored for decades—if you think about it, there’s very little chemistry that can occur between the tubes’ nickel electrodes and mica spacers in a vacuum. Nor between the aluminium plates and polyester dielectric of N.O.S. ‘mustard’ capacitors. However, other parts, such as the electrolytic filter capacitors described earlier, do degrade. They dry out as the electrolyte gradually evaporates over the years, the capacitor loses its ability to hold charge and the capacitance falls-off.

The years aren’t kind to carbon composition resistors either. Their ohmic resistance can drift by as much as 5% per year just sitting on the shelf-and that’s the best case scenario. Resistance can change even more (around 15%) when it’s operated in a high voltage circuit such as a tube amp. Even the act of soldering the resistor into a circuit can result in a 2% change. The reason for this instability is inherent in the design of the component, which is essentially a cylinder (the component body) made of fine graphite particles mixed with a clay binder that are baked to harden them. The baking also glues and electrically connects the wire legs of the resistor to the body. Any change in temperature results in differential expansion and contraction of these materials. Additionally humidity affects the resistive properties of the carbon/clay mixture. This means carbon composition resistors exhibit a lousy temperature coefficient in the order of 1000ppm/°C.

Carbon composition resistor partial section — The instability and poor noise performance of a carbon composition resistor is inherent in its construction—the component legs are pressed into the clay/graphite body and baked to form a physical and electrical connection.

On top of that carbon comp resistors also invariably become noisier with age. The point of contact between the component leg and the component body gradually loosens-up, becoming weaker because of the stresses induced by temperature changes over the years. This degradation manifests itself as ‘fluctuation’ noise where the resistor begins to generate a low frequency ‘fluttering’ or varying noise as well as increasing broad-band white noise. The higher the resistance value of the resistor the more pronounced this noise becomes.

Even a brand new carbon composition resistor generates significantly more fluctuation noise than an equivalent value metal-film resistor. A common repair or modification to old tube amps is to replace noisy carbon composition plate resistors which are under high voltage stress. For this reason we replaced the 100KΩ and 47KΩ plate resistors on the Echorec too.

Phase I Complete, Now For Phase II…

And that just about wraps things up, well, apart from a rare and unlikely, short-circuit 47nF polyester coupling capacitor and a big blob of solder shorting the 1.5KΩ cathode resistor of the record head amplifier circuit to the tube heater. After Ian sorted out these minor issues David Gilmour’s T7E sprang back into glorious life. Now we’re in business—with all the electronics back in tip-top condition it was time to move on to ‘Phase II’, the often misunderstood and much neglected process of checking the condition and alignment of the tape heads…

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The Dark Side of Echorec Restoration