The voltage selector switch on the side panel of the ‘T7E’ selects various tap points on the primary winding of the power supply transformer allowing the machine to operate from European 220V, British 240V or U.S.A. 120V A.C. mains power. The transformer primary winding also doubles as an ‘autotransformer‘ ensuring the shaded-pole motor always sees the specified 220VAC it requires to operate correctly. This is essential as although the motor is synchronous, that is under no load conditions it spins at the same speed regardless of applied voltage, at low voltage it may not have enough torque to start up under load. This is because at reduced voltage the torque of a synchronous motor is reduced by the square of the voltage ratio. What this means in real terms is that if the voltage falls from 220V to 110V the torque decreases by a factor of four and the motor may stall, overheat and burnout.
It should also be kept in mind that the motor speed in a European Echorec will be about 20% faster when operated in America resulting in the overall delay time being 20% shorter. Altering the diameter of the rubber drive wheel or changing the voltage WILL NOT alter the speed of the disc. Echorecs manufactured for the American market under the ‘Guild’ brand name were fitted with motors that had smaller diameter spindles to correct the gearing ratio so the idler wheel was driven at the same speed as European models. In their day Binson, UK used to undertake a modification to add a D.C. motor and vari-speed control circuitry to alter or fine-tune the delay repeats (tempo matching). The motors were high quality units manufactured by Trident Engineering Ltd. Part number: 2-40/1212.
A few vital statistics from my own Binson Echorec model T7E:
Measured drum diameter (d) = 4.7″
Drum circumference = π × d = π × 4.7 = 15″
Measured rotational speed of drum = 71RPM, so every minute, 71 × 15″ = 1065″ of the magnetic band on the drum surface passes across the heads. Converting this to inches per second:
1065 / 60 = 18ips, which is very respectable tape speed for hifi reproduction
The motor spindle circumference can be calculated from:
Drum ω = 71RPM
Drum circumference = 15″
Motor spindle ω = 3000RPM
Spindle circumference = 71 × 15″ / 3000 = 0.36″
The measured the spindle diameter was just over one tenth of an inch, which gives a circumference of about 0.31″
The delay time can be calculated from the measured distance between heads which is 1.3″ (34mm). If the drum speed is 18ips = 457mm/s then delay time for the first playback head is:
First tap = 34 / 457 = 0.074s = 74ms
and the Second tap = 2 × 74 = 148ms
and the Third tap = 3 × 74 = 222ms
and the Fourth tap = 4 × 74 = 296ms
Now here comes the really clever part—these playback head distances are not arbitrary. A considerable amount of thought was put into the positioning of the heads around the drum by Dr Bini, the designer of the Echorec. They were chosen so that the delay times are musically related to one another. If the fourth tap is considered as representing a quarter note then the third tap is a dotted eighth note. This is a very useful and inspiring repeat pattern and has been utilised by guitarists such as David Gilmour and ‘The Edge’ to build some fantastic riffs. Continuing on, the second tap represents an eighth note and the first tap a sixteenth note. These shorter delays can be heard in slap-back and rockabilly music from the 1950s and, again by Gilmour to good effect on tracks such as ‘Time’ (The Dark Side of the Moon).