WebEE

Testing
Before plugging the unit in for the first time, carry out the following tests. Make sure the power switch is off. Set your test meter to it's highest resistance range, and measure the resistance between Live & Neutral, between Live & Earth, and between Neutral & Earth on the mains plug pins. In all cases the meter should read open circuit.

Now set the power switch to ON and carry out the above tests again. The resistance between Live & Neutral should now be between 1K and 2K (transformer primary), the other two tests should still read open circuit.

Set your test meter to the lowest resistance range available, and measure the resistance between the Earth pin on the mains plug and the following points: both transformer mounting screws, the screens of all audio connectors, and any exposed metal parts of the case. In all cases the meter should read less than one ohm.

If any of the above results are incorrect, recheck your wiring thoroughly and find out why! It is fairly easy to incorrectly wire a mains switch so that it shorts across the mains when switched on. Do not plug the unit into the mains until the results of all the above tests are correct.

Ensure that the plug-in IC's (IC1, IC2 and IC3) are not fitted. Plug the unit into the mains and switch on. Set your test meter to 30 Volts DC or greater, and connect it's negative probe to the earth tag on the transformer. Connect the positive probe to the cathode (bar) end of D2 or D3, the meter should read between 20V and 27V. Now connect the positive probe to pin 3 of IC1 socket, the meter must now read between 14.25V and 15.75V.

If all is well so far, switch off and wait five minutes, for the power supply smoothing capacitors to discharge. Now plug in the IC's, and switch on. Measure the voltages at the following points: IC1 pin 1, IC1 pin 7, IC3 pin 13, IC3 pin 14, IC1 pin 8, and IC1 pin 14. In all cases the voltage should be between 5 and 7 volts (it may be necessary to switch your meter down to it's 10V DC range for this). Switch off and assemble the case.

Now install the decoder and test out, as detailed below.

Installation and Use
The ideal viewing room setup is shown in Fig 5, although some compromises may well have to be made in practice. Two rear speakers are really required, if only one speaker is used it's position will be heard. These speakers should be behind the viewer, towards the rear corners of the room. The effect is a little better if the rear speakers are further away from the viewer than the front speakers. Remember that the rear speakers do not have to be of particularly good quality, since the rear channel bandwidth is only 100Hz to 4kHz. The author used a pair of small wooden cased speakers which originated from a stereo music centre, and a low power stereo amplifier. His supply of Dolby Stereo films arrived via an Amstrad SRD400 satellite receiver.

Connect the unit to your home entertainment equipment, as shown in Fig 4. Note that the Audio Out from the satellite receiver or VCR needs to connect to both the decoder and your main amplifier, this is the reason for the suggestion of paired sockets. Set RV1 central and RV2 to minimum. Leave the main amplifier off for now, and switch on the surround amplifier, this unit and the satellite receiver or video recorder.

Choose a mono program (if you have satellite try Sky News), and turn up the volume control (RV2). If the balance control (RV1) is adjusted towards either end the sound should be heard from the rear speakers. When the balance control is adjusted towards the centre it should be possible to find a point where there is virtually no sound from the rear speakers.

Leave the balance control at this position, and select a stereo film channel (Sky Movies or The Movie Channel). If the film is in Dolby Stereo, you should get a significant amount of sound (but not speech) from the rear channel. If you can't get this to work, check your satellite receiver is set to the main stereo channel (AU1 on Amstrad receivers), and check the film really is in Dolby Stereo (the blockbusters shown on Sky in the evenings often are).

Switch on the main amplifier and set the volume to your usual listening level. Now adjust the level of the rear channel to a level where the rear sounds contribute to the overall sound, without intruding. Dialogue should still appear to come from the front, whilst crowds, music and dramatic sound effects should fill the room.

A certain amount of practice will be needed to get the best results from the unit, it's really a matter of trial and error, but it's worth the effort!