Altronics K 5171 Manuale - Pagina 6

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K 5171

ever, as soon as the voltage rises above this

200mV limit, the motor is stopped. When the

motor is running normally, the current through

it is about 40mA, which produces 0.4V across

the 10Ω resistor. VR4 attenuates this voltage

and is adjusted so that the voltage at AN3 is

slightly below the 200mV limit.

Note that the AN3 input is monitored only

during the muting operation. At other times,

when the volume is being set by the Up or

Down buttons on the remote, the clutch in

the motor's gearbox assembly slips when

the potentiometer reaches its clockwise or

anticlockwise limits.

As described previously, pressing the Mute

button on the remote again after muting re-

turns the volume control to its original setting,

by driving it clockwise for the same amount of

time that it was driven anti-clockwise to reach

its end stop. This mute return feature in the

software is enabled by leaving shorting link

LK3 open. This allows the RA4 input (pin 3) to

be pulled to 5V by a 10kΩ resistor. Installing

the jumper shunt at LK3 will pull RA4 to

ground, disabling the mute return feature.

Status LEDs

LEDs1-3 indicate the status of the circuit. The

blue Power LED (LED1) lights whenever power

is applied to the circuit. The other two LEDs,

Acknowledge (LED2) and Mute (LED3) light

when their respective RA2 and RA1 outputs

are driven high (ie, to +5V). LED2 indicates

that an infrared command was received and

LED3 lights when the mute function is active.

Pins 15 & 16 of IC5 connect to the oscillator

which drive 4MHz crystal X1, providing the

microcontroller system clock. This oscillator

runs when the circuit is first powered up for

about 1.5 seconds. It also runs whenever an

infrared signal is received at RB0 or when

a button on the front panel switch board is

pressed and then for a further 1.5 seconds

after the signal ceases.

The oscillator then shuts down and the

processor goes into sleep mode, as long as

a muting operation is not in process. This

ensures that no noise is radiated into the sen-

sitive audio circuitry when the remote control

circuit is not being used.

A 10nF capacitor connected directly across

the motor terminals also prevents commu-

tator hash from being transmitted along the

supply leads, while further filtering is provided

by a 100nF capacitor located at the motor

output terminals on the PCB. This reduces the

amount of noise that gets into the preamplifier

signals when the volume pot motor is being

driven.

ULTRA LOW DISTORTION PREAMPLIFIER WITH TONE CONTROLS

Power supply

The Preamplifier is powered from ±15V rails.

These are typically derived either from two

separate 15V windings on the main power

transformer, or a small secondary 15-0-15

transformer and rectifier.

Our Ultra-LD power supply board, (K 5168)

described in the September 2011 issue, is

suitable for use with a wide range of audio am-

plifiers but more importantly for this project,

provide regulated +15V and -15V outputs.

These 15V rails are bypassed on the preamp

board by 470µF capacitors. There are other

capacitors connected across the supply rails at

various points of the circuit which provide local

bypassing for the op amps on the PCB.

We use both 100nF capacitors and 100µF

capacitors to ensure low impedance at a range

of frequencies. The capacitors connected

across the full 30V supply are rated at 35V or

more.

The 5V supply for microcontroller IC5 is

derived from the +15V rail via a 22Ω dropping

resistor and 5V linear regulator REG1. The

22Ω resistor reduces the dissipation in REG1

and provides some additional filtering, in com-

bination with REG1's 100µF input capacitor.

The power LED, LED1, lights up when 5V

is present and its current is set by a 2.7kΩ

series resistor. If you aren't using our Ultra-LD

Amplifier power supply board, or another

board which provides the required ±15V rails,

don't worry. It's quite easy to build a suitable

regulated supply. For the Altronics A1012, use

a code of 023 or 089 for TV mode, 242 for

SAT1 or 245 for SAT2.

Construction

Fig.10 shows the assembly details for the main

Preamplifier module. It is built on a PCB cod-

ed 01111119 which measures 216 x 66mm.

Begin by installing the resistors (use your

DMM to check the values), followed by the four

ferrite beads. Each bead is installed by feeding

a resistor lead off-cut through it and then

bending the leads to fit through their holes in

the PCB. Push each bead all the way down

so that it sits flush against the PCB before

soldering its leads.

Following this, install the IC sockets for the

five ICs. Make sure that each socket is seated

flush against the PCB and that it is orientated

correctly, as shown in Fig.10. Note that IC5

faces in the opposite direction to the op amp

ICs (IC1-IC4). It's best to solder two diagonally

opposite pins of a socket first and then check

that it sits flush with the board before solder-

ing the remaining pins.

The MKT and ceramic capacitors can now

go in, followed by the electrolytic capacitors

(regular and non-polarised). The electrolytic

capacitors must be oriented with the correct

polarity, ie, with the longer lead through the

pad marked with a "+" symbol. The 100µF

capacitors that are marked on the overlay and

PCB with 35V must be rated at 35V or higher.

If you use ceramic 470pF or 47pF capacitors,

make sure they are the specified NP0 (or

the equivalent C0G) type. Using other types

of ceramic capacitors in these positions will

degrade the distortion performance.

The next step is to install the four transistors

(Q1-Q4) in the remote control section. Be sure

to use the correct type at each location. Q1

and Q3 are both BC327s, while Q2 and Q4 are

BC337s. The PC stake (near VR3), 2-way SIL

pin header for LK3 and 3-way SIL header for

LK4 can now be installed, followed by polar-

ised pin header CON6 and box header CON7.

Crystal X1, trimpot VR4, the 3-way screw ter-

minal block (CON5) and the four vertical RCA

sockets (CON1-CON4) can then be fitted.

Ensure the terminal block wire entry holes

face the nearest edge of the PCB. Use white

RCA sockets for the left channel input and

output positions and red for the right channel

positions.

Switch S4 can be mounted now. Take care

that all the pins are straight before attempting

to insert them into the PCB. Press the switch

fully down onto the PCB before soldering each

pin. Also fit REG1, taking care to orientate this

correctly.

Mounting the pots

Before mounting the potentiometers, the

shafts should be cut to length. The length

depends upon the knobs and the type of box

that the preamplifier is to be mounted into.

The thickness of the front panel will have an

impact on the required shaft length.

Make sure the motorised pot (VR1) is seated

correctly against the PCB before soldering its

leads. Once the pot fits correctly, solder two

diagonally opposite pot terminals and check

that everything is correct before soldering the

rest. The two gearbox cover lugs can then be

soldered.

That done, connect the figure-8 wire to the

motor terminals along with the 10nF capacitor

that also connects to these terminals.

These leads pass through a hole in the board

immediately behind the motor. They are then

secured to the underside of the PCB using

cable ties and then brought up to the top side

of the PCB just behind CON6.

Strip the wire ends and crimp them to the

header pins. The wire from the positive motor

terminal (marked with a red dot) should

connect to the CON6 pin that is closer to IC5.

Insert the pins into the 2-way shell and plug it

into the CON6 header. Before fitting VR2 and

VR3, scrape off some of the coating on the top

of the pot body using a file so that they can

be soldered to. Don't breathe in the resulting

dust.