This unit is designed for monitoring the audio output level across a loudspeaker when carrying out alignment of radios. As no great precision is required, a simple passive circuit arrangement has been used.
The meter will read full scale with about 1 Watt onto a 3R speaker, although the calibration (and indeed linearity) are not important for the intended use.
Circuit Description
The circuit is a voltage-doubling rectifier driving a panel meter. The input signal is AC coupled via C1. D1 holds the signal so that the negative peaks are at ground potential. The mean level is above ground, so the polarity of C1 is important.
The peak level is rectified by D2 and stored in reservoir capacitor C2. The value of R1 has been selected to give the required calibration with a 250uA meter movement. If a 100uA is used, the alternative component values should be used.
Germanium diodes are used because of their low forward drop
voltage. If silicon diodes were used the unit would be less linear.
Construction
The prototype was constructed using a small piece of plain matrix board. Tag strip or stripboard could be used if preferred. A PCB would be overkill for such a simple design!
The meter used on the prototype was a low cost (�3.20) 250uA signal strength meter obtained from Maplin (Order Code LB80B). This is marked "SIGNAL" and has an arbitrary scale marked 0 to 5, making it ideal for the purpose. A higher quality meter could be used, but this would offer no real advantage.
The completed circuit may be fitted into a small plastic case. The circuit board may be retained by using short rigid connecting wires to the meter.
The input may be bought in via a length of two-core speaker
cable. The free end may be fitted with a pair of small crocodile clips for easy
connection to the speaker tags in the radio being aligned.
In Use
The unit is designed for connection across a loudspeaker. If you wish to disconnect the loudspeaker because of the annoying noise, replace it with a suitable wirewound resistor.
Unless stated otherwise in the service information, the correct
alignment point is that which gives the greatest reading on the meter.
Parts List
R1 |
18K 0.25W Resistor |
C1 |
10uF 25V Electrolytic Capacitor |
C2 |
2.2uF 25V Electrolytic Capacitor |
D1 |
OA47 Germanium Diode |
D2 |
OA47 Germanium Diode |
M1 |
250uA Panel Meter |
Plain matrix board |
|
Case |
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Speaker wire |
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Croc. clips (2 off) |
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3R3 2.5W wirewound resistor (optional dummy load) |
Note: If a 100uA meter is used, C2 should be 1uF and R1 should be 47K.
Two people have contacted me regarding the availability of the OA47 germanium diodes. These are listed in the current Maplin catalogue (Order Code QH70M, price 49p each).
One constructor, Charlie B. Cade said:
I followed your online article using 10uF for C1, 2.2uF for C2, silicon diodes 1N914 (I think) for D1 & D2, 15K for R1, and a 200uA meter from and old Heathkit Vacuum Tube Voltmeter. It works just fine.
So, if you cannot obtain the germanium diodes easily, try silicon diodes. 1N914 is electrically almost identical to the more common 1N4148 diode. I would have expected silicon diodes to make the scale more non-linear, but this does not really matter much for this unit. Charlie certainly seems happy with his unit.
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