I have a classic Bang and Olufsen Beogram tangental arm turntable. The five pin DIN plug has completely broken off while in storage and I need to resolder it.
Unfortunately, I am unfamilliar with the wiring convention for equipment of this type. Can someone please help?
There are four insulated wires, red, blue, brown and yellow, plus the earth shield.
All wires carry an audio signal relative to earth. I looked inside an they are all connected to the PCB.
Why are there FOUR live wires when only two are required for stereo?
How can I determine which two go the the phono input of my amp, and/or what the remaining two wires are for?
Two pairs of wires connected to two pickup coils. Use an ohmmeter to identfy the pairs, there should be something like 100 to 1000 ohms resistance for each coil and the resistances should be equal for both coils. Each coil is one of the stereo channels. Th fifth wire should be shield, and show about 0 ohms between wire and the metal of the tone arm and pickup element. If there is a pre-amplifier inside, only tracing the wires will show what they did. If so, check the circuit board for markings, there might be a balanced two wire output for each channel. My B&O is of the simpler type, and without pre-amplifier. I am not going to open it up however. :)
In some turntables, the audio from each side of the cartridge is delivered as a differential signal pair. This pair might not be referenced to ground at all, or it may have some sort of weak ground reference (e.g. a 1-meg resistor from each side to ground, to establish a DC bias level).
This approach allows for the possibility of a balanced input topology at the phono preamplifier, which can give you better rejection of induced hum.
It also allows you to use the more common sort of non-balanced phono input (e.g. RCA jack, with the outer contact grounded) simply by connecting one of the wires in the pair to ground (RCA plug shell) and the other to the "hot" side of the input (RCA plug center pin).
If the wiring is as I suggest above, then you can identify which pairs of wires to together by measuring the wire-to-wire resistance with an ohmmeter. I think that you'd find that for any one of the wires, there would be one of the other three which would show a relatively low DC resistance (the nominal resistance of the coil), and the other two wires would show a rather higher DC resistance.
Match up the pairs this way. For each pair, one would go to the RCA plug shell and the other to the RCA plug center pin. Solder them up, then try out the turntable. If you find that the two channels are now out of phase with one another (very weak low bass, which goes away almost entirely if you switch your stereo to "mono" mode), reverse one pair of wires and try again.
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Dave Platt AE6EO
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