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Old 13th Apr 2012, 6:00 pm   #19
Darren-UK
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Join Date: Apr 2003
Location: Blackpool, Lancashire, UK.
Posts: 4,061
Default Stereophonic and multiple 'speaker machines, general information.

As stated way back at the beginning of this thread, stereophonic (hereinafter "stereo") machines are excluded for the following reasons:

a) They are not the typical machines which newcomers seek help with; "typical" being the crude, mono, efforts from the likes of Dansette, Fidelity, E.A.R and so forth.

b) The typical, cruder, machines mentioned above are usually repairable by newcomers/beginners with little initial relevant knowledge, by means of guidance of forum members and, we hope, threads such as this one. Stereo machines are usually far more complex and thus often beyond the understanding of the beginner. Of course, everyone can learn so such comments are not intended to be patronising.

To expand upon point b) above, stereo machines vary greatly in layout and complexity; take as an example the comparison between a Pye 1005 (look that one up) and, say, a Bush SRP-whatever with its add-on amplifer unit. These variations therefore make detailed explanations herein rather impractical for the beginner, so searching the forum and/or creating a new and dedicated thread for your particular stereo machine would be a better idea.

However, despite wishing to avoid the subject herein it would be unfair and unhelpful to omit giving a basic understanding of what stereo is and how it works so off we go:

Records, cartridges and styli

Confusion, or simply lack of knowledge, concerning mono/stereo records and mono/stereo/stereo compliant cartridges and styli is common. Therefore although touched upon elsewhere it's not a bad idea to elaborate here.

A mono record has tracks giving only lateral (side to side) oscillations. Stereo records have tracks giving both lateral (Channel 1) and vertical oscillations (Channel 2). A mono cartridge therefore needs to handle only a single channel and therefore has only two terminals on its rear. A stereo cartridge is therefore in effect two cartridges in one, hence it will have four terminals on its rear (two per channel or, in the case of an odd few, three terminals: Channel 1 - Common ground - Channel 2). Note that the two channels are usually marked on the rear of the cartridge together with the respective 'signal' and 'ground' terminals, but you need a magnifying glass to see them.

Take note of the wiring colours too before you disconnect a cartridge, or you'll be in a muddle later on. Of the four wires, one will be lefthand signal, another righthand signal, another lefthand ground and the remaining wire righthand ground. For reference the common setup is:

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Blue - Left channel ground.
Red - Right channel signal
Green - Right channel ground.

A fifth, usually black, wire may also be lurking and this is merely an earth connection from tonearm to chassis.

Styli, as will by now be realised, are designed to handle a single channel if mono but two if stereo; the mono stylus moves only in one plane (lateral) but the stereo stylus moves in both planes (lateral and vertical). It is for this reason that you should not attempt to play stereo records with a mono stylus due to the "non compliance" of the latter.

However, some mono record players may be fitted with a mono but stereo compliant cartridge and stylus, meaning stereo records can be played albeit with only mono reproduction.

Multiple loudspeaker (non stereo) machines

A common misconception is that a record player (or, for that matter, a radio receiver) with more than one loudspeaker is stereo but this is not necessarily the case. Quite common are machines with multiple loudspeakers simply wired in parallel, but otherwise are essentially straightforward mono machines. The kindest thing you could say about these is that they give 'pseudo stereo'.

Stereo machines, self contained and adaptable

Back in post #10 we gave a simple explanation of the Push-Pull amplifier but to very quickly recap this type of amplifier will have twin output valves (or transistors), each of which amplifies one phase of the audio signal to ultimately give greater output at the loudspeaker(s) at any given volume control setting.

In a stereo machine the above is in effect duplicated. A little earlier we described how a stereo cartridge could be described as two cartridges in one, with four (sometimes three) wires leading from the cartridge instead of the two found with mono. Put in very simple terms, this situation continues through the stereo machine, via the single premap stage, to the phase splitter and output stages to the loudspeakers. However, it isn't quite that straightforward as a Balance control will be in the circuit and biasing has to be set accurately to prevent distortion and possible damage.

The 'self-contained' stereo machine has everything in one cabinet, although we can include machines with seperate 'speakers in this category (1970s 'Music Centres' for example). Some machines may contain a mere two loudspeakers, others may contain as many as six. An example of the latter is the Pye 1005 (Achoic) and it's variations. It will now be realised that circuit complexity varies widely, hence the reason it is not practical to provide an in-depth repair guide to stereo machines in this guide.

'Adaptable' stereo machines include those which are, on their own, essentially mono but cosmetically and electronically matching add-on amp/speaker units to convert them into stereo were available. Very common among these are the Bush SRP units, mentioned in this guide and widely in the forum, and others were produced by Hacker, Dansette and a few more. The add-on units are relatively scarce but see this thread for an interesting and rare Dansette example.

Stereo adaptable machines, by nature, have differing circuitry again. It's worth repeating here about heatsinks in transistorised examples. To prevent overheating and failure of output transistors, the latter must be run attached to their heatsinks (which may take the simple form of the chassis itself). Common output transistors are the large AD161/2 types, of which there'll be four in a true stereo machine. These are usually found bolted by their flanges to an alloy component forming part of the chassis and acting as the heatsink. Do not attempt to run the machine with any output transistors disconnected from the heatsink (see next paragraph for the exception). If you do, overheating and failure will result.

A further problem with output transistors is the possibility of short circuits through the transistor case to the heatsink. This is not unknown with the AD161/2 types and indeed can occur with any output stage transistor - not just in stereo machines although it seems to happen more often in the latter. If this is suspected it is permissible to check by briefly disconnecting the transistor(s) from the heatsink to see if operation is restored. Do not do this for anymore than a few seconds though.

That's about all that is felt practical to include in this guide about stereo machines. Once again, if necessary you will find various threads concerning various stereo machines in the forum. Otherwise please create a new thread giving details of your machine and the problem(s) you're encountering.
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