Osram Music Magnet 4

 This old TRF set started life as a kit of parts, probably in 1930. Since then it's been "partly restored". The front panel looks a bit garish and I think spoilt by the use of new pan-head screws. This brings to mind an event at Plessey, where I worked, when the Plessey Chairman's brother Michael Clark issued an edict that "from today only pan-head screws are to be used". I think I'll change them sometime...

The wiring, above the chassis anyway, is not original.

Is that the wavechange switch located directly beneath the dial and missing it's knob?


Opening the lid reveals the set's details.

The labels are a little misleading, especially the lower label, which taken to the letter would include tuning?


 The front panel has a very uncluttered look because all the controls are positioned at the sides of the case. Only the dial is shown, and this is marked in Long and Medium wavelengths plus a log scale down the centre.


 Here are the controls. In the left picture is the reaction control and in the right the tuning kob (front) and aerial coupling control (rear) which acts as a volume control as well as altering the set's selectivity.




 The three sets of coils are in these three rather large aluminium cans, together with their associated valves.


 Here's a view with cans removed (They just pull off.. no screws). One valve is missing... that's the detector/LF amplifier which should be an H210. On the left are two immaculate S215 tetrodes also with B4 bases. On the right is a fuse holder which should be fitted with a screw-in bulb.


 Here's the B4-based audio output triode valve.. certainly not original Osram LP2 because it's marked "Triotron Made in Austria, ZD2 2 Volt".

To the right is the solid dielectric aerial coupling variable condenser. Space above the control is for a grid bias battery held in a pair of simple brackets.



On the left is the solid dielectric split-stator variable condenser used for reaction control. On the right, covered in verdigris, is the slow motion drive to the triple gang air-spaced variable condenser used for tuning the set. Interestingly, there's a set of small knurled wheels incorporated into the variable condenser for any fine tuning required if the tuned circuits are mismatched. These are located under the lid and to the left of the dial and in slots in the condenser screening box.





Above... a view under the chassis. Pity about the wavechange switch. There's only marginal evidence of it! Most parts of the original switch are missing. The activating mechanism should pass under the three tuned circuit assemblies and exit at the side of the case through a hole at the lower edge of the right hand side panel.

I removed the end screening can and investigated the situation. Close to the chassis at the bottom of the coil assembly is a pair of spring contacts. By pushing a brass piston towards the coil the two springs make contact and complete a circuit, shorting out a coil. The piston is operated by a ball bearing which sits in a brass socket in the end of the piston. This is duplicated in the three coil assemblies. What's missing is a rod passing through a hole in the end of the chassis. The rod passes over the three pistons in each of which should be located a ball bearing. The rod is shaped so that pulling it presses against the three ball bearings and thus the three pistons. There must be a number of brackets holding the rod in place. These are also missing. The rod must be shaped such that pulling it pushes against the three balls and pushing it back allows the three balls to fall back and release the spring contacts.

Here's the circuit diagram from a weekly wireless magazine article. I'll scan the rest of the article soon...

 Here are scans of the pages...

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