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Old 14th Aug 2019, 5:38 pm   #1
PJL
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Default Osram Music Magnet 4

Just started work on my recent gift to myself. It's the deluxe model so has a quality oak cabinet and is complete and pretty good condition although the front panel has probably had a repaint. Valves are all there and it came with 3 'spares' although I expect these are the originals.

I have been looking out for one of these and wanted one with an original intervalve transformer but as only a minority still have them it is no surprise that both primary and secondary of this one are open circuit. First step was to extract the transformer from the painted copper case in preparation for a rewind which required the oven treatment to soften the bitumen - just hope I have the right gauge wire.

Seems to me that bitumen is probably not the best bedfellow for enamelled copper wire as it must give off some pretty corrosive chemicals.
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Old 14th Aug 2019, 7:28 pm   #2
Keith
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Default Re: Osram Music Magnet 4

My experience in removing bitumen (TV22 LOPT) was using white spirit. It took about a week with the odd poke at the stubborn bits) but I ended up with a perfectly clean transformer.
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Old 14th Aug 2019, 7:53 pm   #3
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Default Re: Osram Music Magnet 4

Hi, these are normally about a 3 to 1 ratio and wound with about 0.05 diameter wire; 1000's of turns.
Most were stick wound, but it is easier to fit Paxolin cheeks to the bobbin for your rewind.

Good luck, Ed
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Old 14th Aug 2019, 8:14 pm   #4
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Default Re: Osram Music Magnet 4

Not much hope of unwinding this to count the turns as the bitumen has soaked into the winding in places. In any case, there is no real former, just a bit of paxolin at the centre that does not even extend over the full width of the winding as it's mostly held together with layering tape. Best bet is to start again with a homemade former from some very thin paxolin sheet I have and some araldite. I know the turns ratio is 1:3 so if I fill my new former to a quarter depth then I know how many turns for the secondary.

I will clean up the laminations and other bit with white spirit.

Ed: Got distracted and didn't press the save button till after your post...I have done a few 6000 turn windings recently! I will measure the resistance of a metre of the wire, I have lots of 3ohms/metre and slightly less 6ohms/metre.

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Old 17th Aug 2019, 8:40 pm   #5
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Default Re: Osram Music Magnet 4

The bitumen has been removed from the laminations and casing and the former made. It fits the laminations but will need a bit of filing down as it's too wide for the can. Seems the design didn't make full use of the available space. Next step is to recover a few metres of wire so I can measure the resistance/metre.

I also seem to remember there is a method for calculating the depth of a winding which I would like to use to estimate the number of turns on the existing coil once I know the gauge from the resistance. Anyone know where I can find it?
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Old 18th Aug 2019, 7:23 pm   #6
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Default Re: Osram Music Magnet 4

If you know the wire coated diameter you simply work on the volume of former available and this defines the turns.

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Old 19th Aug 2019, 10:05 am   #7
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Default Re: Osram Music Magnet 4

Wouldn't the cross sectional area of the winding space do that, rather than its volume?

I would guess that a reasonable 1:3 P:S winding ratio would fall out from the primary occupying 1/4 of the finished winding height and the secondary, 3/4.
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Old 19th Aug 2019, 11:13 am   #8
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Default Re: Osram Music Magnet 4

DOES ANYONE HAVE A WORKING BC710 GECOPHONE TRANSFORMER? It would be a great help if you could let me know the primary and secondary resistances.

Taking the area occupied by each turn as 'diameter x2' is going to give an underestimate of turns for a rough wound coil BUT the diameter must include the insulation and I have no idea what that would be. At the moment the old coil is soaking in white spirit as it is a solid lump of bitumen, copper and tape and I can't even get a few metres off it (it may have been Langford Chapter 5 I remember, I'll have a read).
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Old 19th Aug 2019, 1:08 pm   #9
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Default Re: Osram Music Magnet 4

That's been answered in your other thread:-

https://www.vintage-radio.net/forum/...d.php?t=159042

Read the full description.
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Old 19th Aug 2019, 1:21 pm   #10
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Default Re: Osram Music Magnet 4

If acceptable, just to avoid future frustration when people cannot find the original listing the link leads to:

Gecophone BC710 Valve Transformer

3:1 ratio readings of 1900 and 6000
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Old 19th Aug 2019, 3:00 pm   #11
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Default Re: Osram Music Magnet 4

So I now have various different values:
Primary=1784, secondary=o/c (https://www.vintage-radio.net/forum/...d.php?t=151444)
Primary=360, secondary=1.87K (https://www.vintage-radio.net/forum/...ad.php?t=67429)
Primary=1900, secondary=6000 (ebay)

Seems one may have been wound with a much thicker wire gauge?

The windings are well and truly stuck together as you can see in the picture so I am still having trouble getting a few metres off to measure the reistance. Current guess is there maybe 16,000 turns!
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Old 19th Aug 2019, 3:14 pm   #12
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Default Re: Osram Music Magnet 4

A bit of Googling will find several calculators enabling you to determine how many turns of a given wire diameter will fit into a given cross sectional area. Try various wire diameters in the calculator until you find something which looks sensible. You can then multiply the figure by 0.25 and 0.75 to get the number of turns in each winding.

I have tables showing the diameter of ECW over the insulation, but they're for SWG wire, not metric wire diameters. Might give a clue though.

Does the original winding have inter-layer insulation, or is it just burnt turns I can see?
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Old 19th Aug 2019, 3:21 pm   #13
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Default Re: Osram Music Magnet 4

It does have some taped layers but I suspect this has more to do with the stick winding than for insulation properties. Without an occasional layer of tape, the winding would fall off the side. I guess these were hand wound using a pretty crude method not dissimilar to how we would do it today as hobbyists, just a rotating 'stick' and some guide rails and plenty of 'glue' in this case.
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Old 19th Aug 2019, 3:27 pm   #14
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Default Re: Osram Music Magnet 4

I see that yet another set of resistance figures have appeared in the other thread.

I think you're going to have to design from scratch.
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Old 19th Aug 2019, 5:41 pm   #15
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Default Re: Osram Music Magnet 4

Primary=1.9K Secondary 7.6K.

2 metres of the wire measures just over 13 ohms so a good match to my finer gauge wire which I think I ordered as 46/47SWG. Measured the average circumference and a bit of excel confirms its about 16,000 turns which is about 8,600 ohms. 4,000 primary and 12,000 secondary.

I can actually see this wire so I am inclined to think the insulation is quite a bit thicker than the modern replacement and I am wondering if it makes more sense to use the thicker wire as I am fairly confident it would fit in the former.
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Old 19th Aug 2019, 8:36 pm   #16
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Default Re: Osram Music Magnet 4

Hi, early winding wire was often only single coated. Many of the wires available today are grade 2 (double coated) so actually overall diameter is greater for the same gauge number (which refers to the bare wire).

Micrometer is the only accurate way to size wire, resistance measurements can be hopelessly inaccurate (look up the tempco of copper!


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Old 19th Aug 2019, 10:24 pm   #17
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Default Re: Osram Music Magnet 4

Ed, how do you compensate for insulation thickness when measuring these small wires? Do you remove the insulation first?
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Old 22nd Aug 2019, 7:50 pm   #18
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Default Re: Osram Music Magnet 4

Hi, yes you can try to scrape it off but you may also reduce the wite diameter. Easiest way is to look at wire tables where the modern ones give both bare diameter as well as dia with grade 2 insulation. Earlier tables will probably list the grade 1 coated diameter.

Sorry for delayed reply but internet server is playing up at present


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Old 14th Oct 2019, 12:15 pm   #19
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Default Re: Osram Music Magnet 4

I ended up winding 5000 primary and 15,000 secondary of the thicker wire I had and there was still plenty of room left. After a complete strip down, clean and reassembly following the kit instructions, the MM4 is now working very well. The set came with some spares and included 3x S215, H210, HL210, LP2 and PM254 but unfortunately, 2x S215, the H210 and the PM254 all show low emissions (very low anode current). I am using a 6" speaker and an RS miniature output transformer and it sounds as good as any MW receiver, and I only made one error in reassembling the reaction control 1/4 turn out.

These early radios work surprisingly well and the valves are normally pretty reliable so it is unusual to find so many that are weak.
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Old 14th Oct 2019, 1:10 pm   #20
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Default Re: Osram Music Magnet 4

I was faced with an open circuit intervalve transformer in a 1929 suitcase set.

With no facilities for home rewinds, I instead successfully used a miniature commercial microphone transformer from OEP (Oxford Electronic Products). It was possible to fit this within the original transformer case.

The OEP unit has centre tapped nominally 600 ohm primary and a 3.75 K secondary. Using the whole of the primary results in a turns ratio of 2.5 to 1

For those who might be interested, the size is 36 x 30 x 25 mm, the part number J30A11C and it's stocked by RS as 123-7215. Not cheap at 52, but an instant solution and depending on how you price your own time it might be an option. See the RS website for a data sheet.
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