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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 15th Feb 2018, 11:08 pm   #1
cosmocat
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Default 1933 Marconi 286 pickup

I've recently been given a new project, a 1933 Marconi 286 radiogram (a.k.a. the spider sanctuary) . The case is in a bad way but its remarkably complete and un-molested, so I started doing some preliminary checks to see what I was letting my self in for!
The mains transformer, output transformer and speaker windings all measure for resistance sensibly, and apart from lots of perished cables, so far so good.
I then moved onto the gramophone pickup, and I could not get any signs of output from it.
Removed from its shell it looks like a magnetic type, and I could feel a small magnetic pull from the horseshoe like pole piece and the coil measured 1.3k ohms.
What kind of voltage output would you expect to get from this type of pickup ?
With my scope connected across its coil I could not get any discerable signal down on mV ranges.
Do the magnets go weak and if so can it be repaired ?

Thanks
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Old 16th Feb 2018, 6:33 am   #2
Ed_Dinning
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Default Re: 1933 Marconi 286 pickup

Hi, some of these units use a rubber suspension that goes hard, it can normally be replaced with Si rubber sleeving.
It may also be possible to get some small modern rare earth magnets that may even increase the output over that of the original.

Ed
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Old 16th Feb 2018, 10:02 am   #3
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Default Re: 1933 Marconi 286 pickup

As Ed has said, making the rubber suspension softer will allow the movement required to make a small voltage. I have repaired one of these and it is quite simple to do.

I have an almost identical model to your 286. If you require photos of the front grill, please let me know.

These are nice radiograms - small footprint and sound good. Well worth the effort. Have you worked out how to get the chassis out yet? It took me a few minutes to work it out! It isn't easy as you need to disconnect quite a few wires too - iirc.

SEAN
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Old 16th Feb 2018, 2:41 pm   #4
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Default Re: 1933 Marconi 286 pickup

If you take time out to do a bit of a forum search you'll find that there's been a few threads in the past on re-building these pickup heads.

You won't get much output from it as it is, as the suspension is so rock hard that the armature is unable to move in the magnetic field to produce any.

Whatever you do, don't attempt to play any good 78s on it as it is with zero compliance, as it'll tear them to pieces.
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Old 20th Feb 2018, 10:38 pm   #5
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Default Re: 1933 Marconi 286 pickup

Thanks for the replies. Perished rubbers makes sense with the state of the other rubber insulation in the chassis, I'll get some silicon tube. Is the coil supposed to move or does the needle move in the field to produce an output ?

SEAN - a close up of the grille would be appreciated. I have a 286 AC, not the Q version. Some pictures I have seen suggest there should be a "M" bar across the speaker but mine has no trace of anything having been there, so any more info would help.
Getting the chassis out caused some head scratching until I took the radio panel out at the top and the side panel fell out revealing the chassis screws! it was easy then
Andrew.
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Old 21st Feb 2018, 4:53 pm   #6
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Default Re: 1933 Marconi 286 pickup

Looking at your photo of the turntable and Pickup arm I thought the attached might be of interest if not of any practical use, but it may be of historic interest!
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Old 22nd Feb 2018, 9:15 pm   #7
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Default Re: 1933 Marconi 286 pickup

Thanks for the data sheets, very interesting.
I'm having trouble getting the cartridge apart to change the rubbers.
The two metal pole pieces seem firmly stuck to the base. Are there hidden screws under the base ?
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Old 22nd Feb 2018, 9:20 pm   #8
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Default Re: 1933 Marconi 286 pickup

Can't help on screws, but make sure you put a keeper across the horseshoe magnet whilst it's disassembled!
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Old 24th Feb 2018, 3:07 pm   #9
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Default Re: 1933 Marconi 286 pickup

The pivots of the needle holder are set in rubber tubing which welds the two halves together. It might just need a bit of judicial force with a blunt knife to get it apart
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Old 29th Mar 2018, 6:37 pm   #10
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Default Re: 1933 Marconi 286 pickup

Thanks for your help everybody. I managed to get the pickup apart. My suspicions were correct that lurking under the 4 round indentations were indeed screws holding the pole pieces to the bakerlite shell. They were covered in wax that melted away with a quick touch of the soldering iron.
I have replaced the perished rubber with some silicone tubing.
I am still trying to get my head around the physics of how this is supposed to work as the flapper moves between the pole pieces in an otherwise static magnetic field.
I guess the proof of the pudding is in the eating and so I will try it out on an amplifier when I have the deck motor running.
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