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 Post subject: Horseshoe Magnet Pickup
PostPosted: Nov Wed 29, 2017 2:18 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 316
Location: Worcester ma, usa
I am on a fact finding mission for the HS magnet pickup .What volume strength should be expected after a rebuild of the pickup ? There different types .
I have the "Winged" tone arm on a Philco 23X ,I've read this was made by RCA for Philco . I have replaced the the damping on the needle cradle pins (shrink tubing). The cradle has a repaired crack in it. The only other damping is in the center of the coil ,which is on a Bakelite bobbin . That damping appears to be still good . The pickup plays fine and tracks the record as it should with volume at about 80% sounds good. I start to hear the phono at about 50%. Is this to be expected ? Can I get more volume out of these pickup ? What about the cracked cradle ,could this be causing the lower volume?


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Nov Thu 30, 2017 5:41 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3612
Location: Near Brandon, Iowa
To me, it's not clear from the photo how this unit goes together; but I believe that there is a permanent magnet somewhere in it to establish a fixed magnetic field in which the pickup coil can operate. These magnets predate alnico metal technology and will lose their strength over time; when this happens voltage output from the pickup will fall off (and so will audio volume).

It isn't difficult to "recharge" the field magnet but it will first need to be removed from the assembly.


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Nov Thu 30, 2017 10:18 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2543
Location: England
I too don't understand your pickup. Here are some pics of a similar era HMV (RCA) pickup.

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In the first pic you can see the pickup ready to go back in the head shell. The top coil is a hum bucking coil and it can work without that.

In the second pic the component parts are shown with the horse shoe magnet removed. The armature is mounted with soft rubber around the side shafts and held at the top in piece of soft rubber with a slit. See you said you used heat shrink tubing and this would be incorrect. It needs to be soft so the armature can move easily. Rubber of some sort. In the old days the tube used for bicycle tyre valves was one way but they are probably different now. In the far right image can be seen an armature with the tube still on it and one with it cleaned and removed.

The 2 screws shown on face of the pole piece, are tapped, and used to hold them in position on the bottom plate of the head shell. On the other side are two tapped holes to hold the cover for the rubber sheet for the top of the armature. This is shown 'flat' in the last image but is in fact at right angles and fits between the semi-circular cut outs in the pole pieces.

Perhaps these make it clearer:

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In the second pic can be seen the tapped hole in the armature to take the needle holding screw. The third pic shows the bottom of the head shell with fixings of the pole pieces and where the needle is inserted.

In use the horseshoe magnetic field is focussed through the pole pieces and the armature. Small movements of that disturb the filed and this is picked up by the bottom coil surrounding it.

Done 2 or 3 of these and they have always produced plenty of output with no re-magnetising of the horseshoe needed. A tip is that the magnet has to be on the pole pieces and only has one side with the faces fully machined flat.

Hope this helps Gary


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Nov Thu 30, 2017 1:07 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 316
Location: Worcester ma, usa
Yes there is a horseshoe magnet that sits on the housing and the poles are above the nuts on either side .I have that square piece of damping with the slit but mine is in the center of the coil . It is still pliable, but I was thinking about pouring a new one with molding rubber . This is my first rebuild so I'm proceeding with cause. So the shrink tube is not good.I was concerned about how thin is was but when the phono played I looked else where for the fix. I'll post better photos .


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Nov Thu 30, 2017 1:43 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 316
Location: Worcester ma, usa
I didn't want to remove the coil from the hosing just yet. The coil is on a Bakelite bobbin and the center is filled with rubber(pliable) with a slit in it for the needle armature . I centered the armature between the two poles by losing the nuts and adjusting those small brackets you can just see the tip of the armature in the first pic, right above the coil. Does the "O" on the right side of magnet tell you what pole it is ? I'm looking for some rubber to replace the shrink tubing. Thanks


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Nov Thu 30, 2017 5:15 pm 
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Joined: Jun Sun 15, 2014 11:04 pm
Posts: 758
Location: Coos Bay, Oregon
Recommend replacing the rubber armature pivot pin cushions with soft rubber tubing (The kind that is used to rebuild acoustic phonograph reproducers works great). It is a bit hard to slip onto the pins, but with a tiny bit of liquid soap, the will go on easy. The magnets sometimes get weak and can be re-magnetized, but that issue does not happen often, so best to get everything else into good condition before going that route. The slitted block is an important part for clarity and volume. The more compliance it has, the greater the output. The compliance should be about the same as that of a rubber band. The casting rubber you mention (such as Silastic) will work very well. Just pour into a flat surface with a dam around it, about 3/32" thick. When the rubber has set, razor blade cut it to size and make the slit. A tiny amount of liquid soap on the armature blade will make it easy to assemble. Test and adjust as necessary.
BTW, when dis-assembling this pick-up, you should have marked the orientation of the magnet on the coil and armature block, to be sure you assemble with the same polarity. This may not be a problem, but if the coil and armature block was a little magnetized (happens in a few cases) and you reverse the orientation of the magnet, there will have a slight reduction of the flux.


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Nov Thu 30, 2017 5:52 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 316
Location: Worcester ma, usa
I had some of that tubing ( I have a few Victrola's ) .I replaced the shrink tubing a inspected the square rubber inside the coil again . Its in good shape I did treated it with rubber restore (for pinch rollers). The phono plays about the same ,I don't get any real volume to about about 60% mark and up then it sounds really good . I didn't marked the magnet poles but I lucked out with that" O" on one side and the top outward side of the magnet is shaved down some. So my only option left is to replace the square rubber piece ? How much would the needle effect the output ? I'm sure the record does correct ?


What about the capacitor block , I have the 10K resistor but I have a 0.01uf cap . I didn't have the parts list schematic when I originally restored the radio which as it at 0.015uf. The 0.01uf was suggested. Its not that much of a difference just covering all the bases .


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Dec Sat 02, 2017 8:39 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 316
Location: Worcester ma, usa
Ok I replaced the center damping block of the coil it wasn't difficult at all. There was maybe a small improvement in output, but I'm glad I did it . I am getting 800 Ohms on the coil , maybe I am getting all the output the coil has . A side note, with the volume off I can hear the record playing clearly . I'll take a look at the electronics one more time and then finish putting this 23X back together . Thanks for the help


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 Post subject: Re: Horseshoe Magnet Pickup
PostPosted: Dec Fri 08, 2017 5:13 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 802
Location: Philadelphia, PA
I've done over a hundred of these pickups, here are few observations.

The normal output of a high impedance unit, when connected to it's characteristic load resistance is between .75 to 1v P-P and symmetrical, low impedance units those less than 200 ohms are about 50 to 100mV P-P.

To test the output it's best to use a frequency test record, or if that's not available a good electric record. If you removed the magnet without sliding it on a "keeper" you will lose output, if there is any side to side play in the armature pivot you will lose output, if the armature is not set in the center of the gap with no damper pressure you will lose output.

The magnet really needs to be re-magnetized anytime the pickup is worked on.

Silicon tubing is the best I have found so far to use for the armature pivots. The damper material is another story, I have used many different materials over the years. I started out with a soft very compliant silicon material but found it would allow the armature to lock up against the pole piece if the pickup was handled rough or the magnet was too strong after re-magnetizing. Another material disintegrated after 2 or 3 years, others were just too hard. I now have a material that is compliant and will withstand rough handling and strong magnet fields and stay centered.

My home made magnetizer:
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My test turntable and some of the changeable arms to test virtually any pickup:
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To really do a good rebuild job it can take 1hr to 1 1/2hrs to complete. It is more an "art" than science.

This is the output of an Edison C-2 pickup shown in 1/12th octave mode of a HP analyzer, the source is a Victor Scroll record. You can see the output content from around 100Hz to 6,000Hz
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_________________
Chuck

http://www.myvintagetv.com


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