Philco 640 Shadowmeter Observations - Printable Version

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+--- Thread: Philco 640 Shadowmeter Observations (/showthread.php?tid=14853)

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Philco 640 Shadowmeter Observations - Geoff - 06-26-2016

So I am in the final stages of wrapping up my Philco 640. The last thing I need to do is re-install the shadowmeter. The coil has continuity and I get 1,551 ohms. I’ve seen people on YouTube testing the action of the shadowmeter by hooking the coil up to a 9-volt battery. Here is what mine looks like performing that test:

https://youtu.be/q4uFXKMyKZs

Compared to other examples it does not appear that there is as much movement as there should be. I removed the coil and inspected the magnet. I was surprised to find it was a bar magnet.

[Image: http://i12.photobucket.com/albums/a241/JoeRadio1938/Miscellaneous/DSC03265_zpszkuu1leu.jpg]

Other examples I have seen on websites and YouTube always showed a form of horseshoe magnet. I tested the magnetism at different locations with a paperclip and found one side to be a little stronger than the other. Seeing that it was a bar type magnet I purchased some Neodymium bar magnets off of eBay that were the same thickness, just a little wider and a little longer (eBay Item #321476074749):

[Image: http://i12.photobucket.com/albums/a241/JoeRadio1938/Miscellaneous/DSC03267_zpsgkrp4gla.jpg]

I’ve seen people trying to re-magnetize the original magnet but did not try on mine since it does not seem to last that long (at least from what I have seen on YouTube).

I put one on the side of the shadowmeter housing but did not see much of a difference in meter movement. I then put two of them together and retested:

https://youtu.be/Xb_UX3i8RA0

Not bad. I’m debating whether to file down one of the magnets to the same size as the original and replace it or just leaving the two magnets in place. I think the first thing I’ll do is reinstall it and see how it works as is, then add magnets to the side as needed and see how it goes.

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