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 Post subject: Midwest 20-38 speaker caps
PostPosted: Jul Fri 13, 2018 1:55 am 
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Location: Coopersburg, PA USA
This Midwest has 3 speakers, one 12" and two 6". The two 6" have what looks to be a cap attached. No markings on the caps as to value. I'm not getting any sound out of the two 6" speakers, so I thought I should replace these caps. Any idea on value? The schematic just shows a single speaker.
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Stan Saeger
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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Fri 13, 2018 4:53 pm 
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Location: Near Brandon, Iowa
The mystery parts may be filters intended to pass a portion of the audio signal spectrum to the small speakers. This may have been a "user installed mod" since the small speakers appear to be early permanent magnet units. These weren't unknown when the radio was built (ca. 1938), but they weren't commonplace, either.

Pre-alnico permag speakers tended to eventually lose magnetism, rendering the speaker inop. (Alnico didn't become widely used in commercial audio equipment until after the close of WW2.) You can test for this using a steel tool like a screwdriver held against the field core piece (speaker cone side). There should be strong attraction. If there isn't, you will need to get the magnets "recharged" (or, just replace the speakers). You can further test the speakers by disconnecting the mystery part and feeding in a low-impedance audio source directly to the voice coils.

If the parts are just caps, you can replace them with nonpolarized electrolytic caps of around 16uF @25V or more.


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Sat 14, 2018 12:35 am 
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Location: Bucyrus, Ohio
Mike Simpson, who is the expert on Midwest radios, says that these should be around 5 uf capacitors. I finished mine last winter and used a MKP 400V 4.7 uf capacitor. This worked fine for me but a slightly lower value can also be used.


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Sat 14, 2018 12:47 am 
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No mention of speaker testing. One assumes a battery was cracked across there Voice coil, or an ohm meter uses to ensure they're good? If the caps "dry out" that will cause that I would not use polarised Electrolytic type caps there.

Marc


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Sat 14, 2018 9:03 pm 
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Location: Coopersburg, PA USA
yes, I tested the small speakers with a 9V battery - loud scratching - so A-OK on the speakers.

I removed the mystery caps and now I have sound out of all three speakers.

I will order ~5 uF non-polarized caps to install, since that seems to be the way it should be.

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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Sun 15, 2018 1:50 am 
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There is a cap, I think polypropylene, however will be listed that are intended for speaker crossovers. A lot of transistorised stuff use Electrolytic caps for coupling & in car radios, where things can get really hot, to the point here where it is a criminal offense to leave kids in them. They dry out en masse & kill the volume.

Marc


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Wed 18, 2018 9:21 am 
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Location: Coopersburg, PA USA
I looked on Mouser, which usually has just about everything, and there is no non-polarized electrolytic around 4 or 5mfd with anywhere near 400V, just 50V,

Any suggestions or even a part number from another source?

Thanks..

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Stan Saeger
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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Wed 18, 2018 1:02 pm 
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There will not be 400V on the speaker secondary. https://www.jaycar.com.au/6-8uf-250v-me ... r/p/RY6956. These are what I was going on about, I fail to see why you cannot get similar.

Marc


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Wed 18, 2018 1:24 pm 
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Posts: 342
Location: Bucyrus, Ohio
I agree that 400V is overkill, but being rated above over actual voltage needed shouldn't matter. Correct me if I'm wrong. This was the only capacitor from one of my regular suppliers that fit the value. Antique Electronic Supply part number C-MKP, $1.35.


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Wed 18, 2018 1:40 pm 
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Thanks for the advice Marcc and wyocodak. I have placed an order.

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Stan Saeger
Coopersburg, PA
http://www.saegerradio.com/


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Wed 18, 2018 3:22 pm 
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The reason some of those audio caps look physically bigger is that it is more often all about current, rather than voltage, with a tube radio speaker. Caps, especially those used in motors, quite often have a current rating listed. The fully metal jacketed Plessey (5S10A) that I use for a paper weight is rated @ 20A max 1000VDC WV.

Marc


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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Wed 18, 2018 5:59 pm 
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Location: Beaver Falls, PA. USA
My 18-37 has the optional tweeter which is original, and it is a PM type.

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 Post subject: Re: Midwest 20-38 speaker caps
PostPosted: Jul Thu 19, 2018 2:07 am 
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I did one Midwest where it had a Tweeter & both speakers were electrodynamic (field coil).

Marc


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