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 Post subject: open mica cap
PostPosted: Dec Thu 07, 2017 1:59 pm 
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Joined: Dec Sat 06, 2014 4:02 pm
Posts: 49
Location: Fergus Falls, Minn
Found an open 100pf mica cap in the osc ckt of a '47 Merc radio. Appeared to have never been disturbed but, when the leads were cut, one lead just fell off. It appears that the bad lead didn't extend into the molded body very far, maybe .030".


Attachments:
47 merc 2.jpg
47 merc 2.jpg [ 35 KiB | Viewed 317 times ]
47 merc coil.jpg
47 merc coil.jpg [ 64.07 KiB | Viewed 317 times ]

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Allen L.
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 Post subject: Re: open mica cap
PostPosted: Dec Thu 07, 2017 7:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 31420
Location: Maryland 20709, USA
I expect somebody rotated the mica cap on its leads to read the color code.
That usually breaks the internal connection of the lead with the elements.

- Leigh

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 Post subject: Re: open mica cap
PostPosted: Dec Fri 08, 2017 12:04 am 
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Joined: Dec Sat 06, 2014 4:02 pm
Posts: 49
Location: Fergus Falls, Minn
There was no sign that this radio had ever been worked on, even the buffer cap was original. It may be that bending the leads during installation loosened them in the molded body. I found the bad part by feeding a signal into the osc coil and finding no signal on pin 3 of the 7B8, it is the 2nd grid and acts as the plate for the osc ckt fed by the bad cap. The radio must have worked for a while and quit when the cap opened up at some later date.

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 Post subject: Re: open mica cap
PostPosted: Dec Fri 08, 2017 5:28 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3960
Location: Charleston, W.Va.
Hi Allen,
Thanks for posting that good photo. As Leigh has said, rotating these caps to read them is a common source of damage like this. Sometimes even gently lifting one of their leads from a soldered lug can also damage them. Perhaps yours was indeed undisturbed--if so your experience and photos show that some of them were none too well constructed in the first place. On the other hand, many old mica caps have held up very well. All of which proves that there was considerable variation in quality of construction back in the "old days", just as there is today.

With modern materials and technology, current-manufacture electronic components in theory should be better than ever. But unfortunately this is balanced against cheap construction by unskilled/unsupervised low-priced labor, most especially by some (not all) in China and other Far Eastern locations.

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 Post subject: Re: open mica cap
PostPosted: Dec Fri 08, 2017 5:39 pm 
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Location: Detroit, MI USA
It's also entirely possible that happened during the manufacturing process but the outer shell held the wire end in tight contact with the internal part so it worked. We have no way of knowing the history of the radio at this late date, it could also have been intermittent and cut out when driving over bumpy roads, or the driver could have had to whack the dashboard to make the radio play as the vehicle got older.

Some very strange things can happen to components, and even insulated wire. Over the years I have seen a few cases where a wire is broken inside the insulation, deep inside a chassis with no evidence of any physical damage externally. It had to be a manufacturing issue that eventually let go.

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 Post subject: Re: open mica cap
PostPosted: Dec Sun 10, 2017 5:15 pm 
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Location: Charleston, W.Va.
Mr. Detrola wrote:
. . . or the driver could have had to whack the dashboard to make the radio play as the vehicle got older.

Hi Dennis,
Whacking the metal dashboard was common; I can remember doing so myself on my first car, a 1949 Ford. Usual reason was slight pitting of vibrator points which sometimes required "whacking" to make it start on initial turn-on. :lol:

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