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 Post subject: SW-28a Capacitors: are the micas ever bad?
PostPosted: Feb Fri 23, 2018 6:02 pm 
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Joined: Feb Mon 25, 2013 8:37 pm
Posts: 575
Location: Seattle, WA
Guys,

The troubleshooting of my radio continues. I've found some broken wires and an ecap that was wired incorrectly. The latter was my fault as I replaced it lead-for-lead without checking whether the earlier replacement had been done incorrectly. Thanks to Chuck, I finally found the Candohm resistor, which was also shorted. I am now checking the resistances on tube sockets. I have a whole set of tubes that are off, and they all point to the same set of caps. The pin readings on the tubes vary between 0 and approximately 4k ohms when they should be generally in the 14k-18k ohm range. Naturally, most of them are in the RF area. I previously removed the RF sections and replaced the paper caps. Despite triple checking my work, I may have done something wrong. But my specific question is this: there are also two mica caps in RF section 4 that I didn't replace. Section 4, of course is the worst to get to because the other three have to be removed first. Because these two micas are in the cluster of caps that could be causing my faulty tube readings, I may have to bite the bullet and go back in.

Has anyone had the following micas fail (C70, C86)?

Similarly, are there micas in the IF cans that need to be replaced?

Don


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 Post subject: Re: SW-28a Capacitors: are the micas ever bad?
PostPosted: Feb Fri 23, 2018 6:24 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5901
Don,

Mica caps don't fail as commonly as paper types but they do fail. A typical failure for them is when silver migrates across the insulator and this occurs when there is a high DC differential across the capacitor which is the case with both of those capacitors. So I would not be surprised to see a failure from either of them in a radio that has a lot of hours on it.

Even if C86 is a dead short the reading to ground from the tube pin would be above 6K if C70 isn't shorted so with a very low reading C70 would be my first suspect. Also this is a 2,000 pf capacitor and in many older radios although the parts list will show it as mica it actually isn't even though it is in a flat pack. Mica caps get expensive as the capacitance value increases and you will often find that these caps are in the same basic package as a mica, sometimes differing in color, but an autopsy will reveal it is not a mica type.

Rodger WQ9E


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 Post subject: Re: SW-28a Capacitors: are the micas ever bad?
PostPosted: Feb Fri 23, 2018 6:32 pm 
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Joined: Feb Mon 25, 2013 8:37 pm
Posts: 575
Location: Seattle, WA
Thanks, Rodger

That makes sense. It looks like I get to disassemble the radio again.

Don


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 Post subject: Re: SW-28a Capacitors: are the micas ever bad?
PostPosted: Feb Fri 23, 2018 6:38 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1631
Location: Rochester, NY.
Also beware of Micamold caps that aren't mica, but plastic-encapsulated paper/foil caps:
http://antiqueradios.com/forums/viewtop ... =6&t=37559
I had a Wilcox-Gay FM receiver that had micas shunted by a resistor. The resistance measured across them was lower than that of the resistor value. After separating the leads, I found the micas to be leaky.


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 Post subject: Re: SW-28a Capacitors: are the micas ever bad?
PostPosted: Feb Fri 23, 2018 6:43 pm 
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Joined: Feb Mon 25, 2013 8:37 pm
Posts: 575
Location: Seattle, WA
Hi Wally,

I’ve had Micamolds in other receivers, but it doesn’t look like there are any in this radio. I’m still checking tho.

Don


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 Post subject: Re: SW-28a Capacitors: are the micas ever bad?
PostPosted: Feb Fri 23, 2018 7:49 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1631
Location: Rochester, NY.
Two that leaked badly in my SX-42 were under the power transformer cover in the C51 and C52 positions of an SX-28A.


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 Post subject: Re: SW-28a Capacitors: are the micas ever bad?
PostPosted: Feb Fri 23, 2018 8:07 pm 
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Joined: Feb Mon 25, 2013 8:37 pm
Posts: 575
Location: Seattle, WA
Hi Wally,

C51, C52 are among the ecaps that I've replaced as I'm working my way through the radio.

Thanks for the heads up

Don


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