[ARC5] replacing C3 and C8 of BC453A

[Scott Robinson]

Folks,

About mica caps: while whiskering can be a problem, I have seen more 
failures where the cap goes lossy (gives a low Q resonance) or open. I 
have seen this particularly in my R-390A, and the symptom was a tuned 
circuit that wouldn't peak or had no gain.

It's hard to diagnose these errors in radios with ceramic disc trimmers, 
the kind that go all the way around and just keep going. Even if the 
mics cap in parallel with the trimmer is open, there will still be a 
maximum as you turn the trimmer--at its max capacitance, of course--but 
it won't indicate resonance and your gain will be tens of dB down. In 
the 390, you can look at the rows of slugs; the ones screwed all the way 
in have a problem.

NPO ceramics are find capacitors for these applications, as are silver 
micas.

Peace,


/scott

On 1/30/15 9:35 AM, Kenneth G. Gordon wrote:
> On 30 Jan 2015 at 11:56, Mike Feher wrote:
>
>> Right now I do not know where C3 and C8 are, however, I have seen a lot of
>> defective mica caps. 73 - Mike/N4FS
>
> C-3, 100 pfd, is the white tubular capacitor on the bottom of the "socket" for
> the antenna coil behind the tuning cap. It couples the tap on the antenna coil
> to the RF amp tube's grid.
>
> And C-8, 200 pfd, is the white tubular job mounted on the bottom of the
> mixer-tube's socket. It couples the HFO's coil, L-4, to the trimmer capacitor. I
> have never (yet) found either one of those to be bad. (So far.)
>
> Yet. After many years, what takes place in some silvered-mica caps are
> small silver "whiskers" which extend outwards from the silver layer and which
> eventually short-out "plates" internally.
>
> We are finding more and more mica and silver-mica caps going defective.
>
> If Peter has to replace those, using modern silvered mica caps would be
> appropriate.
>
> Ken W7EKB
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>