[R-390] C553 replacement?

[David Wise]

That’s correct, C553 is a coupling cap into the R-390A mechanical filter.  It’s a catastrophe if it shorts.  Many people replace it even though it’s a Vitamin Q which should still be good.  I kept the old cap and inserted a current limiter, using a Supertex (now Microchip) LND-150 depletion-mode MOSFET.  Just connect drain upstream, and source & gate downstream, and voila, 1mA limit.

In the R-390, C606 sees 1/19th of 180V so it will last forever.  C608 sees about 180-10+5 or 175V.  If it leaks, the regulator will throttle down.  If you suspect this, measure voltage across 6BH6 grid resistor R624; it should be zero.  Slight leakage is worse than severe leakage, because the 6080’s will overheat.

On pdf page 10 of 34 of TM 11-5820-357-35P, C608 is listed as type CP10A1EE224M.  CP is hermetically-sealed paper.  In specification MIL-C-25D, the second E indicates 400 volts.

Dave Wise
SWL (Inactive)

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From: Barry<mailto:n4buq at knology.net>
Sent: Saturday, February 25, 2023 8:08 PM
To: Jim Whartenby<mailto:old_radio at aol.com>
Cc: R-390 Mailing List<mailto:r-390 at mailman.qth.net>
Subject: Re: [R-390] C553 replacement?

I think C553 is only in the R390A.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "R-390 Mailing List" <r-390 at mailman.qth.net>
> To: "R-390 Mailing List" <r-390 at mailman.qth.net>
> Sent: Saturday, February 25, 2023 10:03:27 PM
> Subject: Re: [R-390] C553 replacement?

> A quick look at the R-390 Collins preliminary manual indicates that C-606 and
> C-608 are both rated at 100 vdcw.  Curious that C-608 has 174 volts across it
> according to the voltage table for V-607.
>
> I did not find C553, which tube is associated with it?
>
> Regards,
> Jim
>
> Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy
>
> -----Original Message-----
> From: Barry <n4buq at knology.net>
> To: John Wendler <wendlerjrv at gmail.com>
> Cc: R-390 Mailing List <r-390 at mailman.qth.net>
> Sent: Sat, Feb 25, 2023 8:52 pm
> Subject: Re: [R-390] C553 replacement?
>
> If I'm not mistaken, Dave Medley (and others) suggest that 180v regulator issues
> in the R390 might be due to leakage in C606 and/or C608 and I think those are
> typically Vitamin Q capacitors.
>
> Thanks,
> Barry - N4BUQ
>
> ----- Original Message -----
>> From: "John Wendler" <wendlerjrv at gmail.com>
>> To: "R-390 Mailing List" <r-390 at mailman.qth.net>
>> Sent: Saturday, February 25, 2023 6:01:07 PM
>> Subject: [R-390] C553 replacement?
>
>> The C553 in one of my IF modules is a Sangamo Vitamin Q capacitor, 0.01 uF,
>> 300 V.  It looks good visually and is not a Brown or Black Beauty of
>> Death.
>>
>> What's the group experience with the reliability of the Vitamin Q caps?
>>
>> V/r,
>>
>> JP N5CQU
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