[R-390] C553 replacement?

Mon Feb 27 12:32:32 EST 2023

It is with great trepidation that I venture to comment on this perennial 
subject.  But the fact is that the Vitamin Q and other related capacitor 
designs were the result of a very large, intensive DOD/industry effort 
to research the unacceptably high failure rate of paper dielectric 
capacitors and drastically reduce them.  They succeeded.  These 
capacitors have excellent reliability, and in 50+ years of working on 
large quantities of this old gear, I've seen few failures.

That said, there's certainly no harm in replacing C553 prophylactically, 
but the fact is we probably do get a bit obsessive about it.  Based on 
experience, I agree that the failure rate of these old filters 
themselves is far higher than the failure rate of a Vitamin Q capacitor 
at C553...

73,

John K9WT

On 2/27/2023 1:38 AM, Jim Whartenby via R-390 wrote:
> Well, nothing is forever so I guess there have been some C553 failures but I suspect that chances are much better then even that an R-390A mechanical filter will fail by itself either with the internal isolator turning into a sticky, gummy mess or one of the coil wires will break from fatigue at the feedthrough.  I have seen both over the years but have not experienced a single C553 failure.
>
> If filter failure due to current leakage of C553 is a real concern, why not series connect two 0.02 uF caps of your favorite construction?  The chances of two capacitors that are operating well below their rated voltage failing for leakage at the same time should be minuscule.
>
> As an aside, my experience with Collins equipment is that the mechanical filter most likely to fail will have a narrow passband so in the R-390A, the 2 kc filter has a better chance of failure then the 16 kc filter.  Case in point is the ARR-42 that I am currently working on has a bad 1.4 kc CW mechanical filter but the 6 kc AM filter is just fine.
> Regards,
> Jim
> Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy
>
> -----Original Message-----
> From: Dan Martin <pitfit at comcast.net>
> To: David Wise <d44617665 at hotmail.com>
> Cc: Barry <n4buq at knology.net>; Jim Whartenby <old_radio at aol.com>; R-390 Mailing List <r-390 at mailman.qth.net>
> Sent: Sun, Feb 26, 2023 8:32 pm
> Subject: Re: [R-390] C553 replacement?
>
> I've read these recent C553 posts with interest and a grin.
>
> 10-20 years ago there were stories alleged of hundreds of destroyed mechanical filters due to stock C553 failures. Puzzled operators, concerned about the mysterious loss of a filter, would quickly switch to each of the alternate filters to confirm their function and would thereby sequentially destroy the entire filter bank. Who hasn't heard those stories around a campfire?
>
> Replacing C553 with a Sprague (now CDE) Orange Drop used to be the unquestioned first thing in any refurbishment. Chuck Rippel wouldn't let a 390A leave his shop without an Orange Drop in place for C553.
>
> Now it seems Vitamin Q's, like Golden Retrievers, are our best friends and never turn on us and go bad.
>
> Really, not picking a fight, my friends. Just appreciating the comments re: what had been unquestioned convention "back in the day."
>
> And yes, my '67 EAC has a 20 year old by-gawd Sprague Orange Drop in place for C553, not some  upstart CDE branded version. 🙂
>
> Dan
> WB4GRA
>
> Sent from my iPhone
>
>> On Feb 26, 2023, at 3:16 PM, David Wise <d44617665 at hotmail.com> wrote:
>> 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)
>>
>> Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows
>>
>> 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
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