[R-390] R-390 C101 and C103 Caps

[Al Parker]

Hi all,
    I'd think that using a modern electrolytic that's rated as "Low ESR" 
would cover the worries.
    ESR is the Equivalent Series Resistance and is mesaured at a low AC 
(e.g. 100 kc) freq. and indicates the cap's resistance at that freq.  Low 
ESR would indicate low heating by ripple current, it''ll be less than an 
ohm, usually measured in hundredths of an ohm, but varies with capacitance 
and voltage rating.
73,
Al, W8UT
New Bern, NC
www.boatanchors.org
www.hammarlund.info

"there is nothing -absolutely nothing- half as much worth doing as simply 
messing about in boats."
   Ratty, to Mole


----- Original Message ----- 
From: "Drew P." <drewrailleur807 at yahoo.com>
To: <r-390 at mailman.qth.net>
Sent: Wednesday, January 07, 2009 7:48 PM
Subject: Re: [R-390] R-390 C101 and C103 Caps


>
> On Jan 3, 2009, at 11:06 AM, Perry W. Remaklus wrote:
>
>> I am working on a R-390 and C101 (10uf 600VDC B+ filter) and C103
>> (10uf 300VDC audio decoupler) are both are leaky --- both are paper
>> in metal cans.
>
> Perry,
>
> I don't see any problem using an electrolytic for the audio decoupling
> capacitor.
>
> An electrolytic might cause problems if used for the main B+ filter,
> however.  The R-390/URR (non-A) has just that one capacitor for B+ 
> filtering
> before the electronic voltage regulator circuit.  That cap sees a high 
> ripple
> (not to be confused with Chuck) current and an electrolytic as small as 
> 10uF
> might heat up excessively.  I've also wondered about the outcome of using 
> an
> electrolytic here. You might try one and see if it gets hot and report to 
> us
> your findings.
>
> Years ago I worked on improvements to a machine control system which used
> small solenoids powered by full wave rectified 120 VAC and small 
> electrolytic
> filter caps of about 5uF.  These caps exploded quite regularly from ripple
> current induced heating.  IIRC, the solenoids drew about 400 mA each.
> Methinks the B+ drain of a non-A to be considerably less but still 
> reasonably
> sizeable.
>
> If the electrolytic warms up, then replacement with a cap having a lower 
> loss
> (than electrolytic) would be in order.  Distributors such as Mouser sell 
> poly
> dielectric caps that would work - you might need to parallel a few to get 
> the
> value you need.  Alternatively, you could use a motor run capacitor. These 
> in
> small applications are often poly dielectric these days.  Ceiling fans use
> caps of this sort in about the value you would need.  Check your local 
> Home
> Despot for these or salvage one from a junk unit.  These are AC rated - 
> the DC
> rating would be 1.4 times the AC voltage rating or possibly somewhat more.
>
> Another possibility would be to use an electrolytic of greater than 10uF. 
> The
> ripple current handling ability would be higher.  You would get lower 
> ripple,
> higher average voltage into the voltage regulator circuit, higher peak 
> rectifier
> and power transformer currents which would cause all to run hotter. In
> reasonable proportions, the added heat might not be too objectionable (but
> don't forget to put a cooling fan on the regulator's 6082 pass tubes - 
> these run
> hot as it is).
>
> Drew
>