[Amps] best deal for Mallory caps?

[Jim Thomson]

Date: Wed, 30 Nov 2011 22:07:56 -0500
From: "Carl" <km1h at jeremy.mv.com>
Subject: Re: [Amps] best deal for Mallory caps?

----- Original Message ----- 
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: <amps at contesting.com>
Sent: Tuesday, November 29, 2011 12:35 PM
Subject: Re: [Amps] best deal for Mallory caps?


> On 11/28/2011 7:42 AM, Carl wrote:
>>>   I use my
>>> >  trusty Simpson 260 on the middle Ohms range to read charge and 
>>> > discharge
>>> >  current as I reverse the leads, and compare each cap to the others. 
>>> > A
>>> >  bad one will have a lot less charge and discharge current.  I have 
>>> > three
>>> >  Titans, so I've been through this several times.  Only once when I've
>>> >  replaced one has another one come up bad in the string.
>>> >
>>> >  73, Jim K9YC
>> That test is about 1% better than nothing.
>
> Better than nothing my &$#@   It works VERY well to find a dead lytic in
> a string. Perhaps you have the bucks to spend on every possible piece of
> test gear, but most us have to make do with what we have, and most of us
> have a basic VOM.  If you've got a cap breaking down under load, yes,
> you would need it, but not to find an open lytic n a string.
>
> 73, Jim K9YC

Ummm, an open lytic is not going to show ANY charging on the meter.

I stand by my earlier statement that a 260 is otherwise 99% useless to find 
high levels of leakage that will indicate a problem in the making.

High bucks test equipment isnt necessary, a simple home made ESR meter or an 
old Heath/Knight bench supply and a meter that can read low current....such 
as your 260....is all that is required.

OTOH, many amp owners arent so cheap that they wont do some intelligent PM 
and replace the caps wholesale after 20-25 years.

Carl
KM1H

## My  B+K  875-B will measure AC resistance, down to .001 ohm. [uses a 1 khz tone].  Very  easy
to measure the ESR of a cap.  Real easy to find a dud  from any new batch's of caps.  All  my 2500 uf caps
read .013 ohm...... and all my 2300 uf caps all read  .024 ohm.  The 875-B will all measure 'D' factor. measuring
'D' factor will also  show a dud cap in an instant.   That alone will find  99.9%  of bad  cap rpblems.
 For measuring leakage current, I use a test jig, with a small variac, etc.  Leakage current will be max, with
450vdc applied, and way more with > 450 vdc applied.    Leakage current drops like a rock, when only
90% of rated voltage is applied....[ this is on a new or good cap]. 

## If u bring the vdc up real slow on the variac, measure thee leakage current, then let it sit for while, leakage
will drop way down, at which point, u increase the vdc again, then repeat the process. 

## It's also not required to suck huge amounts of EQ/bleeder current..and cook the caps, like ameritron does.
Drake used 100 K @ 2 watt eq resistors  back in 1963.  Rich measures uses  100 k @ 3 watt mof's  for EQ. he
hand match's em within 5% tol..and sells em dirt cheap..new.    I get the 1%  tol, 100K 3watt mof's  from
mouser.  Then no more hand matching.  100K across each cap is ample, and the resulting bleeder current
will easily swamp out the leakage current  by > 15:1  ratio.... which is what you want. 

## On the drake amps, the  100 k  eq resistor was mounted several inch's  away from the caps.  Most of
mine were still 100K, even though they were carbon types..and that's from a 1977 amp.   replaced em all with
the newer types anyway.     Never had a bad cap  from the  4 x drake amps I own either.   Drake used 8 x 
200 uf @ 450 vdc axial lead caps. No load B+ is 2650vdc.   2650/3600= .736  So caps are being used  at
74% of their max V rating.  Ripple current through any cap will be aprx 2.56 x dc plate current. 

Jim  VE7RF