[NLRS] SMD 10mfd LEAKY

[Dr. Gerald N. Johnson]

On Sun, 2006-05-21 at 10:51 -0500, Bill K0AWU wrote:
> 
> GM Guys
> 
>    My 10GHz tower system is built around a Kuhne 10G transverter that I 
> bought used about 2 years ago. (10mw)
> 
>   Ever since I purchased the unit it has had a -5V DC buss problem. 
> However the unit worked well none the less, heard well and I still had to use 
> a pad between it and the diver amplifier.
> 
>   Consulting the prints, the previous owner had observed the same problem 
> and bias had been readjusted for operation.
> 
>   Friday I started scraping off black paint that had covered all components on 
> the -5V board, that is separate from the main board. That allowed me for the 
> first time to identify the parts involved. Having the parts values/identity 
> allowed me to trouble shoot the board removed from the transverter.  I had 
> discovered earlier that one SMD was temp sensitive. That device ended up 
> being a 10Mfd 16V SMD.
> 
>   The cap was/ is VERY LEAKY ... it was replaced and all is well.
> 
>   My question.... how common is this kind of failure?? This is "nasty" .. This 
> device on the bench PS leaks 700ma at 5VDC .... HAD the -5V buss been 
> more robust the "burn marks on the board" would point to the failure!!
> 
> Bill  K0AWU  EN37ed

Miniature electrolytics have never been very good and some products from
China have been very bad causing a couple of years of computer mother
boards to go down in flames.

A few production runs of TNCs lost the electrolytics in the -5v circuit
because the assemblers couldn't get away from grounding the negative pin
and so for a year or two installed all with reversed polarity which lead
to an early demise.

Point is, be sure to check the polarity of the circuit and the
electrolytic to be sure they match. Electrolytics will work with
reversed polarity but only at low voltage and your high leakage is
characteristic of being reversed.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer