Electrolytic caps for SB 220 pwr supply

[John Godfrey <KE5NZY>]

John,

                My memory on the two SB-220’s I have recapped, is that the originals are 1.4 inches in dia. The ones in the link are 35mm which if you get real technical are 3 hundredths of an inch smaller, but who’s counting. And yes I did install them in the original plastic mounting block, and I used a drop of epoxy to hold them in the way I liked them. I recessed them just a tiny bit so the bleeder/equalizing resisters would stay cooler. I also replaced them with 100K 3 watt resisters based on Richard measures statement from his website…

 

Quote from measures webpage:

Premature Filter-Capacitor Failure

Aluminum electrolytic filter-capacitors are very sensitive to heat. For every 10ºC increase above room temperature, the life expectancy of the capacitor is approximately cut in half.

 

The electrolytic filter-capacitors in the SB-220 are subjected to a high degree of heat during normal operation. The major source of this heat is from the eight, 30k-ohms, voltage equalizer resistors that are adjacent to the eight filter-capacitors. During transmit, a minor source of capacitor heating, results from the 60Hz ripple-current flowing through the capacitor's internal resistance.

 

The capacitor heating problem is compounded because cooling air does not reach the capacitors due to the molded plastic capacitor holders that surround and insulate each capacitor. In some cases, the heat present will partially melt the ends of the capacitor holders that are nearest to the 30k-ohms resistors.

 

The heat that is dissipated can be reduced by c.70% if 100k-ohms, 3W, metal oxide film resistors are used to replace the original 30k-ohms resistors. This simple modification will prolong the life of the (8) electrolytic filter-capacitors.

 

Other resistance values may be used, up to roughly 150k-ohms, provided that the resistors can withstand the voltage that is applied to them and the resistances are within ±5% of each other. I do not recommend using ancient-type 2W carbon-composition resistors for this application. They do not maintain their rated resistance tolerance with age.

 

Richard has a lot of information on the SB-220 and is a really good read in my opinion. I have dealt with Richard on several occasions and found him to be honest. His website can be found at http://www.somis.org

 

 I hope this helps and that your project goes well.

73 de KE5NZY John Godfrey

 

 

From: John King [mailto:k5pgw at yahoo.com] 
Sent: Saturday, December 05, 2009 9:48 PM
To: John Godfrey <KE5NZY> 
Subject: Re: Electrolytic caps for SB 220 pwr supply

 

John, thanks for the information. Will these caps fit in the plastic block in the SB 220, in place of the original caps? I have an SB 220 and a SB 221 as well as an Amp Supply LK 500 ZA amp. My SB 220 capacitors are hissing and need to be replaced. Have you put these caps in an SB 220 and did you use the plastic mounting block?  73, John, K5PGW

 


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