How much capacitance - continued

[Kenneth G. Gordon]

To continue:

If we calculate the capacitance of the original 6 ea 125 MFD 450 
VDC caps in series, ignoring, for the moment, the tolerance which 
was so adequately explained by Tom of Hayseed Hamfest, we 
arrive at a total capacitance of a little more than 21 MFD.

When I recently replaced all the caps in my own SB-200, I checked 
the capacitance of those that were not shorted. The average 
capacitance of the remaining 5 capacitors was 75 MFD.

One, of course, was zero, but I did not include that one in the 
calculations

I replaced those with 6 ea 330 MFC 450 VDC 105 deg C snap-in 
electrolytics.

THEIR total capacitance is 55 MFD, which is a little over twice that 
of the originals.

Rich Measures recommends and sells some very high quality new 
electrolytics specifically for the replacement of the capacitors in 
either the SB-200 or the SB-220. His are 560 MFD at 450 VDC 105 
deg C caps.

Total capacitance using 6 of those is about 93 MFD.

Yet, in both those I used, and in Rich's capacitors, the discharged 
internal resistance is around 10 times HIGHER than the originals.

Therefore, basing the value of our inrush current ONLY on the 
internal resistance of the capacitors ALONE, the inrush current 
using new capacitors SHOULD be LESS than the originals, not 
more.

Of course, there are several other factors which enter into the 
inrush current equation, one of the biggest of which is the 
TRANSFORMER series resistance.

In the SB-220 and the SB-200, Heathkit purposely used 
transformers which are DESIGNED to limit the inrush current to, 
especially, the filament. I do not know for certain that these 
transfomers limit the HV, but I do not see how the same 
transformer can limit the current into one winding without also 
effecting the others.

More later...

Ken Gordon W7EKB

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