[Johnson] Invader 200 Power Supply components question.

[Normiehall@aol.com]

In a message dated 02/20/2003 1:07:02 PM Pacific Standard Time, 
bbc39668@yahoo.com writes:


> It seems that 60uf in that voltage is not available
> the closest I can find is either 75uf and 47uf. I am
> wondering if I can use 75uf with out stressing out the
> transformer and choke. I'm dont think the 47uf will be
> large enough but not sure.
> I really want to avoid using 2 caps series or parallel
> to get 60 uf as I would like to swap all 3 of the
> 60's. Has anyone repelaced them in the invader or been
> in a similar situation?
> Thanks again for any help
> Joe
> 

Hi again Joe,

I don't have the Johnson Invader 200, but do have the manual for the Invader 
2000 in my computer.  I think it is just the 200 with an added high power 
amplifier.  Anyway, I looked at the power supply for the exciter (the 200 
part) and there are three 60 Mfd @ 450 VDC capacitors in the HV part.  It has 
a choke input filter before you get to the caps.  The three caps are in 
series with each paralleled with an equalizing resistor.  Either the 47 Mfd 
or the 75 Mfd would work.  Since you have choke input, this would limit any 
inrush current to the caps and thus not stress the power supply.  You'd just 
get better filtering with the 75 Mfd caps.  The other thing to remember is 
that electrolytic caps have a very wide tolerance range and so you may not 
get the value found on the cap anyway.  I don't have one handy but I think 
they are something like +80 or -20% of specified value.  So it really isn't 
critical at all.

Be sure the equalizing resistors are of equal value since this affects how 
the voltage is split among the three caps.  If one should be high out of 
tolerance that cap would get a much higher portion of the B+ voltage applied 
to all three.

Good luck & 73,

Norm Hall, W6JOD



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