SB-200 Problem

[Multi-Volti]

Bleeders do two things...they sort of improve the regulation at the expense
of dropping the unloaded B+ supply...a tradeoff...by having a fixed load it
keeps the B+ from swinging as wildly between no load and full load...now
only swings wildly from partial load to full load..I seem to recall a rule
of thumb to have about 10% of full load in the bleeders.

The second function is where the name 'bleeder resistor' comes from...it
(they) 'bleed' off the charge after the ac is turned off.

It should be obvious that removing them makes the amp less safe when you
have the cover removed.

But wait!  I just remembered...they are not just bleeder resistors. They are
also equalizing resistors! If you have a  series string of electrolytic caps
with a wide tolerance, the voltage will divide in proportion to the
capacitance...if one is +20% and one is -20%, there will be a significant
difference in voltage across the higher capacitance, possibly overstressing
the cap.

SO, you do want equal value equalizing resistors that happen to also serve
as bleeder resistors across series capacitor strings.
Don't sweepingly dismiss their use in analogy with equalizing resistors that
people used to use in high voltage rectifier strings...back when solid state
diodes were kind of crappy and each one batch have inconsistent PIV rating.

Another thought ... if you had caps that were essentially shorted, the
remaining caps had to withstand the additional voltage.
For best longevity, of the amp and operator, I'd replace the full set of
caps and resistors...do it once, do it right, and you'll save money in the
long run.

I don't know if you can get wirewound resistors that high in value, but they
would be more stable if selected carefully for power rating. I just heard a
figure of thumb to not exceed 40% of their power rating for high reliability
(this is all relative).

Murray

----- Original Message -----
From: rayfri <rayfri at NETWORLD.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Friday, February 14, 2003 8:03 PM
Subject: Re: SB-200 Problem


> Carlos:
> If you cant find reasonably priced bleeders, I wouldnt worry about it.
They
> really arent that critical a component.  When I fixed the sb 200 for my
friend,
> I did not measure or replace the bleeders at all.  I was talking to him
last
> night, and he informed
> me that the sb 200 we worked on is putting out 600 watts without breaking
a
> sweat.
> Ray
>
>
> "Carlos Avellanet [kf4rrn]" wrote:
>
> > Hi gang, thanks for all the inputs on my SB-200 problem specially to
Ray,
> > Mark, David, and Ed.   I was off work today and had time to play.  Three
of
> > the 6 power supply capacitors were dead shorts.  I checked all of the
diodes
> > and they are all good.  The six bleeder resistors however, range from 17
to
> > 23K.  No a single one close to the 30K rating.  I am going to do a
search on
> > the net next to try to find some replacements.  The caps I already
ordered
> > from Bill.
> >
> > I'll keep you all posted in hopes that my experience will benefit others
in
> > the same boat.
> >
> > Thanks,
> > Carlos KF4RRN
> >
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