[Amps] RE: Troubleshooting a SB-200

[Mike Baker]

Hello Steve,
    Having just spent/and am continuing to spend time working on the newer
brother to your amp, a SB-201, here are my thoughts.
    Your key up to key down high voltage seems a bit low especially with
only 285ma. of plate current..
    With new caps the amp I am working on runs about 2400 key up and 2100
key down at full load of 500ma.  That nets me about 750 watts output on my
meters (internal to my MFJ tuner).  Grid drive is in the middle of the white
left half of the multimeter.
    You didn't mention what rig you are driving it with.  I would bet it is
one with a solid state finals.  If your drive is dropping down to 75 watts
indicated from 120 watts the input is probably a bit too reactive. (this is
quite normal because the Q is very low on the input circuit so it doesn't
match up too well to some solid state drivers.)  If you have a small tuner
to use on the input that will likely fix the drop in drive but so might a
change in the length of the cable from the rig to the amp.
    I tend to agree with Carl, it sounds like only one tube is working.  Use
his method of elimination to see if there is much change from one tube to
another.  Just because they light up doesn't mean there is any emission left
in both tubes.  One could just be flat. One other thing to check if you have
access to either a MFJ 259B analyzer or a LRC meter is to check the .02mf
caps in the fill/cathode coupling circuit.  There is a cap across the two
legs of the filament and another that is used to couple drive to the
cathode/filament circuit from the input matching network.  It is connected
between pin 2 and 3 of V2 (the tube on the left side when viewed from the
back nearest the center of the amp.)  It is unlikely that one or the other
has gone bad but not impossible so if you can, check or replace them.  Try
the tube switch first.
    While you are "under the hood" so to speak, take a close look at the PC
choke resistors in the plate circuit.  Good time to take a very close check
with a magnifying glass and look for damage.  If one or both show damage
consider the measures PC choke kit or similar.  It is cheep insurance for
the new tubes you may be installing.  If your line voltage is not dropping
too much the I would take a close look at the power supply caps.  By now
most of the amps in that age group will likely need to have the filter caps
replaced.  Check out the Harbaugh parts web site for a drop in board.  While
you are there, consider the soft key mod board so your newer solid state
rigs can key the amp with very low current and voltage across the terminals.
And did I mention installing a glitch resistor to the HV line going to the
plate circuit??
How smooth is the cooling fan?  Check it for operation and consider giving
it a small drop of lube.  Don't over due it.  One drop a year is plenty.
Too much just makes a mess.  Check the TR relay for contact damage.  If it
looks clean and not badly pitted and arced then you are probably ok there.
    If you haven't read Rich Measures papers on the Heathkit SB-220 amps and
others, check out his web page and take some time to read and digest some of
the information.  Why reinvent the wheel if someone has already spent a lot
of time traveling the road you may eventually end up on and has solutions.
    Don't get disgusted with your amp.  I have no ideal how many of those
little green boxes there are out there still working like the day they were
built, but I am sure the numbers are still high.  They are the RF equivalent
of the Energizer Bunny; they just keep going and going and going.....Cheep
to buy and easy to maintain and upgrade. They were built and designed to be
built by people with simple hand tools and basic test equipment.  Rigs you
could get your hands down into and work on without a 30X magnifier lens and
a anti-static mat.  Parts you can find pretty easily most of the time, not
to mention a large base of folks who have spent lots of enjoyable hours
building and using their amps by Heathkit.  Sort of like a vintage VW! ;>)
Keep us posted on your results and needs.  This is a very helpful group of
folks.

Best 73
Mike Baker  K7DD
k7ddmjb@qwest.net