On Fri, 12 Dec 2003 15:52:49 UTC, (LJ)
wrote:

No Spam (ckh) wrote in message news:ifgU75G3LLdo-pn2-fsI6Eom2wLsY@localhost...
On Wed, 10 Dec 2003 18:37:25 UTC, (LJ)
wrote:

I recently picked up a used Heathkit SB-200 Linear Amplifier. I tried
it out last night for the first time - on 20-Meters. Power out was a
little more than 700 watts, worked fine.

I then tried it on 10-Meters, and looking at the Drake W-4 Wattmeter
on the 2000W output scale, it was pegged! Looking at the reverse
scale, it was pegged on the 200W reflected scale. This tells me that
the amp was probably going into oscillation.

I wasn't too surprised to see this, as I have an SB-220 that just
spits and arcs continuously - it's unusable on 10-Meters (other bands
are okay). I haven't had a chance to install the Harbrach suppressor
kit yet, which I hope will fix that problem (it puts out over 1300W
output on 20 Meters, no problem, so the amp does work.)

I decided to then try 40-Meters on the SB-200, figuring that if
20-Meters worked OK, 40 should be okay, too. Wrong - the same thing
happened - nice 50 ohm load, keying the amp while driving with 60-100
watts, the wattmeter is pegged again - forward and reflected.

I can see why 10-Meters would have a problem, but with 20-Meters
working fine, I'm surprised that 40 would have a problem. I haven't
tried it on 80 and 15 Meters yet, I'll try that tonight.

Any suggestions?

Thanks & 73,

Larry K7LJ

Pretty weird. Unless the watt meter is VERY sensitive at the
frequency of the parasitic, there is something else wrong. No way a
couple 572B's are putting out 2,000 watts

OK, I thought about it and it makes sense. It's oscillating at
some frequency, maybe up about 50 mHz. At that frequency your
antenna exhibits very high SWR. Even if it's only putting out
400-500 watts, the high SWR is pegging the meter on forward and
reflected.

de ah6gi/4
--

Yes, axactly. But why is it doing it on bands like 40 and 10 Meters, but not on 20?

Very strange...

wave a frequency counter around it. Maybe it is generating most of
the power around 42 mHz. At that frequency, a 20 meter antenna
"might" have a low enough VSWR to give good readings.

Also, the antenna is "part" of the tank circuit. A 20 meter
antenna might not lead the amp to oscillate.

Thing to do is search for the SB-200 mods on the web. There are
several nice write ups on parasitic surpression. Also make sure
that the various resistors are still within spec. I've found
resistors waaaaay off value in my SB-200.

I'm betting that you find a fifty cent part that's gone bad or a bad
solder joint.

Good Luck!!!!

de ah6gi/4