Heathkit - SB-200 Problem - Help!
K1HL
I found the SB-200 would not tune. Within 30 seconds one of the
572B's blew out and the breakers both tripped. My exciter, when used
without the SB-200, tunes perfectly on 10 - very low SWR, etc. - so
it's not an antenna issue.
It's got me stumped. Built the linear in 1970 and never had a problem
til this.
Tnx - Harry
Tim Wescott
Used to work, doesn't work now -- sounds like a bad switch.
Exciter works fine, amp blows toobs -- sounds like something on the 'big
sparks' end of the amplifier.
No degradation noticed (or mentioned, at least), then suddenly things go
"blam"! -- sounds like a bad switch.
Bad contact on one of the bandswitch segments on the high power side of
the amp? That'd make the tuning be way off, which would load the tube
way wrong, which would make 'that expensive smell'.
Dunno -- things to check.
Tim Shoppa
Had you ever used the SB-200 on 10 meters before?
For a good length of time, the FCC required that amps fail to operate
on 10M (really 11M was the target of the law but 10M got swept into
it) until a specific (available to "hams only" as if anyone else
should have a linear) mod had been applied. I forget when that law
went into effect but could it have been as far back as 1970?
Anyway when 10M opens up, 100W will generally do wonderfully (just my
two cents :-)!)
Tim.
Roger Basford
chance that the neutralisation wasn't set right for Ten and the
amplifier went into self-oscillation, destroying one of the 572s.
Roger/G3VKM
Michael Black
I would have said something like that, but he said "built in 1970" so
unless he's off by about 7 years, it can't be the issue.
I thought the law was still in effect.
The problem was, anything that covered 10meters would cover 11meters,
they being adjacent. The issue was that while CB amplifiers were illegal,
lots of small companies would sell them as ham amplifiers, despite the
relatively low drive requirement (the same level that a CB set would put
out) and often not high power amplifiers. Even forty years ago, one
could look in the Lafayette catalog and see "illegal for Class-D CB"
amplifiers on the CB page, keeping Lafayette out of trouble at the time,
yet practically telling people "this is just what you need to boost your
power output on CB". And of course, since something illegal is about
making money, the amplifiers were often junk, so not only was there the
issue of high power where low power was the norm, but bad signals in
and out of the band.
Michael VE2BVW
Barry
news:qdWdnViKZK5MM-rQ...@brightview.co.uk...
> I'd say, that as it was working fine on the lower bands, there is a
> good chance that the neutralisation wasn't set right for Ten and the
> amplifier went into self-oscillation, destroying one of the 572s.
>
> Roger/G3VKM
Being a cathode driven amplifier, Heath did not use neutralization in the
SB-200. My best bet is a bad switch.
73, Barry WA4VZQ
Art Sowers
SB-200 was grounded-grid. Did not have neutralization (I had a SB-200).
Art Sowers
We need more diagnostics to come up with better suggestions (I read the
other responses, too).
1. How do you "tune" your amplifier? Are you looking at RF output as you
move the plate tuning knob?
2. Like someone else asked: had you ever had it on 10 meters in the past?
3. How did you perceive one of the 572s blow? (flash of light? filament go
out? noise? smoke/smell?).
Look inside, see if you have a cold solder joint on 10 meter position of
bandswitch?
Get one new 572B and restore to test on other bands, first.
Don't wait 30 seconds before you remove drive.
Use less drive (carrier) next time.
Another possibility is that there was some kind of "short" that developed
inside the tube.
Next time check for output on at least one other band before you try 10
meters again.
Kenneth Scharf
SB201 was the re-designed amplifier that covered this. However, being a
kit Heath complied with the law by leaving out the 10 meter switch label
on the front panel and not including in the instructions the steps to
connect the wires to the band switch. All the required parts were
actually in the kit. If you sent the company a photo copy of your ham
ticket (or even a QSL card) they would send you the missing page in the
manual. But it only required a half a brain to figure it out on your own.
Other companies complied with the law in different ways. One linear
only required removing a screw from the band switch that served as a
stop to prevent the amp from going into the 10 meter position! My
Kenwood linear required adding one or two capacitors in the front end
and adjusting a coil. I also had to add a decal to the front panel for
10 meters (I used a dymo label).
Kenneth Scharf
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--
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K1HL
K1HL
Thanks everybody for your ideas. Turns out that the most likely
answer had to do with one of the two 572B's being internally
defective. I ordered another matched pair (goodbye $100) and tried
again; but as suggested, first to confirm it worked on 15 and 20,
which it did. Then, with great trepidation went to ten meters, this
time keeping the exciter at a low output. Tuned quickly, never more
than 5-10 secs before giving it a rest. Waddya know, the darn thing
tuned just like it does on the other bands. Began to raise power,
watched the plates for cherry - everything good with output to about
400W. Since the incident I have carefully etched fine pencil marks on
loading and plate tuning so I can go back to resonance very quickly.
Even did same on the exciter.
Sometimes you never know exactly what happened, but Art Sowers hit it
right. Amp with previous set of tubes always ran on 10M. When it
failed filament went out, breakers tripped, flash of light, pop of
noise - probably bad tube that worked ok at lower frequencies.
Thanks again!!
Harry K1HL
Kenneth Scharf
them or what label is on the bottle, they are all made in the same
factory in China today. I suppose some of the importers may test them
better and have in their contract that they can return tubes that fail
their tests to the factory for credit, so where you buy them from might
make a difference, but you are still getting the same tubes.
If you are REAL lucky you might still be able to find some NOS US made
572B's, but they won't be cheap. The Svetlana 572B's are NOT the same
spec's as the ones used in most ham linears. They are AUDIO tubes!
They will work in a ham linear but you must keep the plate voltage at
2000 or less, maybe apply some bias, and be happy with less power.