If the arc was one of Ameritrons EID events the plate choke may have opened,
a fairly common occurance as the tube is prone to parasitics. Also check the
safety choke on the back of the Tune cap.
Carl
KM1H
----- Original Message -----
From: "Hal W5GHZ" <w5ghz@yahoo.com>
To: <amps@contesting.com>
Sent: Tuesday, October 19, 2010 3:07 AM
Subject: Re: [Amps] AL-1200 debug at a distance: we might have a
winner(W5GHZ)
It seems to be a common failure mode, Kevin. That arcing was most likely
what you heard. If you can, clean up the mark left by the arc with some very
fine wet/dry sand paper or emory cloth, about 800 grit. It should polish the
metal, not scratch it. That will lessen the chance of another arc in the
same place. An arc always leaves a carbon path and that leaves a low
resistance (relative) path for another arc to occur. If the arc isn't too
deep, you can often clean it up.
Hal
--- On Tue, 10/19/10, Kevin Normoyle <knormoyle@surfnetusa.com> wrote:
From: Kevin Normoyle <knormoyle@surfnetusa.com>
Subject: [Amps] AL-1200 debug at a distance: we might have a winner (W5GHZ)
To: AMPS@contesting.com
Date: Tuesday, October 19, 2010, 12:47 AM
Hal W5GHZ suggested: "Check out the two Zener diodes above the filament
transformer mounted on the wall.
The high duty cycle may have burned out one or both."
Other people seemed to point in the same direction, as they suspected I was
saying "no grid or plate current" when
keyed. (which was true: zero.)
I unconnected the filament transformer and did some measurements.
The AL-1200 schematics apparently aren't up to date in the manual online. I
have a 2002 vintage rig, and the transmit
relay was changed then. That new little board requires 14v and 28v so they
changed the full wave rectifier shown in
their meter board schematic to something with two diodes that gives 14v and
28v (plus two 2200 uf electrolytics).
At first I thought those two little diodes must be what I was looking for.
(not! although output from there serves to
hold in one of the two older style relays on power on).
I also checked all the relays for movement and they seemed good. You can't
see the new transmit relay, but I could feel
it clicking softly.
The two zener's are stud mounted with insulators on the mid wall, I guess
for heat dissipation. The back side is right
by the tuning cap. I didn't want to mention that I heard a buzzing in the
amp and backed off on the drive at one point.
But the amp didn't die then, so I thought that wasn't a contributor to when
it failed (looks like there's a little
arcing on the brass tab that protects arcing in the tuning cap though..I'm
wondering if that's what I heard, although it
kind of looks old).
Last year we had a long discussion about AL1200 and rtty and temp and I had
done a lot of measurements on temp rise (in
my setup) while CQing for a half hour straight into a dummy load, with temp
probes so I could gather and plot temp rise
data. I was confident I wasn't going to cook the tube from that data. (for
my cooling situation).
So back to the zeners. They don't seem mounted in the best place for heat
dissipation. Maybe they actually get hotter
because of where they are mounted! They had very dried out thermal compound
on the insulators.
The first zener is shorted by the CW/SSB switch. The second zener is in
series with that one. So if the second one
opens, you're dead for sure? (no bias?)
So I measured the voltages, and the second one was definitely open. The
first one at first I guessed was okay since it
measured 5.6v.
Looking at the current Ameritron parts list on
http://www.ameritron.com/Product.php?productid=AL-1200
they list them as 10W diodes (if mounted with low thermal resistance)
DIODE, ZENER, 7.5V, 1N2971A, DO-4,10W, STUB MOUNT
The 5.6v on that first zener dropped to .8v when the amp was keyed, so I
suspected that Zener was bad somehow also.
I totally removed the zeners to measure to be sure.
The second one was an open in both directions.
The first one was 360 ohms in both directions.
So both were bad, but in different ways? (one open, one semi-shorted)
I won't know for sure that this is the problem till I get replacements, but
this is encouraging.
So maybe Hal doesn't get full points till we know for sure.
Thanks everyone! It'll be great if this is the problem (and not a $1300 tube
replace)
Hey: any suggestions on a better replacement than the 1N2971A?
Should I get a better zener diode manufacturer than what Ameritron sells?
Maybe just better attention to the thermal
mounting?
Be interesting to know how common this is.
-kevin
AD6Z
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