Amps
[Top] [All Lists]

[Amps] AL-1200 debug at a distance: we might have a winner (W5GHZ)

To: AMPS@contesting.com
Subject: [Amps] AL-1200 debug at a distance: we might have a winner (W5GHZ)
From: Kevin Normoyle <knormoyle@surfnetusa.com>
Date: Mon, 18 Oct 2010 22:47:34 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
  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

_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>