The zeners are more than sufficient for that amp, just replace them and then
try to find why they shorted. Youre not out of the woods yet with the tube.
The shorted zener would still allow idle current and act as a zero biased
tube.
Also measure the safety diode on the edge of the PS board.
Carl
KM1H
----- Original Message -----
From: "Kevin Normoyle" <knormoyle@surfnetusa.com>
To: <AMPS@contesting.com>
Sent: Tuesday, October 19, 2010 1:47 AM
Subject: [Amps] AL-1200 debug at a distance: we might have a winner (W5GHZ)
> 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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