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

Tue Oct 19 10:32:45 PDT 2010

Fortunately that tube has a fairly robust grid so short periods of drive  
without plate voltage probably will not damage the tube. Prolonged operation  
most certainly would damage the tube. We are talking about meter pegging 
grid  current here so it would be quite noticeable to the operator.

73,
Gerald K5GW

In a message dated 10/19/2010 9:32:10 A.M. Central Daylight Time,  
w5ghz at yahoo.com writes:

Gerald,

Wouldn't that cause the grid to fail rather  quickly?

Hal W5GHZ

--- On Tue, 10/19/10, TexasRF at aol.com  <TexasRF at aol.com> wrote:

From: TexasRF at aol.com  <TexasRF at aol.com>
Subject: Re: [Amps] AL-1200 debug at a distance: we  might have a 
winner(W5GHZ)
To: km1h at jeremy.mv.com, w5ghz at yahoo.com,  amps at contesting.com
Date: Tuesday, October 19, 2010, 8:38  AM

Carl and all, if the plate choke opened, there would be no  plate voltage 
or current alright. But, with drive, the grid current would be  there in 
abundance.

73,
Gerald  K5GW

In a message dated 10/19/2010  8:25:37 A.M. Central Daylight Time, 
km1h at jeremy.mv.com writes:
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 at yahoo.com>
To:  <amps at 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 at surfnetusa.com> wrote:

From: Kevin Normoyle  <knormoyle at surfnetusa.com>
Subject: [Amps] AL-1200 debug at a  distance: we might have a winner (W5GHZ)
To: AMPS at 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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