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Re: [Amps] AL-80 ... POW!

To: <k8bb@comcast.net>
Subject: Re: [Amps] AL-80 ... POW!
From: R.Measures <r@somis.org>
Date: Sun, 27 Feb 2005 10:42:51 -0800
List-post: <mailto:amps@contesting.com>
On Feb 26, 2005, at 9:21 PM, <k8bb@comcast.net> wrote:

> I have an AL-80 (not an A or a B), S/N 808. I was attempting to load 
> it on
> 17m in the 15m band position. (In retrospect, I know I should have 
> known
> better than even to have tried this ... )

17m had nothing to do with the fireworks unless the top of the HV-RFC 
has a burned band of turns.
>
> I tuned it to about 250w out, but with almost 50w drive. SWR was 
> around 1.5.
> IG read around 100ma. I switched the meter to IP, just to check. IP 
> read
> just over 300ma, then it made this sound: fzzzt - POP!!! I immediately
> turned the amp off.
>
> I didn't really smell anything particularly bad, but maybe just a 
> little
> bit. And I vaguely recall maybe seeing a white blue/white flash 
> through the
> (added) cooling holes. I also noticed that when I switched the meter 
> into
> the IG position (amp off), it (gently) read negative full-scale. After
> several minutes, it read zero. I put the meter in the V position and
> switched it back on. It went, "Fzzzt." (not loud) and the meter jumped
> around a little bit, but did not register appropriate voltage. I 
> turned it
> off after about one second.

> I opened it up and I see these things:
> - There is a little "spot" (looks like damage?) on the plate choke, 
> but it
> still has continuity (about 2.4 ohms).

Remove a few turns at the top, strip the wire with a butane lighter, 
steel-wool it, and resolder.
If you have a dip-meter, check for HF resonances before "firing up".

> - R6, a 1.5 ohm, 3W, 1% wire-wound-looking resistor has a hole burned 
> in it
> and I can see the little wire coil inside. This resistor is listed in 
> the
> schematic as being for IG metering.

If the R is okay, it will still fly.  Measuring grid-I in a g-g amp is 
of dubious value since the grid-I meter has no role in tune-up, 
However, the grid-I meter shunt R is needed to carry grid current..
>
> What do I do next? I cannot imagine that the POW (that sounded like it 
> came
> from the RF compartment, not the HV supply) could have been the 
> resistor
> burning up.

I have seen the top of a HV-RFC arc to the cover of the output 
compartment during a choke fire.

> Coincidence? Is it possible that the Plate Choke arced without
> seriously damaging itself? Or is it indeed damaged?

Burned insulation is not a good situation.

> And hy did the amp
> continue to "Fzzzt." when I turned it back on, very briefly? What other
> "usual suspects" should I look at? I'd be glad to try to take a 
> picture of
> the PC if anybody is interested in trying to determine if it is NFG. 
> Where
> would I get a replacement?

They are easy to fix.
  I would add a 10-ohm. 10w glitch resistor to the HV +, plus I would 
put 3a glitch diodes across the meter movements.  Better suppressors 
might be a good idea.  [ ref:  http://www.somis.org/bandsw.html ]
>
> Please be gentle ... I am learning ...
>
> Don Chisholm
> K8BB
> Pontiac, MI
>
> P.S. Before posting this question, I searched the archives pretty 
> thoroughly
> and I learned that this little gem is not exactly the Zenith of 
> amplifiers,
> as far as design and reliability are concerned, and I didn't really 
> find any
> troubleshooting advice for this particular model. Instead, I found a 
> lot of
> public (though very personal, at times) debate as to what exactly 
> might have
> caused this. I am not particularly interested in exactly what caused 
> this,
> just how to repair it - I can take care of preventing it on my own.
>
>
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>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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