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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