Re: [Amps] AL-1200 Pop

[R.Measures <r@somis.org>]

On Jul 6, 2004, at 7:19 PM, Ed Briggs wrote:

> It could be lots of things. But I recently had this problem and it 
> turned
> out to be the tube.
>
> Symptoms:  Pop in standby, at power up.
>                    Visible flash, apparently coming from RF compartment

  I do not see how a defect inside the tube could cause a flash in the 
RF out compartment.

>                    Occationally, damage to B- clamping diodes.
>                    No carbon or other charring
>
> Differential Diagnosis:  Remove tube; apply power.
>                                    Never happened with tube removed

  That shoots down the bad blocker C theory.
> Alternative theories:
>                    Parasitics

 Ed --   The 3CX1200A7 does have twice as much feedback C as an 8877 
and 33% more feedback C than a 3-500Z.  Does this AL-1200 have a 
"neutralization" circuit?
.
>                    Dust, humidity, spiders and other bugs,
>                     finger grease on the tube chimney
>                     bent chimney clips
>                     Cracked or damaged blocking capacitor
>
> Actual Cure:
>             Replacement of the tube.

  If the replacement tube happened to have slightly less VHF-gain than 
the original tube, it would have less of a tendency to regenerate at 
VHF and cause HF tank arcing.  On the other hand, VHF parasites 
normally increase the Ωs (ohms) of the suppressor resistor, but this 
one was reportedly within tolerance.  .
> Apparently, Ameritron has seen this before and replaced the tube w/o 
> charge
> (under warranty). The tube was returned to Ameriton
> for testing and 'tested good' but promptly failed when re-inserted in 
> my
> amp. Ultimately, my tube went back to Eimac.

  My guess is that the tube was not defective, and that it had more 
VHF gain than average.
> (Yes, I know, you shouldn't have seen a flash for a tube arc, but 
> there was
> an arc. It may have been a reflection
> of the step start relay arcing,

  Not likely.  Step-start currents are mild by design. /   cheers

> or perhaps the light of the internal arc
> through the ceramic)
>
> Happy hunting
>
> Ed
>
>
> ----- Original Message -----
> From: "Dana Roode" <K6NR@ARRL.net>
> To: "AMPS" <amps@contesting.com>
> Sent: Sunday, July 04, 2004 9:25 PM
> Subject: [Amps] AL-1200 Pop
>
>
> > I just finished replacing the open frame relay in my old AL-1200 with 
> > the
> > Ameritron relay PCB mod.  Put it all back together, seemed to work 
> > fine,
> > tuning on 40m.  Moved to 160m, had the Amp in Standby while I set the
> output
> > power on the exciter.  Heard a loud Pop - exciter power only.
> >
> > I turned the Amp off for awhile, then tried it again.  It still works
> fine.
> >    I opened it up looking for a visibly bad component, none found.
> >
> > Any ideas what this might have caused a Pop in Standby?  Should I 
> > worry?
> >
> >    Dana
> >
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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