[Amps] AL-1200 Pop

[R.Measures]

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=20
> turned
> out to be the tube.
>
> Symptoms:  Pop in standby, at power up.
>                    Visible flash, apparently coming from RF =
compartment

=F0  I do not see how a defect inside the tube could cause a flash in =
the=20
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

=F0  That shoots down the bad blocker C theory.
>
> Alternative theories:
>                    Parasitics

=F0 Ed --   The 3CX1200A7 does have twice as much feedback C as an 8877=20=

and 33% more feedback C than a 3-500Z.  Does this AL-1200 have a=20
"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.

=F0  If the replacement tube happened to have slightly less VHF-gain =
than=20
the original tube, it would have less of a tendency to regenerate at=20
VHF and cause HF tank arcing.  On the other hand, VHF parasites=20
normally increase the =BDs (ohms) of the suppressor resistor, but this=20=

one was reportedly within tolerance.  .
>
> Apparently, Ameritron has seen this before and replaced the tube w/o=20=

> charge
> (under warranty). The tube was returned to Ameriton
> for testing and 'tested good' but promptly failed when re-inserted in=20=

> my
> amp. Ultimately, my tube went back to Eimac.

=F0  My guess is that the tube was not defective, and that it had more=20=

VHF gain than average.
>
> (Yes, I know, you shouldn't have seen a flash for a tube arc, but=20
> there was
> an arc. It may have been a reflection
> of the step start relay arcing,

=F0  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 at ARRL.net>
> To: "AMPS" <amps at 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=20=

>> the
>> Ameritron relay PCB mod.  Put it all back together, seemed to work=20
>> 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=20
>> worry?
>>
>>    Dana
>>
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>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org