Neither did I, which I why I spent a month screwing around with everything
else imaginable. Finally decided to replace the tube, problem went away. Put
the old tube back, problem came back. Incidentally, it took a little time
for this to happen - in other intermittent. But after being able to repro
this several times, back and forth over the course of two months, it's
fixed.
I actually went through all the interior surfaces with high power
magnification looking for charring, insulation breakdown, etc. but the
problem would *never* occur if I had no tube in the socket (just high v
applied with no tube)
I can only speculate about possible reasons for a flash in the rf
compartment a) it wasn't really in the rf compartment but was a reflection
of the arc in the soft start relay. The interior of the AL-1200 is such that
this is at least possible (shiny metal surfaces, cutout leading to the area
of the relay etc.) b) something, somehow permitted the arc to occur on the
outside of the tube (defective ceramic surface?)
I should add that Ameritron was very helpful, but there is a limit to what
you can do over the phone with an intermittent problem.
When I originally posted to this DL about my problems, I received a flurry
of emails from people who had encountered similar problems; some had them
resolved by changing the tube, some by cleaning the RF compartment.
I mention it here because it might help someone who is pulling out hair,
changing meter protection diodes and mains fuses.
Ed
----- Original Message -----
From: "R. Measures" <r@somis.org>
To: "Ed Briggs" <edbriggs@optonline.net>
Cc: "Dana Roode" <K6NR@ARRL.net>; "AMPS" <amps@contesting.com>
Sent: Wednesday, July 07, 2004 9:53 AM
Subject: Re: [Amps] AL-1200 Pop
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
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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