[Amps] issue with AL-1200 output -UPDATE NUMBER 2-

[Steve Bookout]

Hello Merv,

I understand the concept, I just didn't know of any amp mfgr doing that.

That one is toast.  I could easily make one out of some copper I have 
and re-install with a 'just slightly' closer spacing than the plates.

Or, I may just take my chances like I've done for many years.  I do know 
people who DO arc their caps with regularity; I am not one of those guys.

73 de Steve, NR4M


On 6/26/2017 8:48:PM, Merv Schweigert via Amps wrote:
> That small piece formed a arc gap,  it prevents the tuning capacitor 
> from arcing
> under some fault condition of mistuning etc.
> It is suppose to arc there instead of the tuning cap arcing and 
> destroying the plates
> its usually set just a little smaller gap distance than the gap 
> between the cap plates.
>
> You take your chance,  either set it to the correct gap and have a 
> saftey measure,
> or take your chance at arcing the capacitor some time when the amp is 
> mistuned.
>
> If yours arced all the time at 400 watts,  it would appear the spacing 
> was set to small.
>
>> Hello all,
>>
>> Good news, I guess.
>>
>> That mystery board is a sophisticated T/R board assembly that was 
>> introduced many, many years ago.  It still is not documented in the 
>> schematic on the Ameritron page.
>>
>> That aside, I found the problem.  I will try to describe as best I 
>> can, but if anyone wants a pic, let me know.
>>
>> Anyway, on the PI/PI-L input cap, C1, there was an issue.  On the top 
>> of these air variable caps the aluminum blades are fixed (stator.)  
>> The main aluminum frame mounts to the chassis and the movable blades 
>> of the capacitor (rotor) are mounted on a shaft, which, in turn is 
>> mounted to the main capacitor frame.  The fixed blades are mounted on 
>> a two threaded brass rods and separated by aluminum spacers.  These 
>> brass rods, with fixed blades attached are mounted to ceramic 
>> insulators, and these insulators are then attached to the main 
>> frame.  This makes a solid assembly that will allow the rotor to 
>> accurately variably mesh with the stator blades.
>>
>> Now for the issue.  The ends of the threaded brass rods are used to 
>> make mechanical connections to the rest of the plate tank circuit.  
>> The nut is removed and the copper strap, wire, etc is placed over the 
>> end of the brass rod and the nut replaced.  90% of us have probably 
>> seen this.    The front end of one of the rods, had a piece of silver 
>> plated copper strap about 5/16 inch wide x 3/4 inch long, secured on 
>> the shaft.  It went no where. It was positioned coming off the brass 
>> rod in such a manner that it spanned across the ceramic insulator and 
>> was parallel to the fixed alum end of the capacitor.   The way it 
>> was, there was only about .050 of an inch between the silvered copper 
>> strap and the cap body, which is at ground potential.   The RF 
>> voltage in that cap at anything over about 400 watts out was high 
>> enough to arc and keep arcing as long as power was applied.   The 
>> copper strap was significantly melted and cap frame had burn marks 
>> all over it.   I have no idea why that one small piece of copper was 
>> there in the first place.  I could not find any purpose for it.
>>
>> I took it off, fired the amp up and in small increments brought it up 
>> to almost full output, about 1400 watts.
>>
>> Down side is the tube seems 'well worn'.  Maybe the arcing didn't 
>> help.   100 watts in, 700 ma plate and 250 ma grid into a dummy load 
>> = 1400 watts out on 15 meters.
>>
>> Thanks for all the suggestions on getting to the bottom of this.
>>
>> 73 de Steve, NR4M
>>
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