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

[Carl]

It is just an example of a Hammy Hambone approach to a problem for an amp 
that literally took years to correct various stability problems.

Carl



----- Original Message ----- 
From: "Steve Bookout" <steve at nr4m.com>
To: <amps at contesting.com>
Sent: Monday, June 26, 2017 9:19 PM
Subject: Re: [Amps] issue with AL-1200 output -UPDATE NUMBER 2-


> 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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>>>
>>
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