Hi Roger,
Fan is good. I can beleive the overtemp case because the amp spent a bit of
time in the shack of a guy who is known for testing amps to destruction in
contest and then a short stint at a guy who runs rag-chew AM. What suprises
me is that I don't see more evidence of damage. Overall the bandswitch
looks fantastic. No signs of arc anywhere that I could get my dental mirror
down into anyway.
In doing a bit of band testing, it looks like there is a low output problem
specific to 21 Mhz. I suspect the choke is fried on the bottom end from use
on 12m. Think I have an MFJ AL1500 plate choke in the junkbox as a spare
and so if that's the problem, the solution is "at hand."
And for the seals, I should have been more specific in my choice of words.
The tubes hi-pot ok so there's not been a tube hermitic seal breach.
I was talking about the silicon rubber boots that serve as the air duct
shrouds on each tube. Looks like the amp is around 23 years old based on
the cap date codes. Not sure if you could get this delamination of the
silicon rubber onto the tube in this manner absent higher temps or not.
Just thought the missing solder on the plate clips was a pretty funny find.
73/jeff/ac0c
www.ac0c.com
--------------------------------------------------
From: "Roger" <sub1@rogerhalstead.com>
Sent: Thursday, September 16, 2010 12:29 AM
To: <amps@contesting.com>
Subject: Re: [Amps] Famous Alpha plate clip desoldering issue
>
>
> On 9/16/2010 1:01 AM, Jeff Blaine wrote:
>> I picked up an old 76pa which seems to be in otherwise FB condition.
>> 2400v idle on the plate cooking away those pipes who have not been used
>> in a couple of years prior to some initial power testing tomorrow.
>>
>> Only strange thing I noticed on the inspection was that every single one
>> of the braid connections to the plate cap had delaminated. The braid is
>> tarnished so I assume it's silver plated - hard to tell the condition of
>> the solder connection otherwise. But the braid was lose enough that the
>> cap could be removed from the braid just by bending the braid slightly.
>>
>> I got to thinking about what could have caused that. Came down to one of
>> three possibilities:
>>
>> 1. The famous alpha plate clip desoldering issue, known world-wide to all
>> Alpha owners but not to nubees who expect these things to be soldered in
>> the traditional way.
> I've owned a 76PA for 8 or 9 years and never had such a problem
>> Not to worry though because....
>>
>> 2. The amp creates a virtual short when keyed down due to it's massive
> Massive out of a 76? It is a lot of power out of small tubes though.
>> high power output only to have the joint break again with key up. A
>> technology known only to Alpha and is top secret such that RF Concepts
>> was unable to dig the secret from the ETO data archives. The secret
>> sauce lost for all time.
>>
>> 3. OR - The dogs were whipped mercilessly at one time, so hard that their
>> anode temp rose high enough to desolder, AKA SB220 pins can sometimes do.
>> Say it's not so!
> Then there is the possibility of the fan failing. What do the sockets
> look like?
>> Hoping some of the learned 76 owners may have some insight.
>>
>> There was some what appeared to be "melted" gasket material visible on
>> each tube.
> Gasket material? The only gasket material in mine is the silicone rubber
> chimneys and those are not likely to melt.
>> Maybe 1/4 in long and a tiny fraction of an inch in height and depth.
>> I say melted because it was stuck firmly to the tube and rounded and
>> shiny on the exposed surface.
> Where on the tube. Anode, base...?
>> Interesting because the sealing material will break with age but in
>> that case it generally has a rough and "flat" appearance.
> I've never had the seals fail on any of the 8873, 8874, 8875, or 8877
> tubes.
>
> 73 and good luck,
>
> Roger (K8RI)
>> 73/jeff/ac0c
>> www.ac0c.com
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