[Amps] PT-2500A and toroid stack

[Carl]

The toroids could have been damaged and they are the standard T225-2A of 
which I have plenty a lot cheaper than Amidon.
Send me a private email if interested.

Carl
KM1H


----- Original Message ----- 
From: "bob" <n612dw at gmail.com>
To: <amps at contesting.com>
Sent: Sunday, December 09, 2012 3:56 PM
Subject: [Amps] PT-2500A and toroid stack


>A tome on a PT-2500A (2x 3-500) -seeking advice
>
>
>
> I am the owner of a late model PT2500A (S/N 233) which I bought out of an
> estate last winter.  It never even had it's tubes put in place as it still
> had the original shipping strings tying up the terminal strip and choke in
> place from the factory and no scratches etc from the xfmr ever being
> installed.  Was in storage since bought new.  Truly a brand new vintage 
> amp
> - replaced all the bleeders with new (better and higher values), slowly
> formed up the caps, got the tubes glowing red for hours without lots of HV
> (trying to getter them).  cool... life is good... been using it on and off
> for a few months - quite happy.  Ya gotta love the instant-on feature of 
> the
> 3-500's compared to the 3 minute warm up time of the ceramics I have in 
> the
> other amps.
>
>
>
> however, during the last cw contest I casually operated in, it went up in
> smoke when tuning up on 80 meters (one of the few times), typically I run 
> it
> at about 1000-1200 watts out with about 75-90 watts in which is more than
> sufficient without beating it. More drive and it readily puts out more.
>
>
>
> The 160 and 80 mtr L coils are toroidal around a couple of stacked sets of
> beefy toriods (2.25"x1"h cores with a 3x stack and a 2x stack 
> respectively)
> cores.
>
>
>
> The teflon tubing around the ends of the toriod core migrated thru and let
> it arc to the toroid cores. Lots of good smoke came out - I couldnt catch 
> it
> and stuff it back in.
>
>
>
> post-mortem, this seems like an assembly defect from the factory, since 
> the
> screw holding the entire stack of 5 cores together on the end caps was
> tighted so tight on the fiberglass rod going through the middle that the
> resulting physical compression caused the migration of the magnet wire
> through the teflon tubing at the ends the stack.  While one or two turns
> were burned through, there were others where it has almost completely
> migrated thru the teflon insulating tubing and would have been a failure
> there soon as well.  Was just a matter of time.
>
>
>
> Man, what a PITA to get this assy out of the amp !!  Anyway, now I have to
> come up with  a fix.  Obviously, I will correct the compression issue when
> it gets re-installed (trivial, just machine some washers etc so the 
> majority
> of the compression is on the rod, not the stack ends).  Cost isnt an 
> issue,
> as I prefer a quality fix than the cheapest.
>
>
>
> Clearly the coils have to be rewound, but I have some questions that I 
> seek
> answers and informed opinion from the "Oracles of Amps".  I havent had to
> repair a toriod stack like this before.
>
>
>
> 1- Can the toroid cores be re-used?  There is the one that arced, and
> another looks slightly discolored (darker red) from the normal Red color 
> of
> the others (but it could have been that way since new - dont recall).  Can
> the arc itself cause a change in permeability? I believe heat can.  In a
> nutshell, can I just clean 'em up and re-use? or should I just go new?
>
>
>
> 2- if I have to replace them, how can I be sure of the same composition?
> They are Red t225 x 1.4" ID cores 1" in height, but I dont know the 
> material
> composition.  I can readily find T225-2B cores - would this be correct?
> What is the composition of the existing - anyone know? There are two 
> coils,
> one is a 2 stack and the other is a 3 stack core.
>
>
>
> 3- where can I find some good teflon tubing that has a good thickness? 
> much
> of what I can find is very thin stuff.  Has to fit over #12 magnet wire.
>
>
>
> 4.- I was thinking of winding some fiberglass dielectric insulating tape
> around the stack to add some add'l dielectric strength between the cores 
> and
> the windings. Will this be a problem in terms of containing heat?
>
>
>
> 5. - since I have to rewind it, anyone have a good source for high quality
> magnet wire?
>
>
>
> 6. - measuring what I have prior to taking it apart is seems a good idea,
> since then I can match up the L values prior to inserting it back into the
> cramped RF deck.   what is the best approach to doing so?   I thought I
> would use my AIM 4170 and run a plot over the 160-40 mtr band on each tap.
> what are the recommendations for making this kind of measurement where the
> objective is to replicate an exising implementation?  If the cores are 
> heat
> damaged, is this even worth doing as the measurement could be skewed?  In 
> a
> nutshell, what is the best way to replicate it?
>
>
>
> 7. would there be some room for improvements? different core that wont 
> heat
> up as much? etc etc?  any way to improve on the breed here?  suggestions
> welcome.
>
>
>
>
>
> -bob, w9zv
>
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