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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