we have had digital transmissions for commercial broadcast for a long time now.

curiously enough, we have not had to dump all of our pre-90s design radios and stuff to get their signals.

even as a thought exercise, that should tell you that out of the tank and into the ether, signals is signals, and if it works in band X with known type equipment, it's the same thing.  if it's the same thing, you check the envelope and decode like you always did for that emission class.  doesn't matter if they have 20 100-watt triodes in parallel, a PWM FET output, or sparks in a bunsen burner flame, that's just how you encode it.

provide me a few pallets worth, and I'll teach ya

there are some internally fragile tubes that short if you look at them funny... the same audio tube used in the Heath SB/HW series is used in some Hammond organs.  looky here, they are prone to shorting suppressor to cathode. that burns up the limiting resistor if any, the rectifier, and the power transformer.

there are also a bunch of NOS tubes that have gassed up over the years, but sitting in the tester socket for a while can degas them.

two excellent reasons to have a tube tester.  and it's not necessary to have one box to test them all, and in the caddy bind them, if you get a newer tester and an older one at the right price.

I take it as a challenge to get stuff that's feeble or worse back in shape.  unfortunately, most of that has been concentrated in the house and the cars for the past couple years.  it's a personal challenge, and obviously I don't dig deeper in the trash pile of fleabay than I think I can achieve.  the CX7A still has a little power supply issue at 38 volts, and the BH Green critter appears to have an open prod detector coil, so they're still getting a little dusty.  the Yaesu needs a master oscillator cap and an oscillator alignment, but is workable with the secondary freq counter antenna up as a freq monitor.

skills vary.  however, I developed what I have with some elmering, lots of reading, and getting zapped a few times puzzling out circuits.

R390s appear to be quite the mechanical challenge, though.  I may pass on that model.

instead of jury-rigging a few Variacs and manually adjusting power all the time, why not get a constant-voltage transformer (Sola, Topaz, etc.) and put that between what is laughingly called "the line" and your amp?  there should be some floating around, used to be the norm to put one in front of minicomputers.  only issue would be the 50 Hz thing, but they did make 'em.

overhead caternary line rail/streetcar systems are going to spark at all grid supports, crossings, and the like.  that can't be fixed.  there are likely to be periodic sparks as the vehicle goes along when there are point issues as well.  but the pickup arm shouldn't be blasting like a comic book or a bumper car booth all the time, it will notch and weaken the overhead lines.  the motorman has a lot to do with that in how the arm is set, but so will maintenance.

practically all locomotives are now run with electric motors, whether overhead or 3rd rail, or diesel-electric powerplants.  they know how to suppress the motors... otherwise, they'd have to do heavy repair all the time.

so I hope that Amtrak will get on this... but they don't have a lot of spare equipment, and probably can't take an engine down for heavy repair until scheduled.  good luck.

you can exceed most tube specs temporarily without catastrophic results.

not so semiconductors.  when you are looking at the sheet, if they say max temperature, max voltage between any two elements, maximum current, maximum power... they mean it.  for each rating.  for any time beyond zero.

they also mean the derating curves, absolutely.  if it says capacity goes down 50 percent each 10 degrees centigrade you step beyond the 25 C benchmark, that's no fooling either.

if you buy a part that is a derated version of a higher power part, and I don't have my old spec books on hand now for examples, you might get a "batch failed" part in which the test pieces didn't make the higher voltage spec, but work at a lower voltage.  somewhere in between, most of the parts will work over the rated limit.  but you don't know where they would fail, or which ones, unless you are equipped to do curve tracing.

where you can "push the ratings" like the amp you asked about, the average load on the part will not push the component into thermal failure, and none of the instantaneous ratings are violated in any event.  so they're probably depending on your not running continuous two-tone and crossing the dissipation limits of the device at the heat sink.  you can generally add up all the instantaneous ratings of any semiconductor and find if you ran it there, you violate something like power dissipation or thermal dissipation by a country mile.

could you use a spark gap rig so I can claim you on worked all bands?

you can also get them from Fastenal.com as blind self-clinching press-fit nuts.

http://www.fastenal.com/web/Search.ex?action=productSearch&searchterm=press+nut

lucky, nothing I've bricked at work is worth more than 12 grand.  but I did get (a major international company) to do a total international recall on control cards for Internet switches.  had to do with using a PC card disk drive in continuous service when it was rated for the lesser of 5 minutes max, or 10 minutes in an hour spun up and seeking.  at the time, that was THE backbone for Da ISH... .

QSY 15 or 20.  that slop was happening in the 70s when I was a novice.  them good ol' boys, hail, they's way too drunked up to turn the knob, heh.

could be a dirt-cheap voltage charger, once the battery is up to voltage, it basically stops charging all of its own.

could be it's a smart charger if you have Lithiums in the radio.

probably a voltage charger with the "charge" LED in series/shunt with the voltage... once they equalize, current draw drops, light goes out.

-- opinion based on taking apart chargers in the past, finding nothing fancy, and moving on... no specific knowledge of this unit, which probably did not take thousands of hours of engineering time.

the cans look a lot like Trimm ... with the cushions, they weren't bad, but they sure weren't AKGs.

you need to know what it came out of, and then you might find some data online that identifies the pinout.  Nortel is, of course, gone and of no help, but if somehow a manual made it online, that's kewl.

the other thing to do is lift the cover and play detective.  there will be lower voltages plus and minus, possibly an AC interlock, and if you have a big ol' heat sink with one or more devices leading to a pin, that's probably golden.

a note again... when you're talking Telco, that is a positive-ground kind of place.  their red wires in the CO are actually negative voltage.  you need to put a reasonable load on any "rectifier" and use your Fluke to be sure you know where positive and negative are.

any telephone company power supply (they're called rectifiers in the trade) has an output in the 53-54 volt range.  they're made as small as one amp, and as large as hundreds of amps.  the ones I've used desktop at work have good regulation (they are paralleled in practice) and good ripple.  the big guns, being filtered by zillions of amps of battery filtering, may not be as clean.  but you won't be using one of those.

telco "central office battery" is based on positive-ground 48 volt lead-acid battery stacks, so the power supplies have to run a little hotter to charge the batteries.

yes, I said positive ground.  in the CO, red is negative voltage, and ground is "backwards."  you will need to keep that highly in mind if you use telco rectifiers.  your Fluke is your friend, watch it carefully