[Milsurplus] TCS Question

[Don Davis]

Interesting thread about increasing plate voltage for more power out in the
TCS.  I think most folks have the right answer, but there are a couple of
really good reasons to be circumspect about ANY mods of this type to ANY
gear.

1.)  Derating of component parameters.  All comm'l, military, space,
automotive, etc equipment use derating standards of one type or another.
Derating is operating a given part at some fraction of it's rating.  This is
done to be able to predict the MTBF (mean time between failures) or some
other statistical probability that your piece of gear will meet the mission
needs under extreme conditions.  The more you derate applied voltage,
current, power, the longer time between failures and the greater the
reliability.  Good source is MIL-HDBK-217E or F if one has access to Rome
AFB pubs.

If you have a power supply running at 450 volts all day long at temperature,
what capacitor voltage rating do you need for "good" ham radio reliability?
450 volts?  500 volts?  600 volts?  Depends.  For the military gear of the
WWII period, they are pretty much the same from radio to radio, and they
provide decent ham radio reliability.  If you up the voltage, the
reliability goes down.  Simple in priciple, a bit harder to calculate,
especially give parts that are way past their useful life.

2.)  Heat - the mortal enemy of electronics.  As I recall the TCS
construction, the cases don't breath very well, and adding dissipation
(heat) to the radio will decrease it's life and make failure more probable.
Remember that the dissipation will increase at a rate approaching the square
of the increase - depending on circuit bias methods.  So, a 10% increase in
voltage might give a 15 to 20% increase in dissipation.  Simple rule of
thumb is that chemical activity in electronic parts doubles for every 10 deg
C rise in temperature.  Failures are usually the result of some chemical
reaction in the parts.  Also note that the derating decreases as temperature
increases.  A capacitor derated at 75% at room temp might be derated to 50%
at 85 deg C, for example.

Good mod to do is to add airflow to older radios to extend life.  Bad mod is
to increase the heat load.  Again,  all of these parts are way beyond their
useful life, so if a 60 year old raio works out of the box without repairs,
it's almost a miracle.

Note that none of the above addrersses the propriety / morality /
advisability of modifying old hardware.  If one makes mods, however, he
should be aware of the potential trouble, and what form it may take.  I have
mixed feelings, having hacked a bunch of nice stuff in the 50s and 60s, and
having spent the last 10 years re-buying un-hacked versions of the same
units.  Hacking is BIG fun, and very rewarding to get more "performance" out
of an old radio.  BUT, the older I get, the more fun I have in finding all
of the connectors, antennas, bits, pieces needed to assemble a working unit.
Very satisfying.  In olden days nobody worried about it, you ripped out the
connector, and soldered wires to a pigtail... you've all done it!

Your milage may vary...

73

Don  AD6PB    (designer of high-rel space power systems)

PS:  In the eraly 70s I put a really nice 427 TP into a fairly stock 57
Fairlane.  Yes, it went 140, and yes, it broke.  Often.

Others said...
>
> >I am not intending to modify or change the original design of the TCS =
> >transmitter, will drop the idea of running the second PA tube, but
because =
> >I do not have the original AC power supply, I am building a external =
> >modern supply and their are legitimate questions on what can safely be =
> >supplied as a plate voltage with this radio, if by increasing the plate =
> >voltage by a hundred volts or so  you have the benefit of a couple more =
> >watts how can that hurt?
>
> I'd be careful about arbitrarily increasing the plate voltage by 25%.
> The guys that originally designed this stuff were not stupid. The
> answer to this questions lies in the schematic.
>
> This seems somewhat akin to asking "What will happen if I try to put a
> supercharger on this 1955 Chevy? Will it do 140 MPH, or will it break
> down?"
>
> The best answer to your question is: Why would you want to do this?
>
> With all due respect, the result of this experiment will likely be one
more
> hacked up TCS. You're not likely to get a whole lot of help in this forum
> in accomplishing that result.
>
> 73 John K5MO
>
>
>
>
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