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Chris Jones · July 31st 07, 12:37 AM posted to rec.radio.amateur.homebrew

wrote:

Thank you all around!

I went through a couple of display motherboards, and looked into the
specs of all power devices.

What their applications have in common is being rather high current
and low frequency. You don't find much rated to work beyond fery few
MHz, and with very large capacitances.

Examples:

- power MOSFET capable of switching 600V, 10A peak, with 2200pF gate-
drain capacitance
These can be handy for switching stuff on and off, regulators, DC-DC
converters etc.

- bipolars that can absorb several watts, but no higher than a couple
of MHz

- horizontal finals yjat are included in fairly complex ICs with lots
of ancillary components in feedback and protection circuitry - at
least in CRT VDUs from the last 10+ years.

I doubt this - the vertical output is in an integrated IC, but the
horizontal output is normally just a transistor, often with a built-in
diode across the collector-emitter, but in the direction such that it won't
bother you in any sensible circuit. They are mostly pretty slow, since
they're high voltage.

I have no doubt some RF can be teased out of these, but it seems to me
it would be at the price of major design complications.
Or you could say that you would get a sense of accomplishment when you've
succeeded. Some of the transistors are mot so bad - e.g. I got a few
2SB772 (PNP) and 2SD882 (NPN) (10-Watt 30V, 3A, fT80MHz) transistors out
of a monitor, that'd be OK for a QRP transmitter I think.

The advantage of tube finals was that their high-voltage low-current
and low-capacitance nature lent itself to functioning also at higher
frequencies than those of typical video circuits.
I did find a 2SC3675, which has an output capacitance of only 2.8pF and a
VCEO of 900V. It is slowish (fT=6MHz), so I might use it as a cascode in a
high voltage audio amplifier for driving electrostatic headphones.

The video amplifiers in a monitor have to be pretty fast for high resolution
signals, and often these are discrete transistors to handle the power. I
got some 2SC2682 devices out of monitors - 180V, 3.2pF, fT=200MHz, 10W - a
few of these could make a useful amount of RF, I reckon.

Next I'll look into TV distribution amps. I remember that transistors
in those were designed to linearly pump a lot of different signals up
to UHF, and that they tended to run hot - and 24/7.

Good luck!

Chris