On Fri, 14 Sep 2007 23:37:57 +0100, Chris Jones
wrote:

wrote:


I already found something, thinking a bit sideways: these SS devices
impressed me more on the current- than on the voltage handling side.
I reckoned that scaling to higher powers was mostly done in current,
so I headed for a low power video unit I had: a long deceased bible-
sized 1970's TV + radio combo, with a 2" B/W screen. As suspected, I
found 2-3W low capacitance bipolars that could run into VHF... so
much so I had to bead them to keep them from producing spurs in my
ugly prototype.



On Jul 30, 11:37 pm, Chris Jones wrote:

2SB772 (PNP) and 2SD882 (NPN) (10-Watt 30V, 3A, fT80MHz)

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.


Hmmmm.... I'll look harder. The pile of monitors is still there
gracing a hidden nook of my place.

W.r.t. the high definition requiring high frequency: KVM switches
often quote their bandwidth, which must match what^s required from a
given definition (if a KVM only quotes that, and not pixels, good luck
finding the equivalents). Bandwidths now run in the 200-300MHz range,
which is a good omen.

Where would such transistors be?

In the vicinity of the HV coil?

Or near the CRT cathode?

The video amplifiers are normally on the PCB that plugs directly onto the
back of the tube. The unusual thing about those transistors is that they
are moderately fast and also capable of handling quite high voltage.

Chris

I pulled parts from 17" Viewsonic/Optiquest, and I found a treasure
trove of parts. The horizontal deflection amp is a VPS10S 3-channel
module (each indentical) tested at 100MHz. The data sheet is on
www.datasheet catalog.com. Vcc is 80 or 90 Volts, but Ic is only 75
mA. It's the only heavy-duty-looking thing on the board that mounts
on the CRT.

However, that monitor' shielding cages would be perfect for tube RF
cages, and there are a lot of large sections of it; a couple of square
feet, at least. The video amp is an LM1205N, 3 channel 28 pin device.
It apparently drives 3 2SC1921s, 200 V Vceo, 50mA, 600 mW, Ft=130
devices. Lots of C0G and Z7R decoupling caps on all the boards. This
is a 1997 vintage monitor, and it was a good'un.