In the vacuum tube world,
several types of TV line finals
are reused as HF PA FINALS.



- WHAT ABOUT CRT-TYPE PC MONITORS? -



Are there solid state components there
that could do say 5 to 30W (in) at HF?

Any other components
that would fit well
in a basic CW TX?



I'd be moderately surprised if that were not the case!

Hints, anyone?

Filippo
N1JPR/I2

) writes:
In the vacuum tube world,
several types of TV line finals
are reused as HF PA FINALS.



- WHAT ABOUT CRT-TYPE PC MONITORS? -



Are there solid state components there
that could do say 5 to 30W (in) at HF?

I think you'll find the big power transistors are the horizontal
output transistors, and they have very limited frequency response.
Barely up into the HF range. Not really a surprise, given they
are intended for tens of KHz useage, and nothing more. They may
also have extraneous components built in, that may get in the
way of other useage. And there may be limits on their useage
in non-pulse applications, just as sweep tubes from years ago
had limitations in linear use.

Any other power transistors may also be limited in frequency. On
the other hand, other than the horizontal output transistor there
isn't really anything special about the power transistors in a
monitor. I suspect your chance of finding suitable transistors
are just as good in any piece of consumer electronics that would
have power transistors. So it's more a case of looking in those
scrap printers and those switching supplies and even audio amplifiers,
and looking up the power transistors in the databooks to see if
they have any good frequency specs.

In the old days, tv sets were one of the few consumer electronic
equipment around, those and radios. So if you wanted to scrounge parts,
you had to go for tvs and radios. The former, if they had an
actual transformer in the power supply, could supply a nice
hefty transformer and that was likely a bigger key component
than the tubes.

Now, there is an awful lot more consumer electronics out there,
in quantity and variety. And of course, it gets scrapped. So
there is a lot more chance to find parts of the common variety.
There was nothing all that unique about tv sets, other than they
were available.

Of course, sweep tubes were used in commercial ham equipment because
they were cheap because they were used in a lot of tv sets. So long
as transistors aren't exotic, they tend to be far cheaper than tubes
were, and buying them is as easy and cheap as trying to find suitable
HF transistors in consumer electronics.

Any other components
that would fit well
in a basic CW TX?

Any consumer electronics will supply lots of capacitors and resistors
and garden variety small signal transistors. They may supply some
power transistors suitable for transmitting, but that would depend
on what is used. Some consumer electronics will still have transformers,
and those are worth grabbing. Filter capacitors for the power supply.
A lot will supply various crystals and/or crystal oscillators, though
chances are good their frequencies won't be in an actual ham band. Of
course, find enough crystals on the same frequency and you can start
making ladder filters. Lots of LEDs, and various switches (though
pushbottons are more common in some areas than toggle switches). Pull a
VCR or tv set, and you'll find tuner modules that may be useful in other
things. Any old am/fm radio will have a multi section variable capacitor,
just like the old days, though likely the newer the radio the more likely
they use varactors instead. All the audio amplifier ICs you can want,
be they in radios or on soundcards or modem cars. It just goes
on.

But other than the variable capacitors, few of the parts will be
especially exotic. If you need really high frequency transistors,
you'll have to look for things like satellite tv receivers. Though, I
once got some Mosfets out of FM preamps for car radios. The average
am/fm radio will supply ceramic filters, "narrow" at 455KHz, wide
at 10.7MHz. Find the legendary ssb CB sets in the garbage, and that
should supply a decent filter for SSB, but otherwise you aren't likely
to find narrow filters in consumer electronics. Cellphones, the older
and clunkier the better since they will have useable components, are
a treasure trove of parts, including roofing filters in the range
above 30MHz, and usually a 455KHz narrowband FM IF strip, complete
with a crystal to move from the roofing filter frequency down to
455KHz. But those narrow filters will still be too wide for
even AM. Cordless phones offer up 10.7MHz and 455KHz ceramic
filters for FM use, and again the needed crystal to get from
one IF to the other. You can often extract the IF strip as a module
and use it intact. They often use identifiable Motorola FM IF
strips, that can be reused in some other uses. At the very least,
they are a source of "gilbert cell" type mixers.

In the old days, if you wanted exotic components, you had to go for
surplus. Because no consumer electronics would have those needed
SSB filters, or even crystals to make your own filters. That's changed,
a lot more variety is available in consumer equipment.

But likely no matter what you are building, unless it's relatively
simple, you will always find some component that you can't scrounge.

Michael VE2BVW

Thank you Michael! I know a back alley where lots of e-junk lie
waiting for the municipal solid waste disposal operators - or a
discerning scavenger. Now I am a more discerning scavenger.

:-)

wrote:

Are there solid state components there
that could do say 5 to 30W (in) at HF?

check the video finals, usually they can
supply a few watts in the HF range

--
73 es 51 de i3hev, op. mario

Non è Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non è uno che ascolta la radio"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia

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

- 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 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.

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.

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.

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

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?

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

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


..... I forgot..... Thank you Chris!

wrote:
In the vacuum tube world,
several types of TV line finals
are reused as HF PA FINALS.



- WHAT ABOUT CRT-TYPE PC MONITORS? -



Are there solid state components there
that could do say 5 to 30W (in) at HF?

Any other components
that would fit well
in a basic CW TX?



I'd be moderately surprised if that were not the case!

Hints, anyone?

Filippo
N1JPR/I2

Look at RF transistors from the surplus houses if you want to save
bucks. Up to 10MHz or so you can use MOSFETs designed for SMPS service
if you're careful with the gate drive circuit, and they'll be dirt cheap.

Brand spanking new transistors that'll handle a few watts at HF are a
couple of bucks each, you can get up to 10W out affordably -- why do you
want to muck around parting out consumer electronics of uncertain
provenance?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html