20 Jun
2004
20 Jun
'04
11:18 p.m.
Again, I'm thinking in terms of vacuum tubes, and
I can't see how losing
the knowledge of how they worked is going to affect the future, or even
the present for that matter.
I'm guessing that there is at least one piece of equipment in your house
that relies on a vacuum tube to opperate that is not some sort of display
device.
We're so beyond them technologically that they are
irrelevant today.
So you don't want television, or radio, or communications satellites, or
radar, or ...
Lee.
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21 Jun
21 Jun
4:56 a.m.
Again, I'm
thinking in terms of vacuum tubes, and I can't see how
losing the knowledge of how they worked is going to affect the
future, or even the present for that matter.
I'm guessing that there is at
least one piece of equipment in your
house that relies on a vacuum tube to opperate that is not some sort
of display device. We're so
beyond them technologically that they are irrelevant today.
So you don't want
television, or radio, or communications satellites,
or radar, or ... 
10:42 a.m.
Aside from legacy electronics (I have some
pre-transistor-era equipment
lying around), for which transistor versions exist, I can't think of
any. The only thing I'm not sure of is the microwave oven - does the
microwave-generation-thingy depend on vacuum?
We said knowledge was being lost, now I am sure of it.
Yes, the microwave generator in a microwave oven is a cavity magnetron,
which is a type of vacuum tube. It's close to a directly heated diode
with a strange shaped anode block (incorporating the resonant cavities)
and a magnetic field along the axis of the device.
Certainly television and radio do not depend on vacuum
tubes today
(well, certainly not on the receiving end; the technology exists to
transmit with transistors, but I don't know whether it can handle the
power levels appropriate to mass broadcasting).
I am almost sure the trnasmitter output stages are still valve-based, at
least on larger transmitters.
Radar - as above: the power transmitting stage may still be vacuum
tube, but certainly _could_ be transistor; the rest definitely can be.
I wouldn't want to bet on that. I certainly wouldn't want to try to
design a microwave oscillator using semiconductors of sufficient power
for a large radar system.
-tony
22 Jun
22 Jun
6:12 a.m.
We said knowledge was being lost, now I am sure of it.
Tsk. Poor reasoning there :-) The worst case scenario
here is that *someone* has lost some knowledge; but to
get that far you would have to show that hw knew it in
the first place :-)
Actually, I only have a tenuous grasp (for some suitable
value of "tenuous") of how the thing works anyway. I've
only ever possessed one, it currently still seems to
cook as well (or badly) as it ever did so I've had no
cause to examine it in any detail. If it breaks, it will
get looked at (although I suspect, without ever having
checked, that a new megnetron will cost a significant
fraction of the price of a microwave oven). Still,
there's always the chance that the controller or its
keypad will give up the ghost.
I am almost sure the trnasmitter output stages are
still
valve-based, at
least on larger transmitters.
AFAIK they are. I remember reading an article in (I think)
one of the IEE rags some time ago that described some new
generation of semiconductors that could handle the combination
of power and frequency, but they were only being used in
specialised applications because of the cost. I assume that
your average TV transmitter mast will stay with valves for
a while yet.
I wouldn't want to bet on that. I certainly
wouldn't want to try to
design a microwave oscillator using semiconductors of
sufficient power
for a large radar system.
If "large" == "big enough to fry the occasional bird",
I think I'd tend to agree!
Antonio
--
---------------
Antonio Carlini arcarlini(a)iee.org
6:20 a.m.
AFAIK they are. I remember reading an article in (I
think)
one of the IEE rags some time ago that described some new
generation of semiconductors that could handle the combination
of power and frequency, but they were only being used in
specialised applications because of the cost. I assume that
your average TV transmitter mast will stay with valves for
a while yet.
Don't bet on tubes being used in broadcast use for long - the
semiconductor has made serious inroads into the field (as in 50 kW). Same
with radar (although tubes probably have more life in radar than in radio
or TV).
William Donzelli
aw288(a)osfn.org
6:40 a.m.
Don't bet on tubes being used in broadcast use for
long - the
semiconductor has made serious inroads into the field (as in
50 kW). Same with radar (although tubes probably have more
life in radar than in radio or TV).
I don't work in the field (no pun ...) so I have no idea
how soon it will be before new TV (and radio) transmitters
are built using semis. I just assume that the existing ones
won't be replaced any time soon (since they presumably work)
and that the natural replacement rate is relatively low.
I could be wrong about either of these things. I've not
heard that the new digital services require new transmitters
(just new ways of generating signals I guess) and it could be
that we all move to cable/satellite services and ditch
terrestrial anyway. I just don't think it will happen.
Semiconductors have advantages in many areas (power, speed,
robustness) but it may be that these do not apply (or do
not apply so much) in the high power, high frequency arena.
I do expect the main vacuum tube in most display devices will
be gone within the next ten years or so. I would have thought
the tubes at the middle of the chain (the final stages of
the transmitter) would be mostly still around by then.
Antonio
--
---------------
Antonio Carlini arcarlini(a)iee.org
1:50 p.m.
I don't work in the field (no pun ...) so I have
no idea
how soon it will be before new TV (and radio) transmitters
are built using semis.
They have been building them for a while (minor correction to last light's
post - 200 kW solid state rigs are now available), and in the past five
years, they have actually made them quite reliable (now to the point
where they are basically fault tolerant). You can still buy some tube
based transmitters, but I doubt they will be on the market in five years.
I just assume that the existing ones
won't be replaced any time soon (since they presumably work)
and that the natural replacement rate is relatively low.
Extremely low. The tube rigs being replaced will become the standby units,
and the current standby units (many dating to the 1950s) will go out to
pasture.
Semiconductors have advantages in many areas (power,
speed,
robustness) but it may be that these do not apply (or do
not apply so much) in the high power, high frequency arena.
The high power solid state designs use many, many smaller transistors
working in parallel. It took time for these to work properly, but now they
do. If pieces blow out, due to a lightning strike, they can be hotswapped
out, and I Love Lucy still goes out.
Minor correction: tubes rule over semiconductors in the robustness
arena. It is pretty amazing just how much abuse a tube can take and still
work (although perhaps in a weakened state).
William Donzelli
aw288(a)osfn.org
23 Jun
23 Jun
10:14 a.m.
I just assume
that the existing ones
won't be replaced any time soon (since they presumably
work) and that
the natural replacement rate is relatively low.
Extremely low. The tube rigs being replaced will become the
standby units, and the current standby units (many dating to
the 1950s) will go out to
pasture.
24 Jun
24 Jun
2:56 p.m.
The people I know who are into telephones seem to
let it get out of control and start to try to build
major exchanges in the back yard.
This is a shock - but a good one. Nearly all of the telephone collectors I
know are interested only in the telephones - once past the plug, they
don't give a care. CO stuff is just not important to them, and neither is
longline stuff. Too bad, as so much of today's networking theory is based
on Bell stuff from 70 plus years ago.
I guess that there will soon be another breed of
collector with large transmitter masts and
glowing valves in the back garden :-)
"Will be"?
People have been collecting large transmitters (250 thru 5000 Watt) for
some time. Some hams modify them (not much, basically just a small shift
in frequency) to work on the 160 meter band.
I think my big transmitters just recently lost the weight crown in my
collection to the computers.
William Donzelli
aw288(a)osfn.org
22 Jun
22 Jun
8:41 a.m.
I've only ever possessed one [microwave oven], it
currently still
seems to cook as well (or badly) as it ever did so I've had no cause
to examine it in any detail.
Same here.
If it breaks, it will get looked at (although I
suspect, without ever
having checked, that a new megnetron will cost a significant fraction
of the price of a microwave oven). Still, there's always the chance
that the controller or its keypad will give up the ghost.
That's what happened with my oven. It was bought at a garage sale some
years back, and worked fine for some time (years). Then after a
lightning storm, it started beeping intermittently at odd times when it
shouldn't. After a few days, it occurred to me that if it could beep
when it wasn't suppsoed to, it could turn on the microwaves when it
isn't supposed to. I opened it up and found that the keypad and
control board all culminated in two relays, one to control the fan and
the other the microwave-generator. I checked, and a new board would
cost almost as much as we paid for the oven. So I yanked the whole
thing, wired the fan and magnetron together (I almost always used it on
high anyway, and lower power settings worked by imposing a <100% duty
cycle on the magnetron), and controlled it with an ordinary wall light
switch, on the principle that it's too simple for much to go wrong.
But this still didn't mean I dug into the underlying principles of the
magnetron. For my purposes, it was a black box: mains power goes in
here, microwaves come out there.
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11:24 a.m.
[Microwave ovens]
Actually, I only have a tenuous grasp (for some
suitable
value of "tenuous") of how the thing works anyway. I've
only ever possessed one, it currently still seems to
I don't have one of these infernal devices, but anyway...
cook as well (or badly) as it ever did so I've had
no
cause to examine it in any detail. If it breaks, it will
get looked at (although I suspect, without ever having
checked, that a new megnetron will cost a significant
fraction of the price of a microwave oven). Still,
Most of the time it's the door interlock microswitches (one of them is
_designed_ to short the mains if the others stick closed), the HV
rectifier diode or the HV capacitor that fail.
Be _very_ careful if you work on one of these. They are _lethal_. I've
warned people here about SMPSUs before, but microwave ovens are much
worse. The magnetron runs at a couple of kV and half an amp or so. This
will (not might, will) kill you if you touch it.
Incidentally, the magnetron block is earthed, and is the anode. The
filament (which is a directly heated cathode of course) runs at a high
-ve voltage. The voltage across the filament is only a few volts, but
it's at a couple of kV wrt earth. Just warning you if you decide to try
measuring the filament voltage (in other words, don't do this unless you
really know what you are doing!).
-tony
12:07 p.m.
Be _very_ careful if you work on one of these. They
are
_lethal_.
I have a sort of aversion to even using it: the door seals
rely on a 1/4-wavelength trap to prevent (or rather - minimise)
microwave leakage. I expect that if you whack the door shut
often enough with something solid half in and half out,
you may find stuff outside cooks too :-)
The chances that I'd want to run something like
that with the skins off and the magnetron active
is pretty minimal :-)
The electric oven is just so much nicer by comparison!
Antonio
--
---------------
Antonio Carlini arcarlini(a)iee.org
3:10 p.m.
New subject: Microwave oven hackery (was Modern Electronics, earlier List charter mods & headcount...)
[Tony Duell]
It's been a while since I had it open, but I'm fairly sure my oven's
door switches don't work that way.
[David V. Corbin]
Well, no; I never submitted an example for testing, so it can't be.
[failure
modes]
Most of the time it's the door interlock microswitches (one of them
is _designed_ to short the mains if the others stick closed), So I yanked
the whole thing, wired the fan and magnetron together (I
almost always used it on high anyway, and lower power settings
worked by imposing a <100% duty cycle on the magnetron), and
controlled it with an ordinary wall light switch, on the principle
that it's too simple for much to go wrong.
A classic example of where a little
knowledge can be very dangerous.
Definitely NOT "UL Approved" or "CE Marked". [your solution APPEARS to have bypassed all of the
safeties (door,
etc.)).
I'm not sure where you saw such an appearance, but it is incorrect. I
may be crazy, but I'm not stupid.
I didn't mention them, because they were irrelevant to my story, but I
did not bypass the safety switches. They were not wired through the
PCB I removed; its _only_ connections to the rest of the oven were (a)
a power feed to run the electronics, (b) an SPST relay (or something
else being used as SPST) to control the fan, and (c) a similar relay to
control the magnetron. Oh, and a flexible-printed-cable to the keypad.
(The beeper was on the PCB.)
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5:58 p.m.
New subject: Microwave oven hackery (was Modern Electronics,
[Tony Duell]
It's been a while since I had it open, but I'm fairly sure my oven's
door switches don't work that way.
Then it's very unusual. Normally over here there's at least one swtich in
series with the mains (live wire) to the transformer, and one across the
mains after that one. Normally the first one is all you need (door open,
switch open, no microwaves), but the second one shorts out the mains
should the first one stick closed. This will blow the fuse, of course. If
you have a microwave oven that blows the fuse as you open or close the
door it's a fair bet that the switches are misaligned.
Some ovens have a current limiting resistor, of about 0.25 ohms (!) in
series with the shorting switch. This is often shaped like a fuse and
fitted in a normal fuse holder. Needless to say it must not be replaced
by a fuse!
-tony
[failure
modes]
Most of the time it's the door interlock microswitches (one of them
is _designed_ to short the mains if the others stick closed),
7438
days inactive
7441
days old
14 comments
6 participants
participants (6)
-
arcarlini＠iee.org -
ard＠p850ug1.demon.co.uk -
aw288＠osfn.org -
julesrichardsonuk＠yahoo.co.uk -
Lee.Davison＠merlincommunications.com -
mouse＠Rodents.Montreal.QC.CA