Magnetron testing
Raymond Carlsen
> doesn't produce microwaves. There is 117 VAC across the primary of the
> hi voltage transformer. The microwave diode is good (replaced it :) ).
> I did _not_ measure the voltages coming out of the hi voltage transformer
> since my meter doesn't go that high, but I verified that the secondaries
> have continuity, and the resistance is very close to what is indicated on
> the schematic.
> My thinking is that the magnetron is dead. What should the resistance be
> across the filament and from the filament to ground?
>
Let me say first that a microwave oven is more dangerous to even an
experienced tech, let along a novice, as a color TV. Even with the oven
off, that HV capacitor can hold enough of a charge to stop your heart. It
has killed more than a few people. It -normally- bleeds off if the oven is
working normally, but can hold a charge a long time, especially if the
tube is not working for whatever reason. Experienced techs keep one hand
in their pockets when checking around high voltages like that. It's to
keep the energy from discharging across the chest (one hand on ground,
the other on the source). Troubleshooting without the proper test
equipment puts you at a disadvantage. Now, I'll try to answer your
questions...
The tube filament should measure less than an OHM... it's very heavy.
Notice the heavy guage wire running to it. It is also HV insulation. You
should measure infinity between the filament and ground. When the oven is
running, there will be up to 5000 volts -negative- on those leads. If the
capacitor is charged up, those filament leads hold the charge.
Ovens are not created equal, design wise, but generic info is as follows:
The transformer runs the filament of the tube and generates, along with
the HV diode and a big oil-filled capacitor, the necessary High Voltage.
The plate of the tube is grounded and the HV is impressed on the tube
filament (cathode) as a negative potential of several thousand volts at a
lethal amount of current. The primary of the transformer is switched on
and off by some type of control circuitry... a relay, triac or
whatever... by means of a microprocessor. Simple ovens use a simple
switch and mechanical clock timer. Since you are still alive, I assume
the transformer never comes on. (Pardon the sick humor, but I'm
scared for you.) Look to the control circuits on the primary side. There
is usually some kind of over-temperature "fuse" in line with the AC to
the transformer... it opens up if the tube gets too hot. These
thermostats must be replaced when open. Most ovens have a schematic of
sorts on the inside of the cover. It never shows the control circuitry
except as a black-box, but does show the wiring of the rest of the oven.
If you run the oven (with a cup of water as a load) for a minute or so,
the tube should get warm, even if the HV never comes up. Sometimes there
are holes in the side of the tube and you can see the filament. I suspect
a control circuit problem (bad triac, etc.) that never lets the oven turn
on... no filament or HV. It could be as simple as an adjustment on one of
the door switches. There are several, used as interlocks, so the oven
can't be run with the door open.
I wish for you a long and happy life. Take care!
Ray Carlsen Univ. of Washington, Seattle
############################################################
Luke: I... don't believe it!
Yoda: THAT is why you fail.
############################################################
Sam Goldwasser
> Hello! I bought a microwave at a yard sale. Everything works except it
> doesn't produce microwaves. There is 117 VAC across the primary of the
> hi voltage transformer. The microwave diode is good (replaced it :) ).
> I did _not_ measure the voltages coming out of the hi voltage transformer
> since my meter doesn't go that high, but I verified that the secondaries
> have continuity, and the resistance is very colse to what is indicated on
> the schematic.
> My thinking is that the magnetron is dead. What should the resistance be
> across the filament and from the filament to ground?
> Thanks
> Willis Chung
Filament should be a fraction of an ohm. Filament to ground should be
infinity. Capacitor could be open though this is not a likely failure.
There is a good chance your magnetron is bad.
Microwave Repair Notes are available at:
http://www.paranoia.com/~filipg
using a Web browser along with whole lot of other interesting stuff)
maintained courtesy of Filip Gieszczykiewicz.
--- sam
Willis Chung
my limitiations. That's why I'm here typing ;)
The magnetron is shorted. Low resistance from filament to ground.
Dalbani lists a bunch of magnetrons, but skips over this one. Oh well.
Maybe I can use the controller for something!
Thanks again!
-
WILLIS CHUNG CQB...@prodigy.com
Sam Goldwasser
<snip - all excellent comments, especially on safety.>
He did say that 117 VAC was measured on the primary of the HV transformer,
it is not likely to be a controller or interlock problem.
Here is safety info:
General Safety Guidelines when working on line powered equipment including:
TVs, monitors, and microwave ovens.
These guidelines are to protect you from potentially deadly electrical shock
hazards as well as the equipment from accidental damage.
Note that the danger to you is not only in your body providing a conducting
path, particularly through your heart. Any involuntary muscle contractions
caused by a shock, while perhaps harmless in themselves, may cause collateral
damage - there are many sharp edges inside this type of equipment as well as
other electrically live parts you may contact accidentally.
The purpose of this set of guidelines is not to frighten you but rather to
make you aware of the appropriate precautions. Repair of TVs, monitors,
microwave ovens, and other consumer and industrial equipment can be both
rewarding and economical. Just be sure that it is also safe!
* Don't work alone - in the event of an emergency another person's presence
may be essential.
* Always keep one hand in your pocket when anywhere around a powered
line-connected or high voltage system.
* Wear rubber bottom shoes or sneakers.
* Wear eye protection - large plastic lensed eyeglasses or safety goggles.
* Don't wear any jewelry or other articles that could accidentally contact
circuitry and conduct current, or get caught in moving parts.
* Set up your work area away from possible grounds that you may accidentally
contact.
* Know your equipment: TVs and monitors may use parts of the metal chassis
as ground return yet the chassis may be electrically live with respect to the
earth ground of the AC line. Microwave ovens use the chassis as ground
return for the high voltage. In addition, do not assume that the chassis
is a suitable ground for your test equipment!
* If circuit boards need to be removed from their mountings, put insulating
material between the boards and anything they may short to. Hold them in
place with string or electrical tape. Prop them up with insulation sticks -
plastic or wood.
* If you need to probe, solder, or otherwise touch circuits with power off,
discharge (across) large power supply filter capacitors with a 2 W or greater
resistor of 100-500 ohms/V approximate value (e.g., for a 200 V capacitor,
use a 20K-100K ohm resistor). Monitor while discharging and/or verify that
there is no residual charge with a suitable voltmeter. In a TV or monitor,
if you are removing the high voltage connection to the CRT (to replace the
flyback transformer for example) first discharge the CRT contact (under the
insulating cup at the end of the fat red wire). Use a 1M-10M ohm 1W or
greater wattage resistor on the end of an insulating stick or the probe
of a high voltage meter. Discharge to the metal frame which is connected
to the outside of the CRT.
* For TVs and monitors in particular, there is the additional danger of
CRT implosion - take care not to bang the CRT envelope with your tools.
An implosion will scatter shards of glass at high velocity in every
direction. There is several tons of force attempting to crush the typical
CRT. Always wear eye protection.
* Connect/disconnect any test leads with the equipment unpowered and
unplugged. Use clip leads or solder temporary wires to reach cramped
locations or difficult to access locations.
* If you must probe live, put electrical tape over all but the last 1/16"
of the test probes to avoid the possibility of an accidental short which
could cause damage to various components. Clip the reference end of the
meter or scope to the appropriate ground return so that you need to only
probe with one hand.
* Perform as many tests as possible with power off and the equipment unplugged.
For example, the semiconductors in the power supply section of a TV or
monitor can be tested for short circuits with an ohmmeter.
* Use an isolation transformer if there is any chance of contacting line
connected circuits. A Variac(tm) is not an isolation transformer!
The use of GFCI (Ground Fault Circuit Interrupter) protected outlet is a
good idea but will not protect you from shock from many points in a line
connected TV or monitor, or the high voltage side of a microwave oven, for
example. A circuit breaker is too slow and insensitive to provide any
protection for you or in many cases, your equipment. These devices may
save your scope probe ground should you accidently connect it to a live
chassis, however.
* Don't attempt repair work when you are tired. Not only will you be more
careless, but your primary diagnostic tool - deductive reasoning - will
not be operating at full capacity.
* Finally, never assume anything without checking it out for yourself!
Don't take shortcuts!
Sam Goldwasser
> Thanks again!
Try also Dalbani and Premium Parts.
Note that most Magnetrons are interchangeable if they will fit
mechanically. Don't give up!
--- sam
MCM Electronics (VCR parts, Japanese semiconductors,
1-800-543-4330. tools, test equipment, audio, consumer
electronics including microwave oven parts
and electric range elements, etc.)
Dalbani (Excellent Japanese semiconductor source,
1-800-325-2264. VCR parts, other consumer electronics,)
Premium Parts (Very complete VCR parts, some tools, adapter
1-800-558-9572. cables, other replacement parts.)
Raymond Carlsen
> I just tried to fix a microwave oven, but I gave up hope because of
> the dead magnetron (3 kiloohms between anode & cathode...)
>
Sometimes the manufacturer puts the HV diode inside the magnetron... I've
peeled open a few and replaced the diode with an out-board type and fixed
the oven. Magnetrons are available here in the states from numerous
after-market vendors like MCM Electronics. Some are as cheap as $40 US.
Sam Goldwasser
> > Note that most Magnetrons are interchangeable if they will fit
> > mechanically. Don't give up!
> Eh ? Who wants to exchange a magnetron ? I think they are nearly
> as expensive as a new microwave oven...!? (Here in germany it seems
> to be impossible to get a (new) magnetron for less than 200 DEM
> (about 140 US$); a new microwave should be available for 250 DEM...)
> I just tried to fix a microwave oven, but I gave up hope because of
> the dead magnetron (3 kiloohms between anode & cathode...)
In the U.S., the typical replacement magnetron is about $40 and that
is worth it. MCM Electronics, Dalbani, Premium Parts, and others.
--- sam
> bye
> Stefan!
Sam Goldwasser
> Only ONE diode !?
> I repaired a few microwave ovens, but all of them used a HV rectifier
> circuitry like that:
The ovens I have worked on lacked this diode (D1).
> AC----*---||---*---CATHODE---+ -
> | | | Filament
> *---|<|--* ___|___
> D1 | / \ Magnetron
> - /_____\
> D2 V |
> - | Plate
> | |
> AC-------------*---GROUND----+ +
> (AC=2300 volts)
I wonder about this circuit. With no load between Cathode and GND,
this is just a half wave power supply with 2300 V * 1.414 DC across
the cap. D1 shown across the cap will be reverse biased all
the time. With the magnetron conducting, it appears as though the
voltage on the cap will decrease and have more ripple. (D2 will appear
to be more and more leaky as a result of the load). At some load,
the ripple will bring the cap voltage below zero at which point
D1 will bypass current around the cap. I guess this would help
heavy load power handling.
Note that the circuit configuration without D1 is called a 'half wave
doubler' since the peak voltage across the magnetron is nearly twice
the peak voltage of the AC - AC+cap.
> Why do _I_ never get schematics for the things I repair :-(
You are not selective enough about what is broken? Schematics make
life too easy. What challenge is there in it if you actually know
what is going on?
--- sam
> bye
> Stefan!
Sam Goldwasser
> > I just tried to fix a microwave oven, but I gave up hope because of
> > the dead magnetron (3 kiloohms between anode & cathode...)
> >
> Sometimes the manufacturer puts the HV diode inside the magnetron... I've
> peeled open a few and replaced the diode with an out-board type and fixed
> the oven. Magnetrons are available here in the states from numerous
> after-market vendors like MCM Electronics. Some are as cheap as $40 US.
3 Kohnms does sound like an unlikely resistance if the magnetron were
defective (the tube, that is). I would expect it to be real low if the
filament was touching the anode.
--- sam
Sam Goldwasser
> > > > I just tried to fix a microwave oven, but I gave up hope because of
> > > > the dead magnetron (3 kiloohms between anode & cathode...)
> [that was one of my msg's...]
> > 3 Kohms does sound like an unlikely resistance if the magnetron were
> > defective (the tube, that is). I would expect it to be real low if the
> > filament was touching the anode.
> I removed the magnetron from the oven and tested again - 3 kohms...
> But don't ask me what has happened to it...
Interesting...It is definitely bad, however.
--- sam
> bye
> Stefan!
Robert Rolf
;>> > > I just tried to fix a microwave oven, but I gave up hope because of
;>> > > the dead magnetron (3 kiloohms between anode & cathode...)
;>[that was one of my msg's...]
;>> 3 Kohms does sound like an unlikely resistance if the magnetron were
;>> defective (the tube, that is). I would expect it to be real low if the
;>> filament was touching the anode.
;>I removed the magnetron from the oven and tested again - 3 kohms...
;>But don't ask me what has happened to it...
Sounds to me like you have a small wisker of material shorting it out.
Try slamming the tube on a table on each of 5 sides (the sixth isn't
an option). You've got nothing to loose, do you? Use reasonable force
since its mostly Cu and Al, and BeO.
You might also try discharging a loaded HV cap across the tube to
burn it off. Heck, you could use the existing HV supply. Just disconnect
and INSULATE one filament lead, and using suitable insulated pliers, touch
the other lead to the filament pins. You should get a good snap as you
fry the wisker.
Remember this HV stuff is LETHAL, so take suitable precautions and anything
you do is at your own risk.
Stefan Huebner
> Try slamming the tube on a table on each of 5 sides (the sixth isn't
> an option). You've got nothing to loose, do you? Use reasonable force
> since its mostly Cu and Al, and BeO.
AFAIK BeO is a ceramics material with a very good heat conductivity,
right ? Where is it used in standard 600..800 watts magnetrons ?
(Would like to know because it is known to be quite unhealthy...)
> You might also try discharging a loaded HV cap across the tube to
> burn it off. Heck, you could use the existing HV supply. Just disconnect
> and INSULATE one filament lead, and using suitable insulated pliers, touch
> the other lead to the filament pins. You should get a good snap as you
> fry the wisker.
I tried it, but it did not help.
bye
Stefan!
y
Chris Orgill
Stefan_...@rookie.fido.de (Stefan Huebner) wrote:
> > > > I just tried to fix a microwave oven, but I gave up hope because of
> > > > the dead magnetron (3 kiloohms between anode & cathode...)
>
> [that was one of my msg's...]
>
> > 3 Kohms does sound like an unlikely resistance if the magnetron were
> > defective (the tube, that is). I would expect it to be real low if the
> > filament was touching the anode.
>
> I removed the magnetron from the oven and tested again - 3 kohms...
> But don't ask me what has happened to it...
>
> bye
> Stefan!
Mine has gone intermittent, I can't decide whether its the HT or the
magnetron device. Can magnetrons go intermittent ? I am talking about
working for a few hours and then not working until left overnight.
Certainly not overheating.
Chris Orgill