Whirlpool microwave quit, cold turkey.

This is the message I recieved from my hostess for Thanksgiving dinner.

"I found all my 'stuff' on the microwave. It is not heating anything up and it
is making a large humming motor noise. The model is MH7100XY. Whirlpool.
Manufactured in 1993. Part# 3874224B. Its a Microwave Hood Combination."

Thanks and Happy Thanksgiving to all.

David Farber
David Farber's Service Center
L.A., CA

--
Where's the KABOOM, there was supposed to be an earth shattering KABOOM.

Sonic Boom

Jay B wrote in message <19981125235047...@ng-fi1.aol.com>...
>but if its already shorted, why short the terminals??? jay
>

A loud hum usually means a shorted HV diode or magnetron or other short
circuit on the far side of the HV cap.

WARNING: Microwave ovens are the most dangerous consumer appliance to
service due to the up to 5,000 V at high current inside. Even with the
power off, there is a HV capacitor that can produce quite a kick. This
capacitor must be discharged before touching anything in the microwave
generator circuitry.

Don't even think about tackling it yourself unless you have the appropriate
background and have thorougly studied the Microwave Repair FAQ at the site
below as well as the info at: http://www.gallawa.com/microtech/, or a book on
microwave oven repair. There is one by Homer L. Davidson that may be
available at your public library.

Especially the SAFETY warnings and guidelines.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Mirror Info: http://www.repairfaq.org/REPAIR/F_mirror.html
| Sam's Laser FAQ: http://www.misty.com/~don/lasersam.html

>Subject: Re: Whirlpool microwave quit, cold turkey.
>From: lec...@home.com
>Date: 11/25/98 10:17 PM Central Standard Time
>Message-id: <365CD709...@home.com>

Thanks for the tip and for the special safety precaution information.

Happy Thanksgiving,

David

>From: s...@stdavids.picker.com (Sam Goldwasser)
>Date: 11/25/98 6:05 PM Pacific Standard Time
>Message-id: <SAM.98No...@colossus.stdavids.picker.com>

Hi David, I've seen microwave ovens crap out in the same manner you
describe, no heat and a loud hum from the transformer. Most likely the
magnetron itself is at fault, it shorts out, putting the high voltage
capacitor directly across the transformer. I assume the transformer
saturates the core because of the capacitor effectively across it's
secondary, which causes it to hum loudly. You can check the magnetron
with a DVM for shorts, unplug the oven and short across the
capacitor's terminals to discharge the cap (safety first). Disconnect
the leads from the magnetron (marked "F" and "FA"). Put your meter on
the Rx10K scale and measure from any one of the 2 leads to the
magnetron's case, you should read infinity, you may even want to put
your meter in the megohm range to check this. Any reading at all
indicates the magnetron is bad. I had a friend's oven measure 1.2K
between the magnetron leads and case, replacing the magnetron solved
the loud hum and the oven heated once again. The magnetron doesn't
necessarily have to read a dead short, any reading at all means it is
shot. I know you probably heard this a thousand times, but please be
careful, discharge the cap anyways to play it safe, those things bite
hard! Good luck and hope this helps!:)

Joe

As they say at www.compercraft.com: "The only he (the S.E.R FAQs) can't fix is
a broken heart." :-) I wish.

I was a little confused by what the schematic was indicating and what the
feedback from most of the techs was. I earlier replied with a post stating that
the diode measured open. A lot of techs said it's supposed to read open because
it is actually a series of stacked diodes. This makes sense. However, there is
a page in the Whirlpool schematic (the little folded up paper one that is
inside the unit) that actually says you can test the diode with a meter which
is powered by at least a 9 volt battery on the 1K range. So at that point I
believed that the diode was open because my meter usually doesn't have problems
checking diodes, stacked or otherwise. If you look at the NTE cross reference
specification page, the HVR-1X diode crosses to an NTE517. The data sheet
indicates that this is a "Low Forward Voltage Drop Diode". I guess "Low" is a
relative amount. Anyway, I read the FAQ's on checking the diode in a microwave
oven. It illustrated a little circuit with a 15V supply and a series resistor.
So I did this test with a 220 ohm series resistor and the voltage across the
diode measured 7.2 volts at 15ma of current. This of course would be exactly
where it is supposed to be, between 6 and 10 volts.

So now my attention turns back to the magnetron. I earlier checked the
resistance from the filaments to ground and it measured open on the 2 meg scale
of my meter. I wonder though if it would be better to check the reading from
each filament directly to the magnetron case. By the way, this oven is mounted
into the stove top and I don't have it in the shop. I just removed the diode
for testing.

Thanks for your responses.

Well www.compercraft.com doesn't load on my browser, does that need fixing too?

Regards,

Joe

Also, I know that all the micro switches should be checked, and then
the high voltage rectifier.

Please don't forget to discharge the big capacitor with a 100 K 1
Watt resistor before touching anything. You would have to use highly
insulated leads to do this...!!!

Leave the resistor connected while doing the work. The cap can build
up charge electrostaticly.

I have worked on and designed very high voltage supplies many times.
Proper precautions are important to take. The cap in the microwave
oven is deadly dangerious!

--

Jerry Greenberg
http://www.zoom-one.com/electron.htm
=============================================

Quick Reply To: jerr...@hotmail.com
We Specialize In Elecronic Componets

WebSite 1: http://www.total.net/~jerryg
Website 2: http://www.zoom-one.com

If It Works, Don't Fix It !
If It Don't Move, Slap It !
If It Don't Make Noise, Shake It !
If It's Broke, Then Fix It ... !
If You Don't Know How To Fix It, Give It Out !

============= Message Separator ================

FarberBear wrote in message
<19981205201531...@ng127.aol.com>...
|Hi again.
|
|I bought a new magnatron although I didn't find any faulty
components. I
|installed it and it is stll not working. The only irregularity I can
find is
|that the HV transformer reads about 340 ohms from the secondary
winding to the
|filament winding. It is my understanding that these windings should
be isolated
|from one another. Any thoughts about this situation?
|
|David

|>From: jfe...@megsinet.net (Joe)
|>Date: 11/28/98 8:59 PM Pacific Standard Time
|>Message-id: <3660d0c1...@megsnews.megsinet.net>

> Hi again.

> David

With the output leads disconnected, there should be no connection between
them - the filament winding is just a couple of turns separately on the core.

If you mean still in-circuit, 340 ohms would mean a high resistance short
in the diode, magnetron, or HV cap - or elsewhere - unless you meter applies
enough voltage to forward bias the diode - about 6 V minimum.

Did you ever do the test of disconnecting the transformer secondaries and
powering it up?

|| +------------------------+
||( 3.3 VAC, 10 A, typical |
TP Relay or || +------------+------+ | Magnetron
_ Fuse I __ Triac || | +-|----|-+
o------- _---+---/ -- ----/ ----+ || +------||----+ | |_ _| |
| )||( HV Cap | | \/ |
AC I \ I=Interlock )||( __|__ | ___ |
Line | TP=Thermal Prot. )||( 2,000 VAC _\_/_ +----|:--+
o------------+-------------------+ ||( .25 A | HV |'--> Micro-
||( typical | Diode | waves
(Controller not shown) || +------------+---------+
_|_
- Chassis ground

The resistance readings on the transformer were taken out of circuit. Ok, the
exact values, I have the transformer right in front of me out of circuit:

Filament: 0.0 ohms.

Primary: 0.4 ohms

Secondary: 108.6 ohms.(The schematic says this is normal range.)

One side of secondary winding to filament: 311 ohms (the 340 ohms I previously
stated was an estimate from memory)

Other side of secondary wiring which is actually ground, to filament winding:
202 ohms.

Thank you Sam for the wiring diagram. Unfortunaltely my AOL doesn't seem to
put the carriage returns in the proper place so it's quite cryptic.

I tested the HV diode with the circuit described in the FAQ. Just to review, 15
volt dc supply in series with a 240 ohm resistor. The voltage on the diode
measured about 8 volts as I recall which was in the normal range.

If I remove the secondary connections and power it up, I guess I could measure
the filament output. But suppose there is high voltage leaking into the
filament winding? Do I burn up my meter? I forgot to bring my HV probe. (My
"patient" is 185 milles away from home. I am a weekend guest.)

Thanks for your patience with this long drawn out post.

I agree with Sam. With the transformer disconnected, there should be
no continuity between the filament winding and the HV winding. A 202
Ohm resistance between the low or ground end of the HV winding and the
filament is tantamount to 202 Ohms across the diode when connected up.
Virtually the same as a "shorted diode" that Sam also mentioned
earlier.

Michael Caplan CET
------------------------

I have the transformer in my shop now. I was able to remove some of the glue
where the filament winding was held firmly to the transformer core. I then
proceeded to wiggle the wire and watch the resistance readings fluctuate from
200 ohms to infinity but was not able to locate the source of the short. So I
then gently tried to unwind the 3 turns of 18(?) gague wire. The last little
piece that came out looked nice and crisp and burned. I should mention that the
case of the transformer is actually the other end of the secondary winding or
ground. So the fact that the filament winding was touching the transformer case
was the problem.

I feel sort of resourceful now and was wondering if winding three turns of some
new 18 guage wire through the core is the smart and more importantly safe thing
to do? I have saved the old connectors and insulating material that was used on
the part of the wires that extended outward from the transformer.

Thanks.

You basically seen the worst it can do - assuming the use of similar fire
resistant(??) materials.

It sure is lovely to be at my bench with all my meters. After completely
removing the filament winding and double checking the primary and secondary
windings as far as dc resistance, it checks ok. So before undertaking the task
of threading the new filament wire through the core, I thought I might check to
see how much AC is coming off of the secondary winding. So I hooked up my
isolation transformer (you never can be too careful) to the primary and hooked
up my high voltage probe to the secondary. I put the meter on AC volts and
slowly turn up the voltage. I watch the meter carefully and it peeks out at 1.9
x 1000 AC volts. Then I click the Amp button on my isolation transformer and
it's measuring 3, that's right, three amps of current. If my math is correct,
that would be 115V x 3A or 345 watts being dissipated with no load. Now I
don't really want to let this transformer sit here sucking 345 watts of power
with no load unless I know why. Should I be worrying about power factor here?
lol. I don't suppose the fact that the filament wire is absent has anything to
do with this.

Is it time for a new transformer?

No, this doesn't indicate a problem (I just added this to the next rev of the
FAQ). However, if the filament winding was in the same channel as the HV
winding, you do need to confirm that the HV winding is not damaged - that the
arcing wasn't taking place to it instead of the core!

Testing the HV transformer using an AC current meter:
----------------------------------------------------

Where the HV transformer doesn't blow a fuse but overheats or produces
insufficient output, this test may be useful. If you have a clamp-on AC
ammeter, the transformer can be powered up to see if the primary current it
draws is reasonable with no load.

WARNING: Up to 3,000 VAC on HV terminal - AND possibly other windings if there
is a short in the transformer somewhere. Use a 3 prong cord with H and N
connected to the primary and G firmly screwed to the transformer core/mounting
structure. (Or, just remove the 3 secondary connections and power it through
the existing wiring using the normal oven controls.) The meter's clamp needs
to go around H or N but not both. Stand well clear when you apply power!

Use of a Variac is recommended but not essential. However, here are the input
current readings using a Variac at various input voltages for the HV
transformer from a typical mid-size microwave oven:

Input VAC Input Amps
------------------------
80 .3
90 .6
100 1.1
110 2.0
115 3.0
120 >>3.0

Above about 100 VAC, there was also a noticeable hum (though not nearly as
great as with a secondary short).

No, these readings do not indicate a problem. Microwave oven transformers are
designed with as little copper as possible. And, yes, the non-linear increase
in current indicates that the core is saturating with no load.

If your readings are similar to these, the transformer is likely good.
Shorted turns would result in much higher current at all input voltages.

Hi Sam, I guess that blows my idea of making an isolation transformer
with 2 identical microwave oven transformers. What I was going to do
is cut the welds on both transformers, remove the high voltage
secondary from one, remove the primary of the other transformer and
put it in place of the old secondary, then have a friend of mine
reweld the welds. Is a sense, run the normal primary with the other
transformer's primary in place of the old HV secondary, hence a 1:1
isolation transformer without the dangers of the high voltage. I know
it may be a lot of work getting the transformers apart, but would this
be a wasted effort, due to the core saturating with little or no load?
Just trying to satisfy my curiosity!:)

Joe

However, I sometimes thank my whimpy isolation transformer when a device
wants to draw too much current - automatic current limiting of sorts!

I went to the hardware store and bought some solid core wire that I believe is
normally used in house wiring. The writing on the wire says " 16 AWG, 600V, 105
C, UL approved." And there are a bunch of other numbers. The conductor appears
to be copper. Obviously this is not wire rated at 5,000 V. However, the old
filament wire that I removed doesn't have any writing on it, it appears to be
of the same gague, the conductor is silver colored, and the insulation seems
comparable. I measured the length of the old wire and threaded through the same
length of new wire. I managed to pull three turns through the core with the
same length of wire. There is no way you could squeeze any more turns through
the tight space of the core. I hooked the transformer back up to my isolation
transformer and I got a reading of 2.74 ACV on the new filament winding with no
load of course. The FAQ's mention a typical reading 3.3 ACV and I would assume
that would be under operating load. So I pushed as hard as I could to wind
another half a turn around the core. And this extra half turn brought the ACV
up to 3.29. So I guess the question that remains, how does the unmarked
original filament wire compare to the replaced wire I put in? Could the
unmarked wire be that much better even though it is basically the same
dimensions? Perhaps an undectable few thousandths of an inch in insulation can
make a big difference in insulation strength?

Thanks.

Maybe someone has some extra HV high current wire they could email to you. :)

>I'm sure your 3 or 3-1/2 turns would be fine. However, I wouldn't trust that

David, after reading your Usenet post, this was after answering your
E-mail, I have to agree with Sam. The insulation on this wire is rated
at 600 volts, that it will handle safely without breaking down the
insulation, but I wouldn't trust it with 5000 volts between it and the
core. My transformer has what looks to be like thick (Sam, you can
call this one), sheets of mica (I know it isn't mica, more like a
plastic material) between the sides of the winding and the core. I
know it is a tight fit putting the winding there in the first place,
but if you could jam some kind of better insulating material between
the winding and the core, I'd feel better about operating the
transformer. I just don't feel good about holding back 5KV with vinyl
insulation. My best bet would be to find some magnet wire at 16 gauge,
(you only need about, my guess 4 feet, the insulation will be thinner
physically, but it will enable you to jam some fish paper or other
good insulator between it and the core. Sam, you are welcome to jump
in and name the insulating material, I'm not sure what it is on my
transformer!:)

Joe

Sam, wouldn't it be better if the transformer in a microwave oven ran
immersed in oil, much like a pole transformer. On our power system on
a single phase line, the insulation of the transformer has to
withstand 7600 volts, on a 3 phase system, the transformer(s) have to
withstand 13200 volts! <G> ;-)

Joe

Sure, you willing to pay more for the oil, hermetically sealed container
for the transformer, and have it last 100 years instead of fail just out of
warranty? :) Realize that the transformers used in microwaves are just barely
designed to work at all - see the previous part of this thread about
core saturation when lightly loaded.

I sometimes wonder about that 600 V wire. I have seen some types with really
thin insulation and other with what could be mistaked for insulation rated
for 20 KV! I have never tested either, however.

Thanks for your helpful comments,

David

>Date: 12/12/98 12:12 PM Pacific Standard Time
>Message-id: <SAM.98De...@colossus.stdavids.picker.com>