Notes on the Troubleshooting and Repair of Microwave Ovens


  7.4) Making measurements inside microwave ovens

WARNING: In general, I DO NOT recommend making any sorts of measurements on
the high voltage components of a live microwave oven.  I only include this
section for those who really want to know the details.

You may be temped to break out your Radio Shack DMM and start poking away
inside a live microwave oven.  DON'T!  This isn't like a CD player!  Most of
the time, no measurements of any kind on the oven while it is operating will
be needed to identify and correct the problem.  However, where this is not the
case, here are some guidelines to a long life:

WARNING: ALWAYS pull the plug and discharge the HV capacitor BEFORE doing
anything inside!  Never be tempted to make any changes of any kind while
the oven is on - not even if your meter is being consumed by 5 foot flames!
First, pull the plug and discharge the HV capacitor!

* High voltage - DON'T even think about this unless you have a proper high
  voltage probe or meter, or a proper microwave oven tester - AND KNOW HOW TO
  USE IT SAFELY.  Even professionals have been killed performing measurements
  of this type using proper equipment!  Luckily, current measurements can
  provide enough information to help make a diagnosis.

  WARNING: The high voltage components inside a microwave oven are at a
  NEGATIVE potential with respect to the chassis.  DO NOT be tempted to
  interchange the probe and ground wire if you are using a high voltage
  probe on a meter with a POSITIVE input (e.g., for testing CRT HV) and no
  polarity switch!  The ground cable doesn't have anywhere near the required
  insulation.  Get the proper equipment!

* Magnetron current - Place a 10 ohm 10 watt resistor in series with the HV
  diode cathode and ground.  Measure the voltage drop across this resistor.
  Sensitivity will be 10 V/A.  Normal anode current is around 300 to 400 mA
  for a typical oven.  This will be -3 to -4 VDC across the 10 ohm resistor
  with respect to chassis ground.  SET EVERYTHING UP AND THEN STAND BACK and
  don't forget to DISCHARGE the HV capacitor after making the measurement:

  - If it is around this range, the magnetron is probably fine.

  - If it is very low or 0, magnetron is bad or HV is not working.  Note that
    a shorted as well as open magnetron also results in no current.  If the
    magnetron is shorted, it bypasses all current to ground.  If the magnetron
    is open, the HV capacitor charges up and then there is no more current
    through the HV diode (but there will be an initial transient).

  - If it is much too high (whether fuse blows or not), capacitor is shorted.

  7.5) Testing the high voltage components

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

Assuming the oven passes the above test for interlocks and door alignment, the
triac (if used) may be defective.  There could also be a wire shorting to the
chassis.  However, the most likely problems are in the microwave generator. 

An ohmmeter can be safely used to quickly determine if the capacitor, HV diode,
or magnetron are a dead short (as well as for an open magnetron filament).

Use an ohmmeter to test the diode and capacitor.  While connected in circuit,
the resistance in at least one direction should be several M ohms.  (Try it in
both directions, use the higher reading).  Test the magnetron from the filament
to chassis - it should be high in at least one direction.  Test the filament
for continuity - the resistance of a good filament is close to 0 (less than 1

Where the capacitor and diode are combined into one unit, it should be possible
to test each component individually and replace only the one that is found to
be defective if the entire assembly is excessively expensive or not available.

These may be considered to fail/no conclusion tests - they can definitively
identify parts that are bad but will not guarantee that they are good.  Parts
may test ok with no voltage applied but then fail once operated in-circuit.
Connections may open up when they heat up.  The magnetron may short out when
full voltage is applied.

Don't overlook the wiring as no heat or erratic operation can result from
simple bad connections!

An alternative way of determining if the problem is in the control circuits
(triac, relay, wiring) or microwave generator (HV transformer, HV capacitor,
HV diode, magnetron, wiring, etc.) is to connect the HV transformer primary
directly to a line cord and plug.  Tape the removed wire lugs to prevent

Plug the transformer cord into a switched outlet strip which includes a fuse
or circuit breaker.

Put a cup of water into the oven cavity to act as a load.

* Power the oven via its line cord.  Initiate a cook cycle.  It should go
  through the normal cycle (of course no heat) without blowing the fuse or any
  unusual sounds.  If there is a problem in this case, something in the
  controller or its wiring is shorted.

* Now, initiate a 1 minute cook cycle on HIGH and with the oven running,
  switch on the HV transformer.

  - If the transformer or other HV components are faulty, the outlet strip
    fuse will blow or circuit breaker will trip.  Or, if a lamp is plugged
    into the outlet strip at the same time, it will likely dim significantly
    due to the heavy load before the fuse or breaker cuts out.

  - If the problem is with the triac or its drive, the oven will now heat
    normally.  When the cook cycle is near its end, switch off the outlet
    strip.  Check the water's temperature.

More complete information on testing and replacing the individual components
is provided in the next few sections.

  7.6) Testing the high voltage diode

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

The HV diode can fail shorted (most likely) or open.  It is not likely for
there to be anything in between as so much heat would result that the diode
would not remain that way for long.

* A shorted HV diode will likely result in a loud hum from the HV transformer
  when a cook cycle is initiated.  The main fuse will probably not blow.

* An open HV diode will result in AC instead of DC across the magnetron with a
  peak negative value (the only one that matters) about 1/2 of what it should
  be.  The result will likely be little or no detectable heat but no other

The resistance measured across the leads of the HV diode should be greater
than 10 M ohm in at least one direction when disconnected from the circuit.
However, the HV diode is composed of multiple silicon diodes in series to
get the voltage rating.  Its forward voltage drop will therefore be too great
(6 V or more) for a DMM to produce a definitive answer as to whether it
actually works as a rectifier.

The HV diode can be tested with a DC power supply (even a wall adapter of
at least 12 or 15 V output), series resistor (to limit current), and your
multimeter.  This will determine proper behavior, at least at low voltages.

The following is the schematic of a simple HV diode tester:

                 240 ohms, 1 W
       + o-----------/\/\---------+------------o +
                                __|__ HV       Good: 6 to 10 V
     15 VDC                     _\_/_ diode    Shorted: 0 to 2 V
                                  |            Open or reversed: 15 V
       - o------------------------+------------o -

The voltage drop in the forward direction should be at least 6 V with a few
mA of current but may be somewhat higher (8 V or more) with a few hundred mA.
If your DMM or VOM has a resistance scale operated off a battery of at least
6 V, you may get a reading in one direction (but only one) without the need
for an external power supply.

Or, assume for now that the diode is good if it is not shorted - which is

  7.7) Replacing the HV diode

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

Most HV diodes have press fit (Fast-On) or ring lugs so replacement is very
straightforward.  Discharge the high voltage capacitor.  Make sure you get
the polarity correct if your replacement can be installed either way.  Putting
the diode in backwards will result in positive instead of negative high
voltage and, needless to say, no heat, but no other symptoms either.

Note: the lugs on your new HV diode may just be crimped onto the wire leads
and not welded or soldered.  If this is the case, take care not to stress them
excessively which might result in bad connections now or in the future.  It
may be a good idea to solder the lugs to the wires as well (though this may be

Where the diode is part of the capacitor assembly, it should be possible to
just replace the diode leaving the old one unconnected (at one end).  This
will probably be much much cheaper than replacing the entire assembly.

  7.8) HV diode ratings

Most replacement microwave oven diodes are rated 12 to 15 KV PRV at .5 A.  A
PRV of around 8 KV is actually required even for a small oven.  Here is why:
Until the magnetron heats up and starts conducting in its forward direction,
what you have is a half wave rectifier/filter formed by the HV transformer
secondary, the HV diode, and the HV capacitor.  The reverse voltage across the
HV diode will be equal to: 2 * 1.414 * (VRMS of the HV transformer).  This can
easily be 6 or 7 KV or more!  Once the magnetron start conducting, the reverse
voltage goes down somewhat.

HV diodes rated at .5 A are adequate for most domestic microwave ovens.  For
example, the largest of these will have a nameplate rating of around 1,800 W
power line input and a HV transformer secondary of 2,500 VAC.  While there are
some losses in the HV transformer, and some power is used by the magnetron
filament, controller, motors, and light, this still leaves, perhaps, 1,600 W
into the HV generator.  However, due to the design of the half wave doubler
circuit, not all the power flows through the HV diode (as would be the case
with a regular power supply.  Thus, even though calculations using Ohms law
(I = P/V = 1,600/2,500 or .64 A) would suggest that .5 A is not enough, closer
to 1/2 of the total current actually flows through the HV diode.

To be doubly sure that your new HV diode is happy, run the oven on full power
(high) for 10 minutes with two quarts of water as a load (or a roast).  Unplug
the oven (while your spouse prepares the veggies), quickly DISCHARGE THE HV
CAPACITOR, and then check the HV diode for overheating.  It might be warm but
should not be too hot to touch.  Unless you have the largest oven on earth,
this test is probably not needed.

  7.9) Testing the high voltage capacitor

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

* A shorted HV capacitor will blow the fuse instantly.

* An open HV capacitor will result in no heat but no other symptoms.

(The following assumes no internal rectifier or other circuitry except of
a bleeder resistor.  Adjust procedures accordingly if your oven is different.)

The resistance measured across the terminals of the high voltage capacitor
should be very high - several M ohms for bleeder resistor.  If it is less
than 1 M ohms, the capacitor is definitely shorted.  Yes, if you measure
0.00 ohms across the terminals (and they are not bussed together on the
case), then the capacitor is positively, without a shadow of a doubt, bad!

A high resistance does not prove that the capacitor is actually functional,
just not shorted with no voltage across it.  If you have a capacitance meter,
check it for proper value (should be printed on the case).  Even this does
not prove that it will not short when full voltage is applied.  Substitution
is the only sure test beyond this.

  7.10) Replacing the high voltage capacitor

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

Make a diagram of the precise wiring as multiple connections are often made to
the capacitor terminals.  The capacitor is usually mounted with a clamp which
is easily loosened.  Sometimes, the capacitor is jammed into a location that
requires moving some other components to extract it.

Replace in reverse order.  Tighten the clamp securely but not so much as to
distort the case.

Where the capacitor assembly also includes the HV diode, it is possible to
just replace the capacitor if space permits leaving the old one unconnected
(at one end).  However, the cost of a generic replacement diode is small
(around $3) so replacing both at the same time is usually best.  However,
you don't need to use the exact combined part - which may be very expensive
or difficult to obtain.  Just make sure the ratings of the capacitor and
diode are correct (use a generic replacement microwave oven HV diode and a
microwave HV capacitor with a uF rating within 10% or so of the old one and
at least equal working voltage).

  7.11) Testing the magnetron

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

* A magnetron with an open filament will result in no heat but no other
  symptoms.  The bad connection may be internal (in which case the magnetron
  will need to be replaced) or external at the filament terminals (which may
  be repairable).

* A magnetron with with a short between the filament/cathode and anode will
  likely result in a loud hum from the HV transformer and/or magnetron when
  the cook cycle is initiated but the main fuse will probably not blow.

* A magnetron with other faults may result in a variety of symptoms including
  erratic or low output power or intermittent operation.  See the section:
  "Comprehensive list of magnetron failure modes".

There is no totally definitive way to determine if a magnetron is good without
actually powering it under operating conditions but the following tests will
catch most problems:

* Magnetron filament.  The resistance should be infinite from the filament
  connections to the case and a fraction of an ohm between the filament
  terminals with the wiring disconnected from the magnetron.

  While measuring resistance from filament chassis, gently tap the magnetron
  to determine if there is an intermittent short.  However, such problems may
  only show up once the filament heats up and parts expand.

  It may be possible to determine if the magnetron filament is actually
  working by connecting just the filament connections to a low voltage
  high current supply on a Variac (e.g., a microwave oven transformer but just
  the filament connections).  The ceramic insulators are translucent and should
  show a glow with a working filament.  The one at the antenna may be visible
  if the magnetron is removed from the oven or with a dental mirror looking
  into the waveguide.  WARNING: Make sure you ONLY have the filament connected!

* Evidence of arcing (visible blackening around ventilation holes in base or
  burnt odor) usually indicates a bad magnetron.

* Melting or other damage to the antenna cover ('bull-nose' or 'bullet') may
  be the result of arcing due to problems in the oven cavity or waveguide
  (perhaps operating with nothing in the oven) or a defective magnetron.

  (This part is only visible with the magnetron removed from the oven).  If
  a problem elsewhere has been corrected, the damaged antenna cover can be
  pulled off and replaced from a magnetron that died of other causes - try
  your local appliance repair shop.  (The shape doesn't matter as long as
  it fits tightly - there are several diameters, however.)  Your magnetron
  may still be good.

Most common magnetron failure modes:

* Filament could be shorted to case - check with ohmmeter.  Anything less than
  infinity means the tube is bad though it could be charring due to arcing
  outside the vacuum in the box with the filament connections.  Tap the tube
  while measuring to check for intermittents.

* Filament could be shorted to itself - tough to test since it is such a low
  resistance to start.  Compare with good magnetron.  (Yeh, right.  If you had
  one, this wouldn't be an issue!)  Tap the tube while measuring to check for
  intermittents.  This fault isn't really likely.

* Filament could be open - check with ohmmeter.  Tap the tube while measuring
  to check for intermittents.  However, loose filament connectors (Fast-Ons)
  are more likely than a broken filament.  Therefore, check directly at the
  magnetron terminals with both lugs pulled off.

* Magnetron could be gassy (or up to air) and arcover internally once power is
  applied.  The filament could expand, shift position, and short once heated.
  There is no easy way to test for these possibilities other than substituting
  a known good magnetron.

* Internal or external arcing resulting in physical damage.  External arcing
  could be at the antenna or inside the filament box.  Internal arcing will
  not leave any visible evidence but the damage will result in the magnetron
  failing totally or running with reduced output.

* Overheating might result from a broken or cracked magnet (reduced magentic
  field) or other internal problems.  While there may be some output power,
  the thermal protector will shut down the oven prematurely.

  7.12) Comprehensive list of magnetron failure modes

(Portions from: John Gallawa (mtek@pen.net)).

Here is a list of typical magnetron failure modes. The percentage of each type
of failure varies. Currently, internal shorts and loose filament connectors
are probably at the top of the list. An internal plate-cathode short may only
manifest itself under the stress of high voltage during operation.

1.  Shorts. (a) Internal plate-cathode/filament short or (b) Internal arcing.

    Symptoms: No heat, loud hum when entering cook cycle, possible blown HV
    fuse (but will not likely blow the main fuse).

    In ovens equipped with fuses that monitor the high voltage system, such
    as some commercial Sharp models and most commercial and domestic Amana
    models, the high voltage fuse would probably blow. But, rarely will a
    shorted magnetron cause the main line fuse to blow. (I suppose the
    transformer absorbs most of the current surge.) In fact, with reference
    to the other symptoms below, there are almost no failures where the
    magnetron causes the line fuse to blow.
2.  Loose filament connectors (these may be repairable).  There will often
    also be visual symptoms at the magnetron:  Signs of overheating, such as
    discoloration; and evidence of carbon tracks or pits on magnetron terminals
    when the connectors are removed.  An intermittent filament (internal) is
    also possible (but not repairable).

    Symptoms: No heat or erratic heat.

    The slip-on connectors can loosen, overheat, build up resistance and
    eventually loose contact. If the the magnetron terminal(s) have not been
    burned too severely, the connection(s) can usually be repaired. We prefer
    cleaning up the terminal, then soldering the filament wires directly to
    the terminal.

    Note: when discharging HV capacitor, since there is no load, it may end
    up being charged to a much higher voltage than is normal.  Be prepared
    for a larger spark if you use a screwdriver to discharge it!
3.  Open filament.

    Symptoms: No heat.

    See note about HV capacitor in (2) above.
4.  In the older glass-dome models, the vacuum envelope can rupture.

    Symptoms: No heat, loud buzz due to arcing when entering cook cycle,
    possible blown HV fuse.
    See comments about fuses in (1) above.

5.  Filament breakdown. Usually occurs after a few minutes of normal operation,
    possible blown HV fuse.

    Symptoms: No heat, loud hum once it occurs.

    See comments about fuses in (1) above.

6.  Low output. Occurs as cathode emission decreases from long use.

    Symptoms: Reduced cooking power.

7.  Moding. Occurs when magnetron oscillates in one or more undesirable
    Symptoms: (a) Reduced or no cooking power, (b) RF interference.  However,
    some food products (with high water content) may cook normally, whereas
    the result with other foods is very unsatisfactory.  RF interference is
    possible but usually only occurs if there is actual structural damage to
    either the magnetron, its RF gasket or waveguide flange, or its RF
    (feed-through) capacitors. 

8.  Off frequency. Physical characteristics can change and cause magnetron to
    oscillate at frequencies slightly higher or lower than 2.45 GHz.
    Same as (7a) above.
9.  RF leakage. Structural failure can cause leakage from magnetron housing.

    Symptoms: Microwave leakage into electronics bay, erratic control panel
    behavior. It can be very frustrating because the symptoms disappear when
    the oven's outer cover is removed. With the cover in place, the escaping
    RF energy is confined, and eventually builds up around the control panel
    circuitry causing unusual symptoms. 
10. Insulation breakdown of the internal leads or at magnetron insulators
    or antenna terminal.

    Symptoms: Arcing, burning smell from magnetron, loud hum, no heat.

11. Cracked magnet(s).

    Symptoms: Reduced or no cooking power, magnetron overheating, occasional
    'snapping' sound. 

  7.13) Where to obtain replacement magnetrons

Depending on the age of your oven the magnetron may still be under warranty.
Check the original paperwork that came with the oven - either the users
manual or a separate warranty document.  Contact the manufacturer if specific
instructions on how to file claims are not provided.  Full coverage on the
magnetron of several years is common.  If you have not sent in the warranty
registration card (right, who actually does this?!), a copy of the sales
receipt or other proof of date of purchase may be required.

Both original and generic replacement magnetrons are available.  Going direct
to the oven manufacturer will guarantee a compatible magnetron but is by far
the most expensive option.  For a typical oven, one without the gold-plated
trim :-), such a replacement may be more than half the cost of a similar
new oven.  In some cases (like Sears), you may need to convince their service
department that you are qualified to be poking around inside one of *their*
appliances before they will consider selling one to you (too many lawyers).

In some cases, original magnetrons may also be available from parts suppliers
like MCM Electronics - at somewhat less rediculous prices.  They will be
identified as 'original' or 'genuine' along with the manufacturer and their
part number.

Generic replacement magnetrons are available for the majority of microwave
ovens.  These will almost certainly be much less expensive than original
parts.  Essentially, there is only one type 'tube' (at least for any similar
power range).  The differences are mostly mechanical.  However, quality may
vary.  In some cases, the generic variety may actually be better than the
original.  See the section: "Comments on replacement magnetron quality" for
some recommendations.

  7.14) Comments on replacement magnetron quality

(From John Gallawa (mtek@pen.net)).

In my experience, mags purchased from after-market suppliers may or may
not be OEM parts (there are not that many manufacturers of magnetrons in
the world). Here's the interesting thing, though: In many cases, these
after-market tubes are actually higher in quality than the original
tube, as in the case of the OEM Sanyo magnetrons, which tend to fail
prematurely. Of course, the opposite can also be true, depending on the
after-market supplier. Some manufacturers, such as Toshiba and Hitachi,
produce both high and low end magnetrons. They sell these under a
variety of specialty names, as well as under manufacturer brand names. I
have seen the low-end tubes in many brand-new microwave ovens.

When buying magnetrons from other than the manufacturer, I have found it
best to go to a supplier who specializes in microwave oven parts (i.e.
AMI, Global Micro-parts, QB products). These sales people are usually
more knowledgeable about the magnetrons they sell, and they can help you
with proper choice and application. 

  7.15) Replacing the magnetron

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

When you receive the replacement, compare it with the original.  It is critical
that the replacement magnetron be mechanically identical: this means that the
mounting configuration (studs or holes and their location), waveguide seating
surface, and the orientation of the filament connections and cooling fins are
the same.  The studs may be removable so that the same assembly can be used
with or without them.  The cooling fins are particularly important as there
must be adequate airflow from the fan for removal of the substantial waste
heat - up to half of the input power to the magnetron ends up as heat.  The
shape of the antenna terminal - cone, bull nose, or square - doesn't matter.

Magnetron replacement is generally straightforward but other assemblies like
the cooling fan may need to be removed to gain access.  Make careful notes
of both the wiring and mechanical relationships.  Usually, the magnetron is
fastened to the waveguide with 4 nuts on studs.  When removing it from its
mounting, do not lose the RF gasket - a metal mesh ring which seals the
connection against microwave leakage.  Reuse it unless your replacement
magnetron comes with a new one.  Transfer any thermal protector to the new
unit.  Replace other components in reverse order and then reattach the
filament and HV wires.

Although the magnetron is a vacuum tube, there is probably no glass in yours
(unless it is quite old) so it isn't really very fragile.  However, a sharp
blow or fall (during shipping as well if not properly packed) could shatter
the filament.  Do keep it (the magnets) away from your diskettes unless you
want them bulk erased!

As for the old one, see the section: "The magnets in dead magnetrons" :-).

  7.16) Testing the high voltage transformer

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

* A shorted winding or short between a winding and the core/chassis in the HV
  transformer may result in a blown fuse, loud hum, overheating, audible
  arcing, a burnt aroma, or simply no heat.

* An open winding will likely result in no heat but no other symptoms.

Disconnect terminals as required to make the following tests:

* The resistances of the primary should be .1 to .5 ohms (.2 ohms typical).

* The resistance of the filament winding will likely be so low as not to be
  detectable with your multimeter.  The only measurement easily made would
  be that there is no short to the chassis.

* Typical resistance readings for the transformer HV secondary are in the 25
  to 150 ohms range (depending on the power rating of the oven) from HV
  connection to chassis.  A typical midsize might be 65 ohms.  An open would
  be an obvious failure.  However, based on the way these are wound, a
  winding-to-winding short would not cause enough of a resistance change to
  be detected with an ohmmeter unless you could compare with an identical
  model transformer from the same lot number.

Testing the high voltage transformer more fully is difficult without fancy
equipment.  Only major short circuits can be identified in the transformer
with an ohmmeter since the nominal resistance of the windings is unknown.
However, open windings (not very likely) can be located and other faults
can be identified by the process of elimination.

Note: in the discussion below, it is assumed that the fuse is blowing due to
a possible short in the HV transformer.  Alternatively, there may be a loud
hum as the HV transformer struggles due to a fault in the HV transformer or
a shorted HV diode, magnetron, or a short in the HV wiring.  Also note that
depending on the severity of the fault, the fuse may not actually blow (at
least not immediately) but there will likely be a loud hum when the HV
transformer is powered.

* Disconnect the primary of the HV transformer and initiate a cook cycle.  If
  the fuse still blows, you have a problem elsewhere such as a defective
  interlock or shorted wire.

* Assuming the fuse does not blow, unplug the oven and reconnect the primary
  of the HV transformer.

* If the other components - HV diode, HV capacitor, magnetron - test out,
  remove the high voltage and filament connections to the transformer, power
  up the oven, and initiate a cook cycle.  If the fuse does not blow, the
  transformer is likely good and there are still problems in the high voltage
  components.  Possibly something is failing only when full voltage is applied.

* If the fuse still blows, then the problem is likely with the triac (if used),
  a shorted wire, or shorted transformer.

* If the fuse does not blow with the secondary isolated, reconnect only the
  magnetron filament (not the HV) to the transformer and power it up again.
  If the fuse now blows, then it is possible that the magnetron filament is

* If your oven uses a triac, remove and bypass it.  Now, if the fuse still
  blows when the oven is plugged in (door closed to enable the interlocks),
  the problem is likely with the transformer.

Unplug the oven, discharge the HV capacitor.

* Check for damaged wires that may be shorting to the chassis.  Repair or
  replace these as necessary.

  7.17) Replacing the high voltage transformer

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

Replacement of a HV transformer is straightforward but other assemblies may be
using the transformer bolts for their mounting and/or may block your way.

Label the wires before pulling off the Fast-Ons if there is any doubt as to
where they go.

If the replacement transformer is not mechanically identical, you may need
to use some creativity in anchoring it and any structures that are attached
to its frame.  However, the transformer must be secure - don't just sit it
in place.

Try not to drop either the old or new transformer on your foot!

  7.18) Testing and repairing the wiring and connections

WARNING: First, with power disconnected, discharge the high voltage capacitor.
See the section "Safe discharging of the high voltage capacitor".

Inspect the wiring - especially between the magnetron, HV transformer, and
other components of the high voltage circuits for signs of arcing and excessive
heating or burning.  Arcing may be the result of the wire scraping against a
sharp sheet metal edge due to poor placement and or vibration.  A bit of
electrical tape may be all that is needed.

Since the magnetron filament in particular uses high current, any resistance
at the press (Fast-On) connections will result in heating, weakening of the
lug, more heating, and eventual failure or erratic operation.  Try to pull off
each of the lugs.  They should not be loose - you should have to work at
removing them.  However, note that some lugs are of the locking variety and
require that you push a little tab to release them.

Check for loose, burnt, or deteriorated lugs in the filament circuit (not just
the magnetron).  If you find evidence of this:

* Remove the lugs and clean the terminals with fine sandpaper or a file.  If
  they are not too badly deteriorated, they will still work even if they are
  somewhat ugly.

* If the lugs and their wire connections appear to be in good condition but
  come off their terminals easily, try squeezing them a little tighter with a
  pair of pliers and reinstall.  Otherwise, cut off the old ones and replace

* If any connections between the lug and the wire or HV diode are at all loose,
  solder it with a high wattage soldering iron or soldering gun.

* Alternatively, use a drill to make a hole in each terminal, and then fasten
  the (tinned) wire directly (or better yet) a new ring lug to the terminal
  with a machine screw, nut, and lockwasher.  Soldering is also possible.

  These approaches will work as long as there is enough metal remaining for a
  solid connection and may permit you to salvage a magnetron or HV transformer
  that would otherwise need to be replaced.

Also check for bad solder connections between the terminals on the high voltage
transformer and the enameled wire used for its windings.  If you find anything
suspect, scrape away the enamel and surface corrosion and resolder with a high
wattage soldering iron or soldering gun.

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]