Question about Panasonic NN-C994S Convection/Microwave Oven

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My PCB transformer primary is always blowing off what could be problem

I tried to replace PCB transformer the one which is behind display board. but always primary blows off. I suspect some sensor to be faulty viz. humidity/temp sensors. Can someone help to locate it and anyone has link to service layoout drawing please

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Re: My PCB transformer primary is always blowing off what...

Is your line voltage correct? That needs to be measured, perhaps even monitored. Is it possible you have installed 120 volt replacements into a 220 volt model? Otherwise, there is a component shorted on the control board, such as a rectifier diode, zener diode, filter capacitor, or transistor. This would call for old-style troublehsooting. It's possible you have a bad sensor, but I doubt it. They could be disconnected or bypassed for test purposes.

Posted on Apr 10, 2007

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My microwave has no power,I have checked all fuses, switches and thermostats, i've replace the main control pcb. still no lights or display

pwr comes in, goes thru fuse to transformer which powers pcb and mag. pcb is not getting pwr. chk ur installation and the output from the transformer

Jan 25, 2016 | Microwave Ovens



What can go wrong The most common problems occur in the microwave generating portion of the system, though the controller can be blown by a lightning strike or other power surge. Bad interlock switches probably account for the majority of microwave oven problems. Also, since the touchpad is exposed, there is a chance that it can get wet or damaged. If wet, a week or so of non-use may cure keys that don't work. If damaged, it will probably need to be replaced - this is straightforward if the part can be obtained, usually direct from the manufacturer. Unfortunately, it is an expensive part ($20-50 typical). The interlock switches, being electromechanical can fail to complete the primary circuit on an oven which appears to operate normally with no blown fuses but no heat as well. Faulty interlocks or a misaligned door may result in the fuse blowing as described above due to the incorrect sequencing of the door interlock switches. Failed interlocks are considered to be the most common problems with microwave ovens, perhaps as high as 75% of all failures
No adjustments should ever be required for a microwave oven and there are no screws to turn so don't look for any!

General system problems The following problems are likely power or controller related and not in the microwave generator unless due to a blown fuse or bad/intermittent connections:
  • Totally dead oven.
  • No response to any buttons on touchpad
  • Oven runs when door is still open.
  • Oven starts on its own as soon as door is closed.
  • Oven works but display is blank.
  • Whacked out controller or incorrect operation.
  • Erratic behavior.
  • Some keys on the touchpad do not function or perform the wrong action.
  • Microwave oven does not respond to START button.
First, unplug the microwave oven for a couple of minutes. Sometimes, the microcontroller will get into a whacko mode for some unknown reason - perhaps a power surge - and simply needs to be reset. The problem may never reoccur.
Note: when working on controller related problems, unplug the connection to the microwave generator (HV transformer primary) from the power relay or triac - it is often a separate connector. This will prevent any possible accidental generation of microwave energy as well as eliminating the high voltage (but not the AC line) shock hazard during servicing.
If this does not help, there is likely a problem with the controller circuitry or its power and you will have to get inside the oven.

Uninvited guests Some cockroaches (or other lower life forms) may have taken up residence on the controller circuit board. It is warm, cozy, safe, and from their point of view makes an ideal habitat. If you got the microwave oven from a flea market, garage sale, the curb, a relative, or friend, or if your kitchen isn't the cleanest in the world, such visitors are quite possible. Creatures with six or more legs (well, some two legged varieties as well) are not known for their skills in the areas of housekeeping and personal hygiene. Clean the circuit board and connectors thoroughly with water and then isopropyl alcohol. Dry completely. Inspect the circuit traces for corrosion or other damage. If there are any actual breaks, these will have be be jumpered with fine wire and then soldered. Hopefully, no electronic components were affected though there is always a slight possibility of other problems.
Totally dead oven First, check power to the outlet using a lamp or radio you know works. The fuse or circuit breaker at your service panel may have blown/tripped due to an overload or fault in the microwave oven or some other appliance. You may just have too many appliances plugged into this circuit - microwave ovens are high current appliances and should be on a dedicated circuit if possible. If you attempt to run a heating appliance like a toaster or fryer at the same time, you *will* blow the fuse or trip the circuit breaker. A refrigerator should never be plugged into the same circuit for this reason as well - you really don't want it to be without power because of your popcorn! If you find the fuse blown or circuit breaker tripped, unplug everything from the circuit to which the microwave is connected (keep in mind that other outlets may be fed from the same circuit). Replace the fuse or reset the circuit breaker. If the same thing happens again, you have a problem with the outlet or other wiring on the same branch circuit. If plugging in the microwave causes the fuse to blow or circuit breaker to trip immediately, there is a short circuit in the power cord or elsewhere.
The microwave oven may be powered from a GFCI outlet or downstream of one and the GFCI may have tripped. (Removing a broken oven lamp has been known to happen.) The GFCI outlet may not be in an obvious location but first check the countertop outlets. The tripped GFCI could be in the garage or almost anywhere else! Pushing the RESET button may be all that's needed.
Next, try to set the clock. With some ovens the screen will be totally blank following a power outage - there may be nothing wrong with it. Furthermore, some ovens will not allow you perform any cooking related actions until the clock is set to a valid time.
Assuming these are not your problems, a fuse has probably blown although a dead controller is a possibility.
If the main fuse is upstream of the controller, then any short circuit in the microwave generator will also disable the controller and display. If this is the case, then putting in a new fuse will enable the touchpad/display to function but may blow again as soon as a cook cycle is initiated if there is an actual fault in the microwave circuits.
Therefore, try a new fuse. If this blows immediately, there may be a short very near the line cord, in the controller, or a defective triac (if your oven uses a triac). Or, even a shorted oven lamp - remove and inspect the light bulb and socket.
If it does not blow, initiate a cook cycle (with a cup of water inside). If the oven now works, the fuse may simply have been tired of living. This is common.
If the fuse still blows immediately, confirm that the controller is operational by unplugging the microwave generator, power relay, and/or triac from the controller. If a new fuse does not now blow when a cook cycle is initiated - and it appears to operate normally - then one of the components in the microwave generator is defective (shorted).
Some models have a thermal fuse as well and this may have failed for no reason or a cooling fan may not be working and the oven overheated (in which case it probably would have died while you were cooking something for an important guest - assuming you would use a microwave oven for such a thing!).
Other possible causes: bad controller power supply or bad controller chip.

Totally dead oven after repair On some microwave ovens, there is at least one cabinet screw that is slightly longer than all the others. This engages a safety interlock which prevents the oven from receiving power if the correct screw is missing or in the wrong hole. Check the length of all the screws and locate the interlock switch behind one of the screw holes. I don't know how common this practice is but have heard of it on some Sharp models.

Dead controller The most common way that the controller circuitry can be harmed is by a power surge such as from a lightning strike. Hopefully, only components on the primary side of the power transformer will be affected.
  • Check the primary of the power transformer - if it is open, there may be a fuse/thermal fuse underits outer insulation. If not, the transformer will need to be replaced. There is a good chance that the surge didn't propagate beyond the transformer and thus the rest of the controlled should be unaffected.
  • In some cases, circuit board traces may have been vaporized (but repair may still be possible by simply jumpering across the crater). Some of these thin traces may be there specifically to act as fuses - and there may even be spares to use for just this situation!
  • Assuming that the main fuse and power transformer primary checks out, then check the power supply for the controller next.
  • As always, also check for bad solder connections.
If the controller power supply is working and there is still no sign of life (dead display and no response to buttons) the microcontroller chip or some other part may be bad. It could be a simple part like a capacitor or diode, but they would all need to be tested. At this point, a schematic of the controller board will be needed - often impossible to get - and replacement controller or even just the main chip may be nearly as expensive as a complete new oven.

on Mar 30, 2008 | Kenmore 80412 Microwave Oven

1 Answer

Hi, I have a JVM1490 that is dead. I have chased

There should be a "mini-manual" (tech sheet) hidden inside the unit
behind the control panel or hidden on the left side behind the grille, which is
very helpful when troubleshooting, testing, and locating components.


It *may* have the part number of the low voltage transformer, possibly on a loose sheet inside the mini manual.

Otherwise, you can find links to helpful exploded view diagrams and part ordering help here.

I recommend that you bookmark that link to your favorites.

There should be a part number on the label of low voltage transformer.

The GE part number will be something like WB##x##### and the Samsung number will be something like DE_#######.

The Samsung part number is almost always cheaper.

Feel free to contact me directly if you need more help.
We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

May 13, 2010 | Microwave Ovens

1 Answer

Wont heat up

DISCLAIMER as in all Microwaves, there is a possibility of DEATH if you are familiar with troubleshooting of a Microwave and it is NOT ADVISABLE to trying to diagnose and repair this product by yourself. Now, that being said. Majority of the time on Maytag (which is built for Maytag by Samsung) the Magnetron will pre-maturely fail. There is always the possibility of a open fuse going to the primary side neutral of the High Voltage Transformer. Also the relay on the control PCB has to close as well for the High Voltage Transformer to operate. Unless you are extremely familiar with Electricity and Electronic troubleshooting, you can get in over your head extremely fast and small mistakes can lead to injury or death.

Mar 26, 2010 | Maytag Appliances Over-the-Range Microwave...

1 Answer

All power stopped working. no lights, etc.

This sounds like a blown fuse, unfortunately it is usually something else broken inside so just replacing the fuse probably wont fix it.You can try it though.The fuse is inside the cover, close to where the cord enters the microwave.It could be a lot of things creating a short circuit in there(loose, or damaged connection wire.bad transformer,or faulty control board would top my list) It's usually on the primary side of the transformer though.If stuff goes wrong with the high voltage side it usually doesn't blow the fuse.

Oct 11, 2009 | Microwave Ovens

1 Answer

Emerson Microwave stopped working

Have 1 of 2 choices, the main fuse or the control board for the front switches and clock. primary of the small power transformer has opened. Unplug the unit and take case off and measure the fuse with an ohm meter. if u read 1 ohm or less then fuse is good. then measure across the main power plug tips. If read open then the power transformer is bad. Need to replace the board as not sure if transformer is available.

Jul 26, 2009 | Emerson MW8102SS 1100 Watts Microwave Oven

1 Answer

Kenmore Micro/Oven fuse keeps blowing

Disconnect the connector to the primary winding of the high voltage transformer. Put a new fuse . If the fuse is not blowing the problem may be in the secondary side of the high voltage transformer. Chek the highvoltage diode, capacitor, magnetron respectively. you can also check the primary winding of the low voltage transformer.

May 23, 2009 | Microwave Ovens

3 Answers

My microwave is a 2005

jl, first, are you sure that you have power? Plug something else in to check. Also, unplug the unit, wait a full minute and repower. Sometimes this resets the units. Second, fuses are generally located inside the keypanel area. With the unit unplugged, remove the screws holding the panel on. Inside, you will find at least one fuse. You will also find thermo sensors. They are black, about the diameter of a dime and 3/8 inch thick with two wires coming out of them. Check them for continuity.

Dec 18, 2007 | Frigidaire PLMV169DC Microwave Oven

2 Answers


There is something on the secondary side which is drawing too much current. I would check all electrolytic capacitors, zener diodes, transistors, and voltage regulators.

Mar 27, 2007 | Panasonic NN-C994S Convection/Microwave...

1 Answer

High voltage surge

We regularly repair these GE / Samsung boards from customers nationwide with just such problems for $32.50. If it's too bad off, we can get this board (# WB27X10466 remanufactured) for $145. A new board runs about $180. We would need the full model number with suffix.

Sep 04, 2006 | GE JVM1490 Convection/Microwave Oven

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