Need transformer for microwave oven

sagor

Joined Mar 10, 2019
512
What makes you think the transformer is bad?
If measuring output voltages, and they are all zero, it may be that the transformer has a thermal fuse inside the tape wrappping around the transformer wires (internal to the transformer). Those can fail for various reasons, not just thermal, but current surge as well. This fuse can usually be seen by the different size primary winding wires if you remove the transformer from the circuit board and inspect the primary pins/wires.
It is possible that there is some other protection device on the primary of the transformer.
 

Tonyr1084

Joined Sep 24, 2015
6,459
1633701706655.png
If you're competent to test the transformer with the machine plugged in - proceed with testing. IF NOT - STOP! THERE'S DANGEROUS VOLTAGES PRESENT INSIDE A MICROWAVE OVEN! Don't start throwing parts at a problem without knowing where the problem is.

First thing I'd look for is 120VAC on pins 1 & 4. If there's no power there then the issue is not with the transformer. If there IS 120VAC there then check pins 5 & 6 for 3.1VAC. If there is no voltage there then one of two possibilities exist: first, the secondary output of 5 & 6 is open (dead). Or the primary is open (dead). Next, check for voltages on pins 7, 8 & 9. If there IS voltage there then that means the primary is good and secondary 7, 8, 9 is also good. Only secondary 5, 6 is dead. If there is NO voltage on any of the secondaries AND there IS 120VAC on the primary (1,4) then the primary is not functioning. DISCONNECT ELECTRICAL POWER - UNPLUG IT! Set your meter to read ohms and measure the resistance between pins 1 & 4. There should be some resistance. If it reads open (no electrical pathway) then either the primary has gone bad or a thermal fuse may be present and may have blown. These are all the steps you need to rule whether the transformer is good or bad.

Too many times I've seen people start throwing parts at a problem hoping to discover what made it work again. The big problems with that is the expense at wasted money on parts that were not bad AND the real issue is "What made that (or those) part(s) fail in the first place?" If you have something shorted it can cause other parts to blow out. If you replace one of those blown parts but have not found the short - guess what's going to happen to your new part! Yes, it's going to blow, and blow quickly. You probably won't even know if it's blown without removing it and testing it. So don't start buying replacement parts until you know what was the cause of the failure and have corrected that FIRST. Then replace only the parts you need to replace.

This process is going to take a lot of time, patience and likely money. By the time you've fixed your microwave oven you've probably spent more money than the cost of a new one. I'm with LowQCab buy a new machine and scrap the old one.
 

Tonyr1084

Joined Sep 24, 2015
6,459
These are all the steps you need to rule whether the transformer is good or bad.
There IS one other possibility, one of the windings may be shorted and causing a low voltage (or high voltage) on the output pins. However, transformers don't often fail.
 

MisterBill2

Joined Jan 23, 2018
9,824
In the photo in post #1 it looks like the insulation on the transformer is turned a dark brown, and that usually indicates some serious overheating. So if you do get a replacement transformer you will first need to repair whatever caused the transformer to overheat that much. Transformers seldom burn up for no reason, so there will be some other part has failed. At that point it may indeed be time for a replacement, depending very much on where you are.
 

Tonyr1084

Joined Sep 24, 2015
6,459
Bill sees browning. I don't. Still, it's sound advice to fully diagnose the problem before you start throwing parts at it. If for some reason the transformer HAS failed, it failed for a reason. If you don't find and fix the cause then the next transformer you put in its place will suffer the same fate. Quite possibly causing even further damage to components that may have been hurt but not 100% blown.

Sound advice. Buy new and use the old for something else. I have a control board and panel mounted in an aluminum frame and use the output that controls the high voltage microwave oven to switch on a duplex outlet. Using the timer I can set it to energize the plug for 99 minutes and 99 seconds total (1 hour, 40 minutes, 39 seconds). I generally use it for my soldering iron. Too often I forget to turn it off and it sits there for hours just burning away. And while I don't often set my iron for an hour or more, I DO set it for 20 minutes. That's usually long enough to accomplish some task. If I need it on for longer periods of time I can do just that. Up to 100 minutes and 39 seconds.
 

MisterBill2

Joined Jan 23, 2018
9,824
Give us a side view photo of the transformer and we will see clearly the problem. And as I see the style of the circuit board attached, that must be a quite old microwave oven, possibly over 25 years old. Presently they do not look like that.
 
Top