Microwave malarky.

[Chris Bacon]

So it was making an odd noise, and not heating the food, light comes on, steel turntable revolves, beeper sounds at end of cook time.

Check HV fuse. Blown. Aha, replaced, it's done that before, should be OK... but no! Food still cold, fuse blown again.

Check the diode (CL04-12) using a multimeter on 20V setting and 9V (actually 8.34V) battery. 0V one way, 3.26 the other. A bit low?

Capacitor: No connection (meter on 2000k ohms setting) between either terminal and case. Between terminals resistance increases from zero to infinity over a second or so.

Magnetron: Meter set to 200 ohms. 0.5 ohms between terminals either way. Meter set to 2000kohms, no connection between terminals and case.

High voltage transformer: Primary, isolated, about 2.2 ohms. Secondary

150. There is another winding that seems to go to the magnetron, I don't know what this is.

So any help on what's up, and what to do about it?

[jon]

Try another magnetron.

[Andrew]

Make and model ?

[Chris Bacon]

Crikey. It's a Bifinett KH1167, which I think was also badged Silverline at one time.

[Andrew]

Ahh. Not many had metal turntables like my Sharp did. That suddenly stopped heating on microwave but worked fine on oven and/or grill. Luckily, I was in the BHF charity shop in Worthing and they had a almost-unused Panny CT552 combi-microwave for £30 which works fine, so the 20KG Sharp lump is heading for the recycling skip.

The Panny has a much nicer and quieter beep, beep, beep at completion. The sharp had a really annoying piezo-electric buzzer that sounded for too long.

[Peter Able]

Magnetron filament winding, connected to the two magnetron terminals. Disconnect at the magnetron and check that this transformer winding and the magnetron filament are OK (fraction of an ohm - but not zero). One side of the filament circuit will be connected to the diode's anode. The other magnetron connection (its anode) is via the magnetron's body. The transformer HV secondary should have one end connected to the transformer body, the current path between this and the magnetron anode being via the chassis.

A blown filament shouldn't blow a fuse, though. Slightly worrying that the fuse has blown before !

PA

[Chris Bacon]

Thank you. OK, I disconnected at the magnetron, and there seems to be a tiny resistance through the few red/black flecked fabric-covered coils I can see in the middle of the transformer. Maybe 0.1 ohm, but my meter isn't particularly precise.

Is there anything else to measure?

It's quite an old machine now, and I have heard that the fuses can go after repeated uses.... coincidentally, I had just used "low power" to simmer things; this seems to be full power but switched on and off every so often....

[Peter Able]

OK. Do you mean that you've replaced the fuse and can use the lower power mode for ever - but when you try full power it fails and blows the fuse? If so, how long does it run full power before it fails.

Has the microwave been stored in a garage? That can result in transformer damage when powered up?

PA

[Chris Bacon]

I've only just learned how to use low power (which seems to me to be merely full power applied in short on/off bursts)! I was only speculating that having the thing switch itself on and off a few times a minute might affect fuse life were it used often... but:

This thing has been in regular use for ages. The fuse blew probably yesterday on "normal" the full power setting. I changed the fuse today, having noticed it was not heating my breakfast, and the replacement blew probably straight away, certainly within a second or two, I don't know how to measure that, but it did not noticably heat up my cold coffee.

I have no evidence at all that it works on "low power" (see para. 1).

No, it has been on the worktop in my kitchen, as I say in general use, certainly daily.

I wondered again about the diode. I am certainly no expert, but I know what a diode does (but not in context). Testing it again, but on an old

12V battery, gives me no throughput one way, but a 7.93V potential the other way (off a 13.2V battery).

[Fredxx]

Do you mean the fuse in series with the transformer, ie 230V?

This may be a number of diodes in series, so >5V drop may be expected.

Sounds good. If it was o/c there would be no HV power getting to the magnetron.

I assume 0.5 ohms refers to the cathode heater, which sounds ok.

The case is the anode, ie at ground potential.

There should be 2 wires to the magnetron heater from the transformer, one wire will be attached to the HV generator.

Initially I thought 2.2 ohms sounded very low, but the ratio between primary and secondary is consistent with 2kV output. ie a square function.

If you disconnect hot side of the secondary and turn it on, does the fuse still blow?

Do take care. There is a lethal amount of energy at a lethal voltage in a microwave.

[Paul]

Is it possible the diode is actually made of diodes in series ?

*******

CL01-12 Diode Microwave Oven High Voltage 12kV 350MA

12kV repetitive peak reverse 350mA average output forward 30A surge, 8.3msec, forward Vf 12V max at 350mA <--- about 12 diodes in series or so at roughly 1V drop on each, cannot be checked on "diode" range of multimeter 2 to 50ua reverse leakage

You would need to make up a little test circuit to verify the forward characteristic.

Paul

[jon]

I had something similar and it turned out to be the magnetron breaking down under load.

[Chris Bacon]

I've got one on order, plus 10 fuses....

[Chris Bacon]

No idea. It's a CL04-12 if that makes a difference. I don't know what a test circuit would be.

[Jethro_uk]

Funny to see this post, as our 15 year old microwave packed up two days ago. Went to take the beans out and they were still cold :(

Light and turntable work (obviously - hence my surprise at cold beans). But at 15 years in there's no point considering anything other than a replacement.

[Brian Gaff (Sofa]

Yes Magnetrons can flash over and fail. Usually they get air in, or get carbonised somewhere, however when I could see the usual suspects were the transformer or the diode. Some of the more complexes ones with a lot of electronics can just fry their control boards, sadly, Toshiba were good at that. Brian

[Brian Gaff (Sofa]

There is no such thing as low power on a Magnetron, they simply put it on and off to simulate the lower energy over time so to speak.

Is the filament reading OK?

Also, I was intrigued by the diode being such a poor conductor in one direction, in your previous post. However as you have not actually killed yourself yet, my guess is there is no eht for whatever reason. Brian

[Brian Gaff (Sofa]

But assuming its not some computer controlled device, there is very little to go wrong, so it might be a simple repair. Brian

[Chris Bacon]

Why do you think I might kill myself?

The filament reads "almost no resistance", if you mean the "winding" on the transformer, the terminals on the magnetron connedct to each other with very low resistance, but not to the case.

A new diode costs only a couple of pounds so I may get one of those as well as the magnetron & fuses I have on order.

[Jeff Layman]

Inverter microwave power supplies are effectively continuous at the power selected. They don't switch on/off in the way that conventional power supplies do, although they do use pulse-width modulation to control the power.

This is from

"Inverter Technology In inverter-equipped microwave ovens, the power transformer is replaced by a circuit board, which converts the 60Hz incoming line frequency to a variable rate of 20 KHz to 45 KHz. A relatively small transformer is then required to increase the voltage to the level required by a magnetron. By varying the pulse width, the output power can be linearly controlled for more precise cooking and defrosting levels. The bulky power transformer is replaced by a small, lightweight circuit board; and, because less heat is dissipated, power efficiency is increased. Conventional technology uses just a single power level, which is regulated by switching pulses. In contrast, inverter technology directly controls the power output. This constant soft penetration of microwave energy prevents the common problems of shrinkage, overcooking, and loss of nutrients. The result is even food temperature and textures throughout."