Please help me recognize clamp-meter's cap ratings for replacement

[BGAmodz]

Thanks nigel ,

 

[BGAmodz]

It would be wise to perform the water test for microwave. I think its outlined in the manual I posted.

Intermittent operation and poor heating is usually seen prior to magnetron failure. Typically, the magnetron thermal is first affected.

Why does magnetron needs such high voltage (2100 VAC) witch i guess is delivered by a step up transformer inside the microwave ?

 

[KeepItSimpleStupid]

No, a magnetron needs a source of electrons which happens to be a low voltage AC supply similar to a CRT filament. It also nned a HIGH voltage DC voltage to accelerate those electrons. See here: **broken link removed**

The very thick wires on the transformer is for the filament. There is also a half wave High Voltage Rectifier and capacitor filter. The High voltage shares a filament connection.

You basically tried to measure 2100 VDC or whatever it died down to. There is usually a bleeder resistor across the capacitor because of the dangers.

A small CRT oscilloscope like the Tek 211 or 212, I think uses about 3000 VDC.

One instrument I worked on with the 15 kV power supply had a 1 Megohm 200 Watt resistor for the bleeder resistor.

 

[BGAmodz]

yes i read that they come with a built in 10 Mohm resistor for self discharge .

 

[unclejed613]

it's not uncommon for the internal bleeder resistor to go bad. when it does, it doesn't affect the operation of the microwave, but it does mean the cap will remain charged for a very long time after the oven is shut off (or even unplugged). so i never rely on the internal bleeder, but i always discharge the cap with two crossed screwdrivers (with plastic handles). after the cap has been discharged, i disconnect the diode, transformer and magnetron.

 

[BGAmodz]

it's not uncommon for the internal bleeder resistor to go bad. when it does, it doesn't affect the operation of the microwave, but it does mean the cap will remain charged for a very long time after the oven is shut off (or even unplugged). so i never rely on the internal bleeder, but i always discharge the cap with two crossed screwdrivers (with plastic handles). after the cap has been discharged, i disconnect the diode, transformer and magnetron.

The internal transformer , is it a step up transformer or step down ??? i don't understand how it works , and i don't see any connection with mains (phase and neutral).

 

[KeepItSimpleStupid]

The transformer is actually both. A step up and a step down transformer.

It's similar to any electron tube power supply, whether a photomultiplier, x-ray set, e-beam evaporator, TV picture tube, valve amplifier etc. It's just medium power like 1 KW. an e-beam evaporator is about 10 KW, a CRT probably (40 kV*20 mA) W

There should be an SCR/TRIAC on the mains that turns on the power to the magnetron. Power is somewhat non-linear, so 10% power is not on for 10% of the time.

So it's no more than a high voltage power supply (about 2100 V) riding on a low voltage AC supply (< 6 VAC) with a power of around 1 KW. There are no electrical grids, screens or deflection mechanisms in a magnetron. I believe there is some magnetic focusing.

There are inverter based microwave designs,

 

[Nigel Goodwin]

a CRT probably (40 kV*20 mA) W

Nothing like that, usually 25Kv at a maximum (and limited to) 1mA - some really large sets might be 30Kv..

There should be an SCR/TRIAC on the mains that turns on the power to the magnetron. Power is somewhat non-linear, so 10% power is not on for 10% of the time.

Mostly it's switched by a realy, TRIAC's are fairly uncommon.

The only reason for it's non-linearity is the time taken for the magnetron to actually fire up, so you have the warm-up time before it starts producing RF.

 

[KeepItSimpleStupid]

Nigel is completly right. I didn't verify a source for the TV anode voltages and currents. True, both relays and triacs are found in the primaries of the HV transformer in non-inverter microwave ovens.

My statement about non-linear power behavior failed to mention the reason which Nigel nailed.

Thanks.

 

[KeepItSimpleStupid]

Tracing the primary to mains:

What may be making it difficult is the complexity of the interlocks. There is usually a mains fuse and a NC interlock switch across the mains after the fuse. Then there are two NO interlock switches in series.

 

[unclejed613]

you will usually find at least one interlock switch on the door latch, and one or more thermal switches in series with the transformer primary. the door interlock can get out of position and not close the contacts when the door is closed, the thermal switches can fail open circuit. if the door interlock acts indirectly by energizing a relay, the interlock relay can fail.
KISS wrote "There is usually a mains fuse and a NC interlock switch across the mains after the fuse." (??????)
such an interlock would blow the fuse every time you open the door..... most common is a microswitch that drives an interlock relay on the control board, since microswitches that would actually be in the primary circuit wouldn't last long, or they would be expensive because they would need to be rated at 10A.

the most common failures in the primary circuit that keep the oven from heating, are:
thermal switch open no power to the transformer, but seems to be operating normally
interlock switch failed, or out of position, depending on which switch, may not get primary power to transformer, or controls act as if the door is open
control board failure, usually bad relay contacts if everything seems to be working normally except the transformer never gets primary power. you need to listen carefully to the relays on the board.

secondary circuit failures:
magnetron, usually filament goes open circuit transformer hums, and cap is charged, but no heating.
cap shorted, or shorted to ground. usually blows main fuse.
shorted diode, again, usually blows the fuse
open diode, cap never charges
shorted turn in transformer, blows the fuse
open primary in transformer. power gets applied to primary, but transformer doesn't hum

 

[KeepItSimpleStupid]

On page 10 of the service manual I posted in #22, shows the "monitor switch". This switch can short the primary of the transformer if the door isn't quite right and the magnetron tries to turn on. That's the switch I was trying to describe. My head is really hurting today and I probably shouldn't even be posting.

NC normally closed. Or closed when the door is open.

So, if power is inadvertently supplied to the magnetron with the door open, the fuse will blow. Usually the other interlocks will prevent power from being applied.

There were a few other interlock switches too and one that was monitored by the microprocessor.

 

[BGAmodz]

Hey enclejed , yoo seem to have a clear idea on how microwave ovens work .
I have fixed a lot of microwaves in the past , and there is mostly these situations :
1) Oven does not turn on :
Usually the electronic control card is faulty , but the power on the card is delivered by the big transformer or directly from mains ???
2) Poor heating /oven turns on but no heating :
In the first case the magnetron needs replacement , but in the 2nd case the magnetron usually works fine , but its hard to know if its the capacitor /half wave diode/transformer that is bad , any ideas ??

 

[Nigel Goodwin]

KISS wrote "There is usually a mains fuse and a NC interlock switch across the mains after the fuse." (??????)
such an interlock would blow the fuse every time you open the door.....

Although it's not very well written, it's actually true - there's usually two switches which turn the mains OFF, and a third (the monitor switch) which shorts the mains out and blows the fuse if the other two fail in any way.

 

[Nigel Goodwin]

In the first case the magnetron needs replacement , but in the 2nd case the magnetron usually works fine , but its hard to know if its the capacitor /half wave diode/transformer that is bad , any ideas ??

No disrespect, but your knowledge is so limited you shouldn't be going any near a microwave oven, you're not safe.

You're also obliged (in the UK) to power and leakage test every oven you repair, with a calibrated (yearly) professional leakage tester.

 

[KeepItSimpleStupid]

which shard to know if its the capacitor /half wave diode/transformer that is bad , any ideas ??horts the mains out and blows the fuse if the other two fail in any way.

1. Physical inspection

2. Wait some time, like 5 minutes and discharge the cap. Wait a few minutes and measure the voltage across it. Only ten, can you try an ohmmeter or capacitance tester.

3. The diode usually shorts. The diode test won't work unless it's shorted. You can test the diode by applying a larger, current limited voltage and measuring the voltage drop. e.g A 9V battery in series with a 9 V/10e-3 A resistor and measuring the voltage across it.

4. The transformer: Look for insulation breakdown and measure the continuity of the windings. The actual ohms can be confirmed in a service manual.

 

[BGAmodz]

Hi guys , i got some news , i have received a bunch of high current clamp meters similar to the one i had gone bad but bigger in size , some of them turn on but missing some capabilities like AC voltage/current readings , and some of them just don't boot at all .
Will see if i can manage to fix them .

I will post pictures of them later .

 

[BGAmodz]

4. The transformer: Look for insulation breakdown and measure the continuity of the windings. The actual ohms can be confirmed in a service manual.

I managed to check the transformer , the primary side got two connections , the secondary got 3 wires , the middle one is connected to the magnetron .
So when i power up the transformer with 220 VAC , i measure the voltage between the middle one with each one of the two secondary wires , i get about 140 VAC .

 

[debe]

 

[debe]

Please read these notes VERY carefully. They are from a Toshiba Wksp manual & note it is VERY dangerous to measure the secondary voltage of the transformer & un necessary. ( Ive been repairing Microwaves for 35yrs & survived by being well informed & Carefull around High Voltage) Hate to see some one Killed due to not knowing what is involved & not knowing. Ask questions & make sure you understand what is being said. Stay SAFE & alive.