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Old 8th Mar 2020, 7:00 pm   #1
flyingtiger
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Default How to test a HT microwave capacitor

Hi, everybody,
I have old Hitachi Microwave-grill oven,
the microwave does not heat up any more, but the grill still works fine.
- I've check the fuses, diode,magnetron and the transformer, seems in order.
- except that I cannot test the HT capacitor, as I only have a small basic digital
multimeter !
- How can I find out if the capacitor is Good/bad with a multimeter ?
- May I replace a 1uf capacitor by a 0.92uf same voltage 2100V & 10Mohm?

Thanks for your kind attention

flyingtiger
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Old 8th Mar 2020, 8:09 pm   #2
Refugee
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Default Re: How to test a HT microwave capacitor

Most microwaves have a fuse hidden in the wiring.
You will need to find and test it.
The capacitor may have gone short circuit so just change it if the fuse is blown.
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Old 8th Mar 2020, 8:19 pm   #3
kevinaston1
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Default Re: How to test a HT microwave capacitor

Microwaves are VERY DANGEROUS items to work on; guaranteed lethal. If you are unsure you are safe, do not touch it.

I have attached a basic microwave service manual, which explains how to fault find and test the various components.
Attached Files
File Type: pdf R209 SM.pdf (524.4 KB, 199 views)
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Old 8th Mar 2020, 9:43 pm   #4
flyingtiger
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Default Re: How to test a HT microwave capacitor

Thanks to the reply,
I did find the HT fuse, linked to the magnetron, & the fuse next to the relay, they are in good conditions.
- but I do not know how a control a HT capacitor with a small basic multimeter!
- When I each pole to the case, there's no continuity.
- when I connect 2 poles together, meter jumps up & disappear.
- I am an architect, have limited knowledge in electronic.
Thanks for help
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Old 8th Mar 2020, 9:46 pm   #5
flyingtiger
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Default Re: How to test a HT microwave capacitor

Thanks a millions,
I'll study it with attention
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Old 8th Mar 2020, 10:41 pm   #6
Silicon
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Default Re: How to test a HT microwave capacitor

I have purchased parts from Magsells.co.uk.
They have a range of capacitors such as 0.9uF and 0.95uF 2100V.

I have no connection with the company except as a satisfied customer.
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Old 8th Mar 2020, 11:46 pm   #7
Graham G3ZVT
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Default Re: How to test a HT microwave capacitor

If the capacitor is not a dead short it will almost certainly be OK. I serviced microwaves for D&G about 15 years ago, and never once saw a different failure mode.

These are in the order of likelihood in my own experience, with the most likely first.

The HT diode goes short or leaky
The HT capacitor goes short
The magnetron goes short anode to cathode or overheats and cracks one or both magnets (can be seen with a careful visual inspection).

All of the above result in the same fault complaint, the oven appears to work and makes more or less the normal humming sound from the transformer, but no microwaves are produced.

When you say you tested the HV diode, how did you do that and what reading did you get that made you think it was OK

Same question, regarding the magnetron.

Can I echo what Kevin said about safety? Unplug the oven and short the capacitor with something with a well insulated handle before disconnecting the capacitor to test.

Limit all your testing to cold resistance / continuity testing with the oven unplugged from the mains, and the capacitor discharged.

Even those of us experienced with working with high voltages follow the above rule with microwave ovens. No second chances.
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Old 9th Mar 2020, 12:15 am   #8
Terry_VK5TM
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Default Re: How to test a HT microwave capacitor

I would add - don't short the capacitor with a dead short for at least two reasons-

If it isn't dead it can stress them to the point of failure a short time later.

There is an awful lot of energy stored in a m/wave capacitor - I've seen the ends blown off various tools doing that (and even two screwdrivers welded together once).
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Old 9th Mar 2020, 6:35 pm   #9
duncanlowe
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Default Re: How to test a HT microwave capacitor

Good tip. Still have the screwdriver with no tip from learning this on a flashgun capacitor in my youth.
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Old 13th Mar 2020, 3:28 pm   #10
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Default Re: How to test a HT microwave capacitor

TAKE CARE! Check the spade connections to the magnetron where the wires are crimped. High current connection here causes high resistance connection. You should hear a slight zizz about a second after applying cooking power if the magetron heater is powered up.

If an any doubt, don't touch it. 2200v at considerable power plus a massive capacitor.. Repaired many back in the shop days but you need calibrated equipment to repair them safely. John.
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Old 4th Apr 2020, 3:27 pm   #11
JonSnell
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Default Re: How to test a HT microwave capacitor

There is a pair of semiconductors that sit across the capacitor and they fail if the magnetron fails.
I have never had a faulty capacitor.
If however you do change it, replace it with ONLY the type described in the service manual otherwise it becomes unreliable and possibly dangerous to use.
Spares are available from espares.co.uk.
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Old 10th Apr 2020, 8:01 pm   #12
flyingtiger
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Default Re: How to test a HT microwave capacitor

Thanks a million for the reply,
My microwave is a Old Hitachi, made in China,
so I did get the exact replacement for
- HT fuse
- Diode
- Capacitor & magnetron
They arrived & replaced them one by one,
Expect the transformer.

Very disappointed,
nothing has changed, except fan turned,
humming from transformer......
STILL NOT heating up !??
Can you tell me what would be gone wrong ; transformer ? who can I verify it
I only have a small Digital multimeter.

Thanks for your attention
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Old 10th Apr 2020, 8:38 pm   #13
Al (astral highway)
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Default Re: How to test a HT microwave capacitor

Hello , I know this is a vintage repair forum but if you’ve done all those things and you have no background in EHT and ‘limited knowledge in electronics‘, I’d walk away from this. You can often find the new generation of inverter microwave ovens in large supermarkets for way under the cost of a new transformer.

And I know not now, when movements and supply chains are disrupted, but nothing is so urgent that you should take these kinds of risks ...

they are one-touch lethal . Not probably.
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Old 15th Apr 2020, 7:52 am   #14
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Default Re: How to test a HT microwave capacitor

The transformer can be tested with just a resistance check, if it's not open circuit then there's a good chance it's ok, of it's shorted turns you will have smoke and burning smell. As the comments above, these are extremely dangerous and only do cold checks round it and be absolutely sure that the capacitor is discharged. They used to have an internal bleed resistor but this is not to be relied on. Many years ago I did repair many microwaves and always shorted the capacitor to be sure even though the oven had been disconnected for long enough to discharge capacitor Drawing huge sparks from a capacitor might also spike any electronics in there by the way
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Old 15th Apr 2020, 7:56 am   #15
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Default Re: How to test a HT microwave capacitor

Btw, a crude way to do a rough test on the capacitor would be if you charge it from a known source say, 50 volts then monitor it with multimeter to see how quickly the voltage decays. This will depend on the value of any bleed resistor and the internal resistance of the meter. I would be brutal sometimes and charge it from a megger and discharge with a screwdriver, but this is not recommended as it's still not very safe nor good for the capacitor!
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Old 15th Apr 2020, 10:54 am   #16
flyingtiger
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Default Re: How to test a HT microwave capacitor

Thanks a million for the advice !
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