Micrwave oven - M8A fuse?

[Mike Tomlinson]

I am attempting to diagnose a Sharp combination microwave oven, it's a
model R-7E53(W)M. This is a conventional/microwave oven and grill in
one unit. The microwave output power is 850W. I am in the UK.

After cooking some food last week using dual-cook (convection and
microwave), the breaker for the circuit that the oven was plugged into
tripped. On resetting the breaker, I found the oven worked normally -
when asked to cook, the fans run, the oven light illuminates, the
turntable turns, but no microwave energy is produced and the food does
not heat.

On opening the oven, I discovered the following:

* the fuse (M8A 250V) in the feed to the HV transformer primary is
blown. The main fuse (13A 250V) is okay.
* the HV diode reads infinity both ways using a DMM
* the magnetron reads a dead short across the filament terminals
* the HV cap reads neither open nor short. If I set my DMM to the 20M
ohm resistance scale, the resistance reading climbs up slowly. I assume
this means the capacitor is charging and is thus likely OK.
* the door interlock switches check out okay.

I am following the Microwave oven guide in the S.E.R. FAQ.

Question 1: the blown HV fuse is marked M8A 250V. Is the M
significant? I'm familiar with T fuses (slow blow), F fuses (fast
blow), but not M. A google has but turned up anything. Can I replace
this fuse with a standard 8A fuse, and if so, should I select fast or
slow blow?

Question 2: Is this likely to be a magnetron failure? The magnetron is
made by Sharp and is marked "RV-MZ A165 WREO" on one line, and "2M226
(16)" on another. Googling, this appears to be a type 2M226-16
magnetron.

Thanks for any insight. If this were a standard cheapo microwave-only
oven, I'd probably chuck it out. As it's a combination oven, it may be
worth some effort to fix.

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[Sam Goldwasser]

Mike Tomlinson <mike@NOSPAM.jasper.org.uk> writes:

I am attempting to diagnose a Sharp combination microwave oven, it's a
model R-7E53(W)M. This is a conventional/microwave oven and grill in
one unit. The microwave output power is 850W. I am in the UK.

After cooking some food last week using dual-cook (convection and
microwave), the breaker for the circuit that the oven was plugged into
tripped. On resetting the breaker, I found the oven worked normally -

When exactly did it trip?

when asked to cook, the fans run, the oven light illuminates, the
turntable turns, but no microwave energy is produced and the food does
not heat.

On opening the oven, I discovered the following:

* the fuse (M8A 250V) in the feed to the HV transformer primary is
blown. The main fuse (13A 250V) is okay.
* the HV diode reads infinity both ways using a DMM

This would be correct for a DMM. You need to test it with a power
supply since the forward drop is more than 6 V.

* the magnetron reads a dead short across the filament terminals

It will be very low ohms though not quite 0.

* the HV cap reads neither open nor short. If I set my DMM to the 20M
ohm resistance scale, the resistance reading climbs up slowly. I assume
this means the capacitor is charging and is thus likely OK.

Only thing would be if it fails at full voltage.

* the door interlock switches check out okay.

I am following the Microwave oven guide in the S.E.R. FAQ.

Question 1: the blown HV fuse is marked M8A 250V. Is the M
significant? I'm familiar with T fuses (slow blow), F fuses (fast
blow), but not M. A google has but turned up anything. Can I replace
this fuse with a standard 8A fuse, and if so, should I select fast or
slow blow?

Question 2: Is this likely to be a magnetron failure? The magnetron is
made by Sharp and is marked "RV-MZ A165 WREO" on one line, and "2M226
(16)" on another. Googling, this appears to be a type 2M226-16
magnetron.

A shorted magnetron or diode would probably not pop the breaker. A
shorted cap might.

When it popped the breaker would be significant. If it happened exactly
when the heating was called on to stop, could be a triac failure also.

Thanks for any insight. If this were a standard cheapo microwave-only
oven, I'd probably chuck it out. As it's a combination oven, it may be
worth some effort to fix.

Yep.

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[Sam Goldwasser]

Mike Tomlinson <mike@NOSPAM.jasper.org.uk> writes:

In article <6wznhxybh6.fsf@saul.cis.upenn.edu>, Sam Goldwasser
sam@saul.cis.upenn.edu> writes

When exactly did it trip?

I'm not completely sure. It was either at the very end of cooking one
lot of food or at the very beginning of cooking the next. There was a
~2 minute interval between. I only noticed power had gone when I opened
the fridge to get something.

This would be correct for a DMM. You need to test it with a power
supply since the forward drop is more than 6 V.

Thanks. I have found the test cct in the FAQ.

When it popped the breaker would be significant. If it happened exactly
when the heating was called on to stop, could be a triac failure also.

The supply to the HV transformer is controlled by a relay, not a triac
in this model.

Update: I replaced the 8A fuse with a 5A fast-blow, and the oven
appears to work. It boiled half a pint of water in about 3 minutes.

The mica splash protector for the waveguide was very dirty (even on the
waveguide side.) Have cleaned it up and will see how it goes.

It's not totally out of the question for the fuse to have just died
of old age and there actually being no problem.

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Sorry.

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[Bob Shuman]

Mike,

Keep your eye on that capacitor as well ... it may just be a matter of time
and this was a warning signal of what is to come. My above the range GE
microwave popped the fuse about 4-6 months before the capacitor went out
completely.

Bob

"Sam Goldwasser" <sam@saul.cis.upenn.edu> wrote in message
news:6wllthy62w.fsf@saul.cis.upenn.edu...

Mike Tomlinson <mike@NOSPAM.jasper.org.uk> writes:
Update: I replaced the 8A fuse with a 5A fast-blow, and the oven
appears to work. It boiled half a pint of water in about 3 minutes.

The mica splash protector for the waveguide was very dirty (even on the
waveguide side.) Have cleaned it up and will see how it goes.

It's not totally out of the question for the fuse to have just died
of old age and there actually being no problem.

 

[Sam Goldwasser]

Hotspur@freeserve.co.uk (Hotspur) writes:

On Mon, 25 Aug 2003 15:57:36 +0100, Mike Tomlinson
mike@NOSPAM.jasper.org.uk> wrote:

RE: Blown fuse to HV transformer.>

I've repaired plenty of these,where in the UK are you?I'm at the
Borders near Holy Island.If it's tripped whilst cooking and you are
fully up to date with microwaves I'll walk you through the repair.You
shouldn't have opened it and tested if you did not discharge th hv.
don't assume door switches are ok,replace them as they are dirt
cheap,it's possible the thermal cutouts at that control the oven/mag
have failed.They are small round discs with the operating temp.on
them.Replace them they are cheap.You need the standard 8A microwave
fuse to replace,buy them in packs of ten for a quid.
I won't put insructions on the group but if you don't get there you
can e-mail me and I'll help you.Frank@Teletronic Services[UK]

Why not? That what this group is for.

So how would a bad door switch or thermal cutout blow only the fuse in-line
to the HV transformer? A bad door switch could blow the main fuse.
A bad thermal cutout could stop it from heating.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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[Mike Tomlinson]

In article <6wllthy62w.fsf@saul.cis.upenn.edu>, Sam Goldwasser
<sam@saul.cis.upenn.edu> writes

It's not totally out of the question for the fuse to have just died
of old age and there actually being no problem.

Understood, thanks, though I was puzzled that it popped the breaker as
well. That was what made me think there had to be another fault.

By the way, the oven is still working well.

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Sorry.

Wasn't aimed at you personally.

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[Hotspur]

On Fri, 29 Aug 2003 13:30:13 +0100, Mike Tomlinson
<nospam@nospam.jasper.org.uk> wrote:

In article <3f4b9ef4.1093147@news.freeserve.net>, Hotspur
Hotspur@freeserve.co.uk> writes

I've repaired plenty of these,where in the UK are you?I'm at the
Borders near Holy Island.If it's tripped whilst cooking and you are
fully up to date with microwaves I'll walk you through the repair.You
shouldn't have opened it and tested if you did not discharge th hv.

I did, following the instructions in the FAQ, and kept a wire croc
clipped across the cap terminals while I was poking around. Did
remember to disconnect it before I switched on though

it's possible the thermal cutouts at that control the oven/mag
have failed.They are small round discs with the operating temp.on
them.

According to the diagram pasted to the side of the cover, these are in
series with the HV circuitry and would not cause a blown HV fuse.

Replace them they are cheap.You need the standard 8A microwave
fuse to replace,buy them in packs of ten for a quid.

Thanks. Where can I get them?

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CPC Preston tel 01772-654455 they sell to none trade customers any

problems I'll get them on my account and send you them next order but
you should be ok,All orders are minus VAT and postage

 

[Hotspur]

A. Top posters.
Q. What's the most annoying thing on Usenet?
CPC Preston tel 01772-654455 they sell to none trade customers any
problems I'll get them on my account and send you them next order but
you should be ok,All orders are minus VAT and postage
Also no matter what the diagram states,and i know the cutouts would

not have popped the fuse,replace the door switches and the cutouts you
will prevent trouble later for the sake of a small outlay unles of
course you want to sell the unit,i still replace them all.