Got a kenwood ST/AL25 microwave, about 3 years old, fitted a new turntable
motor about 2 years ago but appart from that it's been fine,

this morning, no cooking power, well, very slight warming of a bowl of
porridge after about 15 minutes in there (GF kept putting it back in for
another 3 minutes each time it came out cold)

I've taken the cover off, checked the overheat cutouts, fine, big capacitor
is reading just over 1uF and not shorted, but the big fuse from the
transformer to the magnetron has gone,

it dosent look like it blew violently, glass isnt blackened, fise wire is
intact for 3/4 of the length, then a break,

i'll get another fuse or 2, but was just wondering if these fuses are likely
to pop just for fun so to speak, i.e. a little power surge whilst cooking,
or power dropping out for a second,
anything to check before i re-fuse it and power up??

i seem to remember the door interlock switches short something out when the
door is opened, so if the door is pulled open at an angle and one of the
switches closes before the other, it pops a fuse, but if that were the case
wouldent the fuse show signs of a violent rupture?

On Oct 22, 12:56*am, "Gazz" wrote:
Got a kenwood ST/AL25 microwave, about 3 years old, fitted a new turntable
motor about 2 years ago but appart from that it's been fine,

this morning, no cooking power, well, very slight warming of a bowl of
porridge after about 15 minutes in there (GF kept putting it back in for
another 3 minutes each time it came out cold)

I've taken the cover off, checked the overheat cutouts, fine, big capacitor
is reading just over 1uF and not shorted, but the big fuse from the
transformer to the magnetron has gone,

it dosent look like it blew violently, glass isnt blackened, fise wire is
intact for 3/4 of the length, then a break,

i'll get another fuse or 2, but was just wondering if these fuses are likely
to pop just for fun so to speak, i.e. a little power surge whilst cooking,
or power dropping out for a second,
anything to check before i re-fuse it and power up??

i seem to remember the door interlock switches short something out when the
door is opened, so if the door is pulled open at an angle and one of the
switches closes before the other, it pops a fuse, but if that were the case
wouldent the fuse show signs of a violent rupture?

Are you sure youve identified the fuse correctly? Normally the only
fuse is on the mains side, and it is a safety critical component. If
you need to replace that, be sure you get the right type, and check
the power resistor's still ok, if its not you've got a microwave thats
not fit for use unless the interlock is sorted out.


NT

In article ,
NT writes:
On Oct 22, 12:56*am, "Gazz" wrote:
Got a kenwood ST/AL25 microwave, about 3 years old, fitted a new turntable
motor about 2 years ago but appart from that it's been fine,

this morning, no cooking power, well, very slight warming of a bowl of
porridge after about 15 minutes in there (GF kept putting it back in for
another 3 minutes each time it came out cold)

I've taken the cover off, checked the overheat cutouts, fine, big capacitor
is reading just over 1uF and not shorted, but the big fuse from the
transformer to the magnetron has gone,

it dosent look like it blew violently, glass isnt blackened, fise wire is
intact for 3/4 of the length, then a break,

i'll get another fuse or 2, but was just wondering if these fuses are likely
to pop just for fun so to speak, i.e. a little power surge whilst cooking,
or power dropping out for a second,
anything to check before i re-fuse it and power up??

i seem to remember the door interlock switches short something out when the
door is opened, so if the door is pulled open at an angle and one of the
switches closes before the other, it pops a fuse, but if that were the case
wouldent the fuse show signs of a violent rupture?
Are you sure youve identified the fuse correctly? Normally the only
fuse is on the mains side, and it is a safety critical component. If
you need to replace that, be sure you get the right type, and check
the power resistor's still ok, if its not you've got a microwave thats
not fit for use unless the interlock is sorted out.

There can be one on the secondary. It will tend to blow if the
diode or capacitor short out (or I guess the magnetron, if that's
possible). I guess insulation breakdown on the HV side could also
do it.

Not sure if it might also be possible to blow that fuse by running
the microwave empty. I know you can write off the magnetron that
way.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Are you sure youve identified the fuse correctly? Normally the only
fuse is on the mains side, and it is a safety critical component. If
you need to replace that, be sure you get the right type, and check
the power resistor's still ok, if its not you've got a microwave thats
not fit for use unless the interlock is sorted out.

There can be one on the secondary. It will tend to blow if the
diode or capacitor short out (or I guess the magnetron, if that's
possible). I guess insulation breakdown on the HV side could also
do it.

there are 2 fuses that i can see, a standard 1/1/4 inch fuse rated at 250
volts, got a white ceramic centre so cant see the fuse wire, but that's
intact, and is on the filter board that the mains lead attatches to before
going off to the rest of the system,

then there's the big transformer, 2 small-ish wires into it at the bottom on
uninsulated spae terminals, spaced close enough i'd expect anything over 1kv
to jump the gap.
Then 2 high voltage wires out (thick cloth like insulation), one goes direct
to the magnetron, the other is on a spade terminal, this 2 inch 5kv fuse,
then goes to the capacitor, then to the magnetron,

i think there's another wire from the output of the transformer that goes to
the interlock switches,

thing is, the fuse that's blown hasnt blown violently as youd get with a
short, 3/4 of the fuse wire is intact, if i think about it, it's more like a
stress failure,

google brings up lots of people saying that just after the 2 year warrenty
runs out, this microwave stops heating up, most just buy a new microwave,
but some look inside and find the big fuse gone, replace it (at about 2 quid
a go, or 10 quid if you buy one from kenwood) and it works again, (some wrap
some copper wire around the fuse and get it working, claiming it to be safe)

i guess i shall just replace the fuse and see what happens, just i like to
find out what caused the fuse to go before i do that usually,
but i'm guessing as the fuse is mounted direclty on the transformer, each
time the tranny powers up it does so with a thunk, many thousands of thunks
over the years has fatigued the fuse wire till it let go,

In article ,
"Gazz" writes:


Are you sure youve identified the fuse correctly? Normally the only
fuse is on the mains side, and it is a safety critical component. If
you need to replace that, be sure you get the right type, and check
the power resistor's still ok, if its not you've got a microwave thats
not fit for use unless the interlock is sorted out.

There can be one on the secondary. It will tend to blow if the
diode or capacitor short out (or I guess the magnetron, if that's
possible). I guess insulation breakdown on the HV side could also
do it.

there are 2 fuses that i can see, a standard 1/1/4 inch fuse rated at 250
volts, got a white ceramic centre so cant see the fuse wire, but that's
intact, and is on the filter board that the mains lead attatches to before
going off to the rest of the system,

then there's the big transformer, 2 small-ish wires into it at the bottom on
uninsulated spae terminals, spaced close enough i'd expect anything over 1kv
to jump the gap.
Then 2 high voltage wires out (thick cloth like insulation), one goes direct
to the magnetron, the other is on a spade terminal, this 2 inch 5kv fuse,
then goes to the capacitor, then to the magnetron,

i think there's another wire from the output of the transformer that goes to
the interlock switches,

thing is, the fuse that's blown hasnt blown violently as youd get with a
short, 3/4 of the fuse wire is intact, if i think about it, it's more like a
stress failure,

The secondary one won't blow violently - the transformer will limit
the fault current to a much lower value than is available on the
primary side (hence the HRC fuse on the primary).

Out of interest, what's the current rating of the 5kV fuse?
I would think the transformer would have difficulty blowing
it if it's much more than the half an amp it uses, and that
may mean the HV side is very closely fused.

google brings up lots of people saying that just after the 2 year warrenty
runs out, this microwave stops heating up, most just buy a new microwave,
but some look inside and find the big fuse gone, replace it (at about 2 quid
a go, or 10 quid if you buy one from kenwood) and it works again, (some wrap
some copper wire around the fuse and get it working, claiming it to be safe)

i guess i shall just replace the fuse and see what happens, just i like to
find out what caused the fuse to go before i do that usually,
but i'm guessing as the fuse is mounted direclty on the transformer, each
time the tranny powers up it does so with a thunk, many thousands of thunks
over the years has fatigued the fuse wire till it let go,

Might want to visually check the HV diode too. I don't know if the
HV diode will lend itself to being checked easily with a test meter.

And obviously, this is all done with the unit unplugged and the HV
capacitor discharged.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Oct 22, 2:21*pm, "Gazz" wrote:
Are you sure youve identified the fuse correctly? Normally the only
fuse is on the mains side, and it is a safety critical component. If
you need to replace that, be sure you get the right type, and check
the power resistor's still ok, if its not you've got a microwave thats
not fit for use unless the interlock is sorted out.

There can be one on the secondary. It will tend to blow if the
diode or capacitor short out (or I guess the magnetron, if that's
possible). I guess insulation breakdown on the HV side could also
do it.

there are 2 fuses that i can see, a standard 1/1/4 inch fuse rated at 250
volts, got a white ceramic centre so cant see the fuse wire, but that's
intact, and is on the filter board that the mains lead attatches to before
going off to the rest of the system,

then there's the big transformer, 2 small-ish wires into it at the bottom on
uninsulated spae terminals, spaced close enough i'd expect anything over 1kv
to jump the gap.
Then 2 high voltage wires out (thick cloth like insulation), one goes direct
to the magnetron, the other is on a spade terminal, this 2 inch 5kv fuse,
then goes to the capacitor, then to the magnetron,

i think there's another wire from the output of the transformer that goes to
the interlock switches,

thing is, the fuse that's blown hasnt blown violently as youd get with a
short, 3/4 of the fuse wire is intact, if i think about it, it's more like a
stress failure,

google brings up lots of people saying that just after the 2 year warrenty
runs out, this microwave stops heating up, most just buy a new microwave,
but some look inside and find the big fuse gone, replace it (at about 2 quid
a go, or 10 quid if you buy one from kenwood) and it works again, (some wrap
some copper wire around the fuse and get it working, claiming it to be safe)

i guess i shall just replace the fuse and see what happens, just i like to
find out what caused the fuse to go before i do that usually,
but i'm guessing as the fuse is mounted direclty on the transformer, each
time the tranny powers up it does so with a thunk, many thousands of thunks
over the years has fatigued the fuse wire till it let go,

As its just a secondary fuse, You could just bridge it with a _very_
thin bit of wire, like a single strand from extraflex.


NT