My microwaves HV capacitor circuit protector has gone short cicuit, shorting the HV transformer and blowing the internal mains fuse.
If I repace the short circuit protector and HV diode (CPC for £5.29) is this likely to fix it or is this failure due to something else failing, magnetron for instance ?
Don't want to spend £5.29 to pronptly have it blown up.
They do go with a bang don't they ;-) I've seen cases where replacing the diode & protector has been all the work required, but what you're asking is impossible to foretell. £5.20 doesn't seem a big gamble to me though, just think of the qdos when you present the missus with a working microwave.
I would however suggest that microwave repairs is a subject best left to people with at least some idea what they're doing. and not one for a DIY N.G Do you for instance posess a microwave leakage detector?
jim.
Out of interest - would microwave leakage only become an issue if something mechanical was altered - such as the fit of the door or changes to any of the metal screening?
Whilst not advocating any work on a microwave, I can't understand the need for a leak detector in the context of replacing a component. But happy to be convinced otherwise.
Yesterday on TV some expert was saying that we'd have to go back to nuclear power in a big way. Considering the state of the existing nuclear industry in the UK, it doesn't bode well for public health and vitality. Not that it makes the effects of stray microwaves any less significant but who knows, rogue microwaves and "whoopsy" type nuclear fallout might cancel each other out, a bit like Godzilla -V- King-Kong. :)
Firstly the repair FAQs of sci.electronics.repair ('Silicon' Sam Goldwasser is your man) will sort you out.
Microwave ovens are the most lethal appliances in the house by far and away. Not (contrary to popular belief because of the microwaves) [1] but becuase they have a 2-5kV transformer which can deliver that voltage at hundred of milliamps. It is capable of pushing those millamps through the _air_ and through you if the circumstances are right.
The HV capacitor also stores a potentially lethal charge.
RCDs and/or isolation transformers won't help you any.
[1] Even large gaps in the oven housing would not let much out. The door is likely triple interlocked to the generator.
The only real danger in a microwave oven is the capacitor which, if the integral discharge resistor fails, can supply a significant shock several hours after switch off.
RF leakage, unless you do some serious metal chopping, is most unlikely. The door uses a choke seal - the critical aspect is not the shielding material but the distance from the edge of the door to the edge of the bodywork and it is why microwave ovens have relatively large external boxes compared with the internal size. The choke seal acts as a section of waveguide and its dimension is such that standing waves in the gap between door and body make the seal, even if not physically perfect, appear as a short circuit and minimal leakage occurs.
A leakage detector is not needed for any normal maintenance operation on a microwave oven (and most detectors are not calibrated correctly anyway).
protector across the capacitor is short circuit shorting the 2100 volt secondary to ground through the diode. So no worries there.
Also secondary resistance is 40R so not short circuited and magnetron is open circuit as I would expect. Unable to test the diode as I think the forward voltage is in the order 15V which won't read on my meters resistance range.
I also disconnected the secondary of the transformer, held away from anything interesting using insulation tape, and turned on and fuse did not blow meaning transformer is probably OK.
Will probably try and source a protector and diode and give it a go.
Sorry if I patronized you. It is hard to get a picture of who might be on the other end of the postings. Whilst the dangers of many aspect of elecricity are often over-stated this is not the case as you know with microwave ovens.
if you ask an electronics question on a diy ng, dont expect expert answers.
NT
The protector is there to go short ciruit and blow the internal fuse should the HV diode fail, replace both.
Peter
Wondered what the protector was for, obvious now, as if the diode shorted it would connect the capacitor straight across the HT which I expect would produce a rather large and dangerous bang!!.
Anyway got a replacement protector and diode from CPC now and will fit tonight.
Replaced the protector and diode with the CPC part and microwave now works fine.
Thanks for the help and links.
You are generally discouraged from repairing microwaves, the internal fuse on mine went when the bulb blew - replacing both fixed the problem for me.
If you are too tight arsed to risk a few quid then you shouldn't be repairing it in the first place! My micro "blew a fuse" after many years of service (they do weaken/wear out) but by supplier wouldn't supply me with just 1. I had to buy 10 for about £10! I'm now the proud owner of 9 microwave fuses (about 6 amps each) which I'll never use! But it's cheaper than a callout fee!
What, just a standard 6A fuse ?
My replacement 6.3A fuses were 99p for 10 from Maplin. Did think about leaving the bodged in 5A fuse from a normal plug, but if you are doing a job you might as well do it properly....
Anyway the £5.29 part from CPC fixed the microwave fine.
Howdy, I used to fixem for a living. Does the diode read short circuit on an ohm meter? It's probably OK otherwise.
There can be many reasons why that fuse blew.
The way I used to check it was to use a power board with a 10A circuit protector, put a jumper over the fuse and make experiments.
If you turn it on full power with a litre of water inside, u may see arcing and sparking near the magnetron.
This can indicate a faulty magnetron if inside, otherwise, and more likely, the connections to the magnetron have become dirty and/or loose. Also check the fuse holder, they can get loose too.
In certain models the capacitors used to break down under high voltages, but this is rare i suspect.
You can check the remaining circuitry by disconnecting the magnetron and measuring the voltage coming out of the system with a high voltage probe. If it's low, then you've got a problem further down with the diode/cap or transformer.
Once servicing has finished, you can remove the jumper and replace the fuse.
Oh yeah, handy tip for avoiding elecrocution:-
Always grab the power cord at the microwave, trace it back to the plug and pull it out before fiddling. This avoids the wrong plug being pulled out and you working on live equipment :)
Cheers, Mark.
Phil Nettlet> whatever you decide to do DONT fiddle around with it plugged in and always
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