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M1BYT > TECH 18.12.08 03:40l 97 Lines 4880 Bytes #999 (0) @ WW
BID : 2B4426M1BYT
Read: GUEST OE7FMI
Subj: Re: Microwave Ovens
Path: DB0FHN<DB0FOR<DB0SIF<HB9EAS<OK0NHD<OK0NMA<OK0NAG<OK0PPL<DB0RES<ON0AR<
GB7FCR
Sent: 081218/0042Z @:GB7FCR.#16.GBR.EU #:21734 [Blackpool] FBB-7.03a $:2B4426M1
From: M1BYT@GB7FCR.#16.GBR.EU
To : TECH@WW
G8MNY Opined with considerable skill:-
> By G8MNY (Updated Oct 07)
> (8 Bit ASCII Graphics use code page 437 or 850)
> Here is the circuit that is used in most Microwave Ovens, but there are some
> SMPSU versions available now (HF QRM?). It is deceptively simple & masks the
> cleaver features of the circuit.
Hi John,
Interesting and useful stuff...
Our oven started misbehaving a few weeks ago. Its a combined microwave/fan
oven grill. The interior light blew, I was advised - with a bit of a bang. I
pulled it apart fitted a new lamp and a new fuse, but the lamp would only now
come on when the door was closed and the oven NOT running. Then I noticed the
turn table wasn't running at all - not easy to see, as there was no light
working when it ran. I was a bit wary of leaking microwaves trying to meter it
out, with case cover off and unit cook, so ran long wires from the unit to my
meter and used a stick to nudge the 'start' button. It is the first one I've
bothered to try to repair and we were ready to scrap it for what it cost/the
time we have had it.
The long leads and stick didn't really help, so I looked at the unit more
closely and the easy way to work on it, with case cover off, with no risk of
escaping microwaves cooking eyeballs, nor the risk from the EHT - is to simply
pull the 240v feed to the magnetron transformer.
I don't know if its usual, but ours had a PCB mounted separate transformer for
the supplies for the electronics and display. That allowed me to trace the
light and turn table motor failure, back to a problem on the PCB, whearas it
had seemed likely to be a faulty in the switching/interlocking at first
glance.
Once I'd got the PCB out, it's problems were obvious - the back of the PCB had
two tracks which completely blown out. Once repaired with a bit of solid
copper wire links across the blown sections and reassembled, all worked fine.
I never found a cause for the damage. The lamp filament had blown, the glass
had cracked, but there were no signs of internal flash over inside the glass
envelope. The latter being the only thing I could think of to cause that level
of damage.
An interesting fact, completely unrelated to the above which I came across
this week....
You can now buy low energy replacements for those 200w/500w halogen lamps
normally used in PIR security flood lights. The fit straight in, in place of
the lamp. I have no information on how quickly they start producing a good
level of light in cold outdoor conditions, but with four of the above floods
fitted as security lighting - I'll be considering testing one the next time a
lamp needs to be replaced. We have a rather large expanse to light up, which
is very dark in winter.
The above lights are now fed via a single 13 amp socket, to which I have added
a plug in remote control socket. Turn the socket off then straight back on and
all the lights come on and stay on, all by a remote control button. Very
useful when the police knock at the door, when prowlers have been reported and
we have lots of places for such to hide.
I also found (I have probably mentioned this before) a means to give warning
when any light has been triggered. Find a tiny tranformer, wrap a few
insulated turns of one of the cables feeding the lights around the
transformer. That will cause the windings already on the tranformer to produce
some voltage. Add a rectifier and a capacitor to turn the output into DC from
which you can power a small piezo sounder which has a built in oscillator.
Light comes on, sounder makes lots of noise. Choose transformer, number of
turns of mains wire and tapping points on tranformer to suit the output
needed, but nothing is too critical.
The remote control also comes in handy when the wind blows and constantly
false triggers the PIR's - now has anyone found a truly effective solution to
that problem? Anemometer, who's output when it is high, disables the lights?
I've tried bracing the lamp brqckets to prevent them vibrating in the wind.
There... that's my EBT size SB :-)
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
º 73 from Harry, M1BYT @ GB7FCR.#16.GBR.EU º
ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
ºInternational Museums Weekends 14th+15th & 21st+22ndº
º June 2008 www.ukradioamateur.co.uk/imw/index.html º
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º The ARC's FL/RAE courses + Newcomers Introduction º
º to amateur radio:- http://www.ukradioamateur.co.uk/ º
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º QTH: Garforth, W. Yorks.. º Timed 23:41 º
º harry.m1byt@tiscali.co.uk º 17 Dec 08 º
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