25 May
2001
25 May
'01
7:39 a.m.
Have to agree with the warning but I've come in contact with both the RF
(ouch I hate RF burns) and the HV side of the transformer. Most are 2kv in
the older ovens until you get to the 1000watt ovens. The capacitor is
genrally safe if he stays away from it as they have ultra-insulated covers,
like the anode of a CRT.
There was a site on the net that went into all of this but for the life of
me I can't locate it anymore. You think the webmaster cooked his ass working
on his own 'wave?
-----Original Message-----
From: owner-classiccmp(a)classiccmp.org
[mailto:owner-classiccmp@classiccmp.org]On Behalf Of Tony Duell
Sent: Friday, May 25, 2001 1:59 PM
To: classiccmp(a)classiccmp.org
Subject: Re: Microwave oven collectors?
My 17 year old "microprocessor controlled" microwave oven stopped making
food hot the other night. Everything still appears and sounds about the
same, just food stays cold, and when it first happened there
was a bit of
the smell of the gates of capacitor heaven being
open. Should I take a
swing at fixing it, or yield to the cheap new inverter technology?
FIrstly a word of warning. A microwave oven is by far the most dangerous
thing you're ever likely to work on. The power supply can provide about
4kV at a significant fraction of an amp. This will kill you if you touch
it. Not 'might kill you', but 'will kill you'.
It is a _lot_ more dangerous than the EHT supply in a monitor or TV, for
example.
And of course there's always the danger of microwave leakage.
If I wa going to fix it, I'd start by connecting a mains voltmeter (or
test lamp, or something like that) across the primary (mains side) of the
high voltage transformer. Try to 'cook' (have a cup of water or similar
in the oven as a load). If you get mains at that point, then the
controller, interlocks, etc are all working correctly. If not, then the
problem is on the mains side, which is somewhat safer to work on.
Assuming that you are getting mains to the HV transformer, then there are
basically only 4 parts in the microwave generator circuit. Discharge the
HV capacitor with a suitable bleeder resistor before touching anything :
The HV transformer (check the winding resistance when the unit is turned
off and unplugged)
The rectifier diode. This has a very high forward voltage drop (10s of
volts) and can't be checked with a normal ohmmeter or diode tester
The HV capacitor, for which you need a high voltage insulation tester to
make any reasonable tests on it.
The magnetron. You can check the filament continuity (disconnect it from
the transformer), should be very low, and for shorts between the filament
and the anode block. The latter is earthed, the filament runs at a high
-ve voltage wrt the chassis.
There is some more information in the sci.electronics.repair FAQ.
I must say, though, that I don't like working on microwave ovens, due to
the high voltage, high current PSU. Monitors and TVs don't bother me that
much, but the thought of working near a transformer that will supply 4kV
at 0.25A or so worries me.
If you do decide to open it up, take great care.
And as ever I am not responsible for any injury caused if you do attempt
to repair it.
-tony