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dead microwave

I was once asked to look at a heavily used microwave oven at a local residential facility. The description was similar, but the problem was obvious on first inspection. Long finger nails punching through the elastometric contact keyboard. Once the protective membrane on the front panel is punctured, all manner of liquids and gasses can cause damage. I've seen this problem other times, where the customer doesn't want to admit that they damaged the front panel, or somehow considers such damage as normal. It also applies to TV/hi-fi remote controls, printer control panels, calculators, where easily damaged flexible membranes are used in place of hard plastic buttons.

While grease is usually non-conductive, having any grease in the various connectors will produce lousy connections. I suspect a good cleaning, and a low grease diet, might restore the oven to normal operation. The green crud (copper sulfate) does not necessarily signal the demise of the wiring, unless it's extensive. I would not replace any electronics until the keyboard and controller cards are baptized in solvent. It would also be interesting to determine how the grease entered the area. Probably a mechanically mis-aligned exhaust duct.

Opinions:

This one is interesting:

The Bottom Line: Despite the top-flight brand name, attractive styling, and high price tag, this microwave is unreliable and extremely difficult to clean.

Drivel: US DoE can't figure out how to measure microwave oven power output.

Sigh.

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann
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Your tax dollars at work, Sigh^2.

David

Reply to
David

I've seen the corruption of membrane switch panels from users spraying things like window cleaners directly on them. Without any punctures. While talking to the customer I try to ask how they cleaned their stuff without trying to point blame. People don't like to readily admit to the over zealous spraying of cleaners that can and do eventually seep into membrane panels. I cringed in horror once watching my girlfriend cleaning my then 2 year old GE oven, She sprayed the front with copious amounts of window cleaner. I told her from now on she was to lightly spray it on a clean cloth first then wipe the membrane. On everything. Not too many realize this is a serious problem unfortunately.

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Live Fast, Die Young and Leave a Pretty Corpse
Reply to
Meat Plow

Jeff-

My impression is that the referenced article refers to efficiency, not power output. It talks about such things as magnetron filament current and warm-up time.

Somewhere on the web I found a procedure for measuring power. You heat a liter of water for a certain amount of time, measuring temperature before and after. Using a formula, temperature rise correlates to power output.

I think such a procedure is basic, and don't see how it could be challenged other than for "hot spots" in an oven. In my case, the oven has a carousel to compensate.

My old 900 Watt Sharp microwave actually puts out 650 Watts according to the procedure. I adjust cooking times meant for 1100 Watt units, multiplying by 1100/650. The food appears to be getting the right amount of heat.

An old Sharp "Half Pint" is rated for 450 Watts. The above procedure agrees exactly.

Fred

Reply to
Fred McKenzie

Yeah, it's really a tough problem. Never mind IEC-705 which has been around since 1990.

Place about a liter of water in the oven. Measure temperature. Apply power. Measure temperature again after a specific cooking time.

Power = (Volume_of_water) x (Temp_rise) x 4.1868 / (Cook_time)

4.1868 is joules/calorie (specific heat of water) Power = Watts Volume = Cubic centimeters or Milliliters Temp rise = Celsius Cooking time = Seconds

Every oven I've tested (6 ovens) with a liter flask and about a minute runtime has resulted in measurements of 15% to 30% less than the rated power. If a use more water and a longer runtime, even less (because the magnetron decreases in RF output as it gets hot). This is not because the manufacturers all lie, but because the IEC-705 standard method generates inflated results for power output. This might also offer a clue why the DoE can't seem to find a workable procedure that produces the same results as IEC-705.

I've also measured the efficiency of these ovens. All are about 60% (using the above test procedure).

Perhaps what's needed is a multi-million dollar research grant to study methods of measuring microwave oven power output. However, should such a study succeed, I'm certain the method will be ultra complexicated and designed to sell plenty of specialized hardware and certified testing labs. With luck, maybe even a bureaucracy to oversee the muddle.

Hint: Don't use a big heavy Pyrex glass bowl as it becomes part of the thermal mass and wrecks the results. Use thin chemistry glassware or if really careful, light plastic bowls. A 10C to 20C temp rise is sufficient. No need to boil the water. Don't leave the thermometer inside the oven when running. Let the water temperature stabilize for

30 seconds or more at the end of a run as microwave heating is rather uneven. 
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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

We had a Kenmore which had lasted us almost 20 years.

When it gave up the ghost, I replaced it with a Panasonic "inverter" model... one of the higher-end consumer-grade microwaves. This died early this year less than three years of service. Looking at the consumer-grade microwaves available at the time gave me no confidence at all.

I bought an Amana "medium volume commercial setting" model (ALD10T) as a replacement. It's roughly twice the price of the better consumer-grade type, but does seem to have a substantially better build quality, and the magnetron has a 3-year warranty as opposed to 1 year (or less) for consumer-grade models. My hope is that it will have a substantially longer lifetime than today's race-to-the-bottom consumer junk. Biggest negative noted so far (from my wife'w point of view) is that the fan continues to run for 30 seconds to a minute after the cooking cycle is complete - this clears out the steam and odors from inside the oven, but she hates the noise.

The companion model ALD10D has a mechanical timer and dial, and only a single power setting... less to go wrong.

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Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
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Reply to
Dave Platt

Yep. I measure the power input to the microwave with a Kill-a-watt power meter. The power output is using the calorimetric water heater method. I can throw in defrost, pre-heat, warm-up, cool-down, and such if you insist.

What was dropped was:

They're measuring "useful cooking energy" which is similar to the exaggerated numbers produced by "peak music power" in hi-fi.

I was borrowing an FLIR i7 infrared camera to do some unrelated (and paying) tests.

What got my attention was the rather bizarre pattern of heating on a small pizza in the microwave oven. Of course, the carousel and stirrer fan help reduce hot spots, but it still takes time for the heat to move around in a liter of water.

My cheap junk Welbilt something oven claims 950 watts and delivers 800 watts.

That's because IEC-705 was designed to inflate the ratings. See first paragraph at:

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

New and improved IEC-705 is now IEC 60705:

For only $143, you too can learn how governments rate microwave ovens or maybe P-IEC 60705 {Ed.3.2}

for $170.

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

See Jeffery Angus' reply. Try to keep up.....

Reply to
Klaatu

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