2nd Panasonic inverter microwave oven with same stupid fault - Page 1
Forum section: electronics / repair

author: TerraHertz
author_rank: Super Contributor
timestamp: December 05, 2018, 01:59:27 pm
content: Today I picked up another microwave oven, inverter type, from a kerb. Found it before the cord-cutters got to it.It didn't work, but took only a few minutes to fix. The fault is due to really crappy design, and this is the second unit I've found with the exact same fault. So maybe it's worth mentioning. Even though both the fault and fix are purely mechanical.It's a Panasonic model NN-ST641W, 1100 Watts.Symptoms: The timer/clock runs, buttons work. But when you tell it to run there's no fan, no microwave power. The relay on the control board closes OK when it tries to run.Inspecting the three microswitches on the door interlock assembly, one seems to have no means for the switch to close when the door is closed. Because a small piece of plastic that's supposed to be there between the door latch prong and the microswitch button, is missing.It used a thin plastic 'spring', and this broke off. The plastic bit found rattling around in the case, but there's no possibility of gluing the very thin 'spring' fracture.Solution: Use a short length of springy stainless steel, excess from a metal wiretie, to make a replacement for the plastic bit.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: December 05, 2018, 02:29:52 pm
content: Good job. Many home appliances are cost-cut to the bone. I've seen thicker plastic parts in airplane model kits...

author: wraper
author_rank: Unknown
timestamp: December 05, 2018, 02:47:00 pm
content: Not necessarily a bad design. Might be that plastic used in certain batches was not quite right. With current microwave oven prices you should not be surprised there is intention to make production cheap.

author: elecdonia
author_rank: Frequent Contributor
timestamp: December 10, 2018, 07:37:20 pm
content: As an EE myself I could tell several stories about how much time I wasted searching for “electrical” or “electronic” faults when the actual cause was mechanical. Here is the best one from my experience:Riding lawnmower, purchased new in 2004. Very popular model, made by MTD, 42” cutting deck with 2 rotating blades, hydrostatic transmission, 2 cylinder V-twin Briggs & Stratton engine, running on gasoline.After running perfectly for 8 years it gradually got very difficult to get the engine started. It cranked over so slowly that sometimes I had to open the engine cover and give it an extra push by hand to get it to rotate a full turn. Then it would start and run perfectly.So I immediately assumed “weak 12V lead-acid battery.”OK, bought new battery. Problem fixed but only temporarily.Next thought: Starter motor worn out?New starter motor installed: No improvement at all.So I next thought “bad wiring, bad solenoid contactor, ?, it must be some weird bad connection.Wound up replacing every electrical component. Thicker battery cables, another new 12V battery...Still hard to start.Very frustrating!Then: Random YouTube browsing led me to “Redneck lawnmower mechanic.”A swarthy looking dude with a cigarette in his mouth, beer can in one hand, dog laying on the ground next to his messy toolbox, etc.I saw his video titled “what to do if your V-twin lawnmower engine won’t start”His solution: “Remove OHV valve covers, then check and adjust valve clearance.”Well, one of the valves on my engine had so much clearance that the pushrod was almost ready to disengage from the rocker arm.After adjusting to proper valve clearance and firmly tightening the locknuts on the valve adjustment screws, NO MORE STARTING PROBLEM.Mower ran great for next 5 years, finally sold it after moving to a house with a very small yard.So why did excessive valve clearance make it hard to start rather than causing it to lose power and run poorly?It turns out these engines contain a “mechanical compression release” which reduces the torque required to turn over the engine during starting. When rotation speed is very low, an additional cam lobe engages which keeps the exhaust valves just slightly open. Then when engine speed increases after it starts, a centrifugal clutch disengages the starting cam lobe, restoring full closure of the exhaust valves.The valve train clearance must be precisely correct or else there will be either too much compression (my problem) or no compression at all. A strictly mechanical issue.Since then I don’t automatically look for an electrical solution to every fault.-EB

author: TheBay
author_rank: Unknown
timestamp: December 11, 2018, 03:54:28 pm
content: Looks like someone at Panasonic HQ worked out a way to get it to fail just outside of warranty