topic Re: OT: troubleshooting "noise" problems.... in BreakPoint

OT: troubleshooting "noise" problems....

altenbach — Wed, 05 Apr 2023 22:02:58 GMT

So my almost 40 year old Panasonic microwave oven (mid eighties) is still working perfectly. Talk about build quality!

To my surprise, it suddenly was making a faint buzzing sound when turned off. Wow, that does not sound good! Is something shorted out and arching?

Measured the power consumption when off and it was normal (~1W to power the display and clock)
Decided to open it up (Yes, I am fully aware of the dangerous voltages present even when unplugged, so I was careful. There are some high voltage capacitors!).
Inspecting the circuit board, the noise was apparently coming from some buzzer there. (see picture)
I blew some compressed air at it and all noise disappeared.
Better than new again!!!

Re: OT: troubleshooting "noise" problems....

wiebe@CARYA — Thu, 06 Apr 2023 07:39:55 GMT

@altenbach wrote:

Decided to open it up (Yes, I am fully aware of the dangerous voltages present even when unplugged, so I was careful. There are some high voltage capacitors!).

I'm sure you are aware, but when opening a microwave the high voltage isn't the only danger. I guess 'don't try this at home' applies.

I wander if the European 'right to repair' will bring some of the product quality back. It sure sounds good in theory, but commerce usually find ways out of it.

Re: OT: troubleshooting "noise" problems....

Intaris — Thu, 06 Apr 2023 10:12:23 GMT

Reminds me of this:

https://www.youtube.com/watch?v=1OfxlSG6q5Y

Old toasters are really much better than new ones, and the engineering behind it was impressive.

Re: OT: troubleshooting "noise" problems....

Hooovahh — Thu, 06 Apr 2023 13:24:18 GMT

I love the toaster design, and Technology Connections. Whenever the wife hears me watching his channel she will say "What's he saying we're doing wrong now?" (She does actually like using powdered dish detergent but won't admit it)

Re: OT: troubleshooting "noise" problems....

altenbach — Thu, 06 Apr 2023 15:05:55 GMT

wiebe@CARYA wrote:
I'm sure you are aware, but when opening a microwave the high voltage isn't the only danger. I guess 'don't try this at home' applies.

When just removing the cover, the high voltage seems to be the only serious problem (see also), i.e. about 2 electrocution deaths per year on average due to microwave repair attempts. There are even warning stickers all over the place. Other dangers would happen if you "dig deeper", e.g. would start taking apart or destroying the magnetron, which could expose you to e.g. toxic beryllium oxide. So don't use it as target practice. I did have a second person in the room, just to be sure. 😄

Even as preschooler, I had some encounters with 220V in Switzerland, e.g. making my own "extension cords" 😮 . The US 110V is mild in comparison. Yes, the voltage in a microwave capacitor is quite high. 5kV!

Re: OT: troubleshooting "noise" problems....

Hooovahh — Thu, 06 Apr 2023 15:15:54 GMT

@altenbach wrote:

When just removing the cover, the high voltage seems to be the only serious problem (see also), i.e. about 2 electrocution deaths per year on average due to microwave repair attempts.

That's pretty low considering other microwave experiments have killed at least 33 people since 2016.

Re: OT: troubleshooting "noise" problems....

AnalogKid2DigitalMan — Thu, 06 Apr 2023 17:41:29 GMT

I have taken many microwaves apart and even thought I fixed one only to have it go up in flames when I fired it up again 🙂

The squirrel cage blowers in them make for nice high flow window fans and floor dryers.

Wanted to try an experiment with an active magnetron and an ant hill once....

I wonder why dust in the piezo would cause it to buzz?

-AK2DM

Re: OT: troubleshooting "noise" problems....

altenbach — Thu, 06 Apr 2023 19:21:15 GMT

@AnalogKid2DigitalMan wrote:

I wonder why dust in the piezo would cause it to buzz?

Maybe it was a trapped insect. 😄

Re: OT: troubleshooting "noise" problems....

wiebe@CARYA — Fri, 07 Apr 2023 08:36:24 GMT

@altenbach wrote:

wiebe@CARYA wrote:
I'm sure you are aware, but when opening a microwave the high voltage isn't the only danger. I guess 'don't try this at home' applies.

When just removing the cover, the high voltage seems to be the only serious problem (see also), i.e. about 2 electrocution deaths per year on average due to microwave repair attempts. There are even warning stickers all over the place. Other dangers would happen if you "dig deeper", e.g. would start taking apart or destroying the magnetron, which could expose you to e.g. toxic beryllium oxide. So don't use it as target practice. I did have a second person in the room, just to be sure. 😄

Even as preschooler, I had some encounters with 220V in Switzerland, e.g. making my own "extension cords" 😮 . The US 110V is mild in comparison. Yes, the voltage in a microwave capacitor is quite high. 5kV!

I know you're smart enough to remove the power cord, but if you don't, actually microwaving yourself isn't a great idea.

I stopped repairing a 2nd hand laser when I discovered touching the tube with one hand and earth with the other put up to 80kV though me. In my excuse, I was ~16. Lesson learned.

At collage there was a 2V 50A (IIRC) current supply (used to study cracks in metal). Even though in theory it should be perfectly safe, as R should limit P, nobody dared to take the teacher's challenge to touch both ends.

Re: OT: troubleshooting "noise" problems....

JÞB — Mon, 10 Apr 2023 15:10:30 GMT

@altenbach wrote:

So my almost 40 year old Panasonic microwave oven (mid eighties) is still working perfectly. Talk about build quality!

To my surprise, it suddenly was making a faint buzzing sound when turned off. Wow, that does not sound good! Is something shorted out and arching?

Measured the power consumption when off and it was normal (~1W to power the display and clock)

Decided to open it up (Yes, I am fully aware of the dangerous voltages present even when unplugged, so I was careful. There are some high voltage capacitors!).

Inspecting the circuit board, the noise was apparently coming from some buzzer there. (see picture)

I blew some compressed air at it and all noise disappeared.

Better than new again!!!

I used to teach this stuff (advanced electronics theory) and, have made practical application of such knowledge.

What I believe I see there is the step-up transformer and the SCR (black case labled D8) forming the HVPS powering the modulator.

Transformers attract ferromagnetic contaminants over time. This can cause the transformer core reluctance to deviate and even widen the beta loop or, the hysteresis between field expansion and collapse. This actually can cause the core to vibrate at the excitation frequency or harmonics.

Blowing out the crap when the transformer core is not under excitation is the proper remedial action. Won't rust bust or take on dust for years.

WELL DONE!