Dim Microwave Oven Display - Page 1
Forum section: electronics / general

author: bostonman
author_rank: Super Contributor
timestamp: January 13, 2020, 03:53:45 pm
content: I have an LG Microwave Oven, model LMV1813SW /01.The LED display has been dim for a few years . I've looked at various repair manuals, but don't see much about the display. I'm wondering if an easy way exists to remove the display and repair it without disassembling the entire oven (which I'm not about to do because it's too dangerous).Has anyone repaired one of these?

author: andy3055
author_rank: Super Contributor
timestamp: January 13, 2020, 05:30:36 pm
content: Try these sites for parts:https://www.appliancepartspros.com/controller-parts-parts-for-lg-lmv1813sw.htmlhttps://www.searspartsdirect.com/model/5aityrflmr-003204/lg-LMV1813SW-00-microwave-hood-combo-parts?page=3

author: andy3055
author_rank: Super Contributor
timestamp: January 13, 2020, 05:41:20 pm
content: Any thing that works on electricity is dangerous if you don't follow the safety procedures in repairing them. Make sure the plug is pulled out before taking the covers off.Be aware that the huge capacitor terminals will have a charge even after mains power is disconnected.You should leave it off for the charge to dissipate first. Even then, I would not touch any of those terminals on the cap.Do not attempt anything if you are not sure how to go about it in a safe manner.If that is the case, take it to a professional repair person!Any information given by anyone on this site (or any other site for that matter) should be followed at your own risk.

author: bostonman
author_rank: Super Contributor
timestamp: January 13, 2020, 05:47:48 pm
content: All the safety I know, but I also know messing with microwave ovens is not safe.If I could just remove the display, then I may attempt repairing it, otherwise, I'll eventually buy a new one.

author: SeanB
author_rank: Super Contributor
timestamp: January 13, 2020, 07:07:08 pm
content: If it is a VFD, then likely the dim displat is more likely to be failing capacitors in the power supply, along with a failing display itself, as they spend all the time on.As an aside, the controller on a microwave actually uses more power than the rest of the microwave over it's lifetime, simply because it is powered and using around 3-5W all the time, while your typical microwave magnetron operates for 5 minutes a day.

author: macboy
author_rank: Super Contributor
timestamp: January 14, 2020, 03:39:09 pm
content: The display is VFD, not LED. A VFD in a microwave has a hard life, because it is operating 24/7. Any VFD will dim over time for three main reasons:- the filaments wear out, reducing the rate of emission of electrons, reducing brightness of the entire display, and sometimes resulting in blotchy looking display (usually brighter under each filament wire, and much dimmer between wires).- the phosphors wear out, reducing brightness of the most-used digits/segments/characters.- too-low voltage driving the filaments or the grids and anodes, due to (partial) failure of the power supply.You can't do anything about the phosphors. You can increase the emission of the filaments by driving them at a higher current. This will restore some brightness to the display, but will also increase the rate of aging. It will eventually dim again, sooner rather than later. A power supply problem is best case scenario, because it isn't a failure or problem with the relatively irreplaceable VFD display itself.

author: bostonman
author_rank: Super Contributor
timestamp: January 14, 2020, 03:44:19 pm
content: The display began getting dim about three-years after I bought it. I don't know the age of VFD's, but know I've seen microwave ovens that are two-decades old and still have a bright display.I'd bet on capacitors, but again, they would be fairly new.In any case, I'd like to disassemble it, but know it's not worth dismantling it as any error in reassembling my cause leakage.

author: bostonman
author_rank: Super Contributor
timestamp: March 09, 2020, 02:33:23 am
content: Today I disassembled the microwave display and didn't see anything obviously wrong on the circuit board such blown capacitors or burnt components.This is the board I have, but a replacement is too expensive and probably easier to just get a new oven. Does anyone have any suggestions on what would cause a dim display?https://www.partsimple.com/lg-ebr64419603-microwave-main-pcb-control-board.html

author: amyk
author_rank: Super Contributor
timestamp: March 09, 2020, 02:43:43 am
content: A lot of these use capacitive droppers to power the low-voltage side and that cap loses value over time, causing symptoms such as a dim display.

author: bostonman
author_rank: Super Contributor
timestamp: March 09, 2020, 02:54:44 am
content: I never heard of a capacitive dropper, but did some brief reading before responding.I guess the next question is how do I find the capacitive dropper capacitor since this isn't a basic single LED circuit?

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2020, 02:56:26 am
content: The VFD is worn out. I've dealt with a lot of these, they wear out for exactly the reasons mentioned above and there is nothing you can do except for overdriving the filaments to yellow heat for a few seconds to bake off any contaminants.The good quality Japanese displays made by Futaba and Noritake are the ones you see that still look ok after decades. The modern Chinese cheapies start to dim after just a few years. They're vacuum tubes, they wear out. The display in my GE microwave is about 10 years old and is very dim, especially the center digits that are used by the clock.

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2020, 02:58:51 am
content: There's no capacitive dropper, I've never seen one of those used in a microwave. In the picture of the control board linked in an earlier reply you can clearly see a switching power supply with a transformer. It's worth checking the voltage rails on that, you could have some worn electrolytic capacitors but I'd bet on the display being worn out.

author: bostonman
author_rank: Super Contributor
timestamp: March 09, 2020, 03:16:07 am
content: I'd like to measure voltages, but probing around a circuit with high voltage while attached to a disassembled microwave oven isn't worth it.I was hoping to see a blown capacitor, burnt resistor, or cold solder. The oven began dimming after only three or four years, so I didn't think a capacitor was the culprit, and certainly didn't think the display would be bad since I've never seen one go bad (but I've seen more LEDs and stuff than VFDs.Since it's most likely the VFDs, it's not worth checking voltages based on what I stated above.Not to deviate, but microwave ovens are dangerous and need shielding from leakage. When I removed the display panel, I was shocked to see the components exposed. To remove the display panel, I removed two screws on the filter grill on top, one screw for the panel, and it came off.How are the microwaves contained since everything I saw was sitting in open air behind the display panel?

author: Peabody
author_rank: Super Contributor
timestamp: March 09, 2020, 04:23:29 am
content: I have a Sharp Carousel that has the same problem. It doesn't appear to be the segments wearing out because when I use the Power setting, which uses special segments, the dimness is still the same all over the display. Also, I find different brightness levels depending on what else is going on - the door open, the oven on, etc. So I suspect it's a power-related problem. But I took it apart years ago and never found anything I thought I could fix. I can still see it, barely, and given the low cost of replacing the entire oven, it certainly doesn't make sense to order replacement parts, if I could even do that. It's just a poor design. I assume modern ovens use better displays.

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2020, 06:53:17 am
content: Quote from: bostonman on March 09, 2020, 03:16:07 amThe output of the magnetron is fed into the oven cavity, the outer cabinet where the electronics are located is not part of the cavity and does not require shielding. The HV power supply in a microwave is quite dangerous but it is not located near the control panel so working on the control board is no more dangerous than any other mains powered equipment. What I would do for simplicity is remove the entire control board and power it up on the bench with a cheater cord. If you are not comfortable working on mains powered equipment then find somebody to help you or just buy a replacement board or a new microwave. They are very simple machines and are not difficult to service but doing so does require some care.

author: bostonman
author_rank: Super Contributor
timestamp: March 09, 2020, 12:50:28 pm
content: QuoteI never tried any power settings, but did think about the fact all segments are equally dim.I don't have an issue powering it on a bench, assuming I know which connector(s) is AC in, but I don't know how VFDs work and would only probe around on guess work.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: March 09, 2020, 03:57:45 pm
content: If you use an isolation transformer, there may be even less of an issue...

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2020, 05:40:52 pm
content: It's easy enough to find out how VFDs work, they are not complicated devices.https://www.explainthatstuff.com/how-vacuum-fluorescent-displays-work.htmlAn isolation transformer is not going to help here unless you're trying to scope something on the primary side. The power supply is already going to be isolated and this should not require a scope anyway, just measure the voltages across the electrolytic capacitors in the output of the power supply. The AC input should be easy to locate, find the bridge rectifier and follow the traces to the connector, or look in the oven and follow the wires from the power cord to the control board.