Microwave oven repair - Page 1
Forum section: electronics / repair

author: bob91343
author_rank: Super Contributor
timestamp: July 01, 2019, 01:50:39 am
content: I have a Sharp microwave oven that has served me well. Unfortunately the display has stopped operating properly. The oven works fine but the numbers are impossible to read.I have had it apart several times. Occasionally I can apply force to the display and it gets more legible but as soon as I let go it freaks out.I am sure it's the display itself at fault. It's an LCD that is backlit by a few LEDs. I looked over the solder connections to the display and they look good.Any ideas? I am certain that a new part from Sharp will cost as much as a new oven. Can I adapt a generic display of some sort?

author: Psi
author_rank: Super Contributor
timestamp: July 01, 2019, 01:55:08 am
content: I would check the bias voltage on the LCD

author: bob91343
author_rank: Super Contributor
timestamp: July 01, 2019, 02:27:20 am
content: I didn't know there was a bias voltage. Can you direct me as to how to measure and what I should read?

author: fzabkar
author_rank: Super Contributor
timestamp: July 03, 2019, 02:07:14 am
content: VFDs have a bias voltage. Did you check that your LCD's backlight is working?There are numerous circuit diagrams for both LCD and VFD based microwave ovens here:http://www.jordansmanuals.com/default.aspx?Brand=Sharp&Product=MICROWAVE

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2019, 02:57:33 am
content: I have the diagram but am not conversant with LCDs. Yes the backlight, a group of LEDs, works okay.When I open it and handle the LCD it doesn't seem to be responsive to its signals. If I warp it or press on it, sometimes it seems to work better.If there is a bias voltage I don't know what it would be, where it goes, or where it comes from. That may indeed be the problem.

author: fzabkar
author_rank: Super Contributor
timestamp: July 03, 2019, 04:07:35 am
content: This application note describes how the LCD segments are driven:http://ww1.microchip.com/downloads/en/AppNotes/doc2569.pdfhttp://www.cloverdisplay.com/pdf/S5080.pdf

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2019, 06:46:31 am
content: That is good information, thanks. In my cursory look I see nothing about a bias voltage. Perhaps I need to read further. But so far it seems the signals are ac coupled.

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2019, 06:54:31 am
content: Looking at the diagram of my oven, all the LCD terminals go to the microprocessor. Thus, if the connections are good, and the power is proper, either the display or the microprocessor is defective.The oven's functions all work as they should, leading me to believe the trouble is in the display module. However, this particular display module is somewhat complex, including text for power level and a few other things. So the only way to restore proper operation would be to replace the LCD module with the identical part.I doubt I can obtain that at reasonable cost, if at all.I do hold some hope that the problem is elsewhere, such as in the power supply. I will give that some thought.

author: fzabkar
author_rank: Super Contributor
timestamp: July 03, 2019, 07:14:39 am
content: Other than reflowing the solder connections, I don't see what you can do. I would think that the COM pins would be the likelier suspects since they affect all segments. I would also check the power supply, as you say, but I don't hold out too much hope in that regard since the MCU appears to check its own supply voltage via a potential divider. ICBW, though.

author: Gyro
author_rank: Super Contributor
timestamp: July 03, 2019, 09:25:52 am
content: If applying pressure helps, then the elastomer zebra strips between the LCD glass and the PCB glass are possibly contaminated. You should be able to unscrew or unclip the plastic LCD frame to get access to them. Clean the PCB pads and strips themselves with IPA (making sure that you don't leave any cotton fibres on them) and reassemble.This is quite a common problem on DMMs so in the relatively 'contaminated' environment of a microwave oven it is a good possibility.Of course, if there are soldered edge clips attached to the glass instead of zebra strips then this does not apply.

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2019, 05:09:20 pm
content: The fact that there are no properly working segments ought to be a clue. The COM terminal(s) can be suspect.Originally the display worked fine for a number of years. Then it started getting flaky but not bad. I opened it to see if I could fix it and then all the problems started. So I blame something I did but I can't imagine what. Recently the machine got unplugged and when reenergized, the current problem started. After another attempt at repair it got so bad as to be unreadable.The only hope I have is to reflow the solder connections. The display is connected via a short ribbon cable but flexing it has no effect so I am not too optimistic that reflow will help. Perhaps cleaning will do some good.Since the machine otherwise works perfectly I want to avoid destroying it totally. Next time I feel ambitious I will dive in and see what I can ruin. After all, a new oven isn't that expensive but my innate desire to fix the problem is pushing me.

author: bob91343
author_rank: Super Contributor
timestamp: July 04, 2019, 12:47:56 am
content: I opened it once again and cleaned the boards that go to the display. Only a minimal change in operation.I guess I gave it my best shot. I wonder if anyone knows where I can obtain a replacement display.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2019, 02:21:19 am
content: Does it use zebra strips between the glass and the board? If so that would be my first suspect, peel them off and clean both sides with warm soapy water. Otherwise there is not much you can do, might be a fun project to retrofit a TFT or OLED display though.

author: xavier60
author_rank: Super Contributor
timestamp: July 04, 2019, 03:04:54 am
content: An unlikely reason could be a deteriorated polarizing film. This can be ruled out by viewing the display though polarizing film or polarizing sunglasses to see if there is an improvement.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 04, 2019, 03:36:41 am
content: If all else fails, try increasing the supply voltage to the microcontroller. But first take a close look at the voltage divider whose only function seems to be supplying two bias voltages to the microcontroller.

author: bob91343
author_rank: Super Contributor
timestamp: July 04, 2019, 04:29:39 pm
content: I do appreciate the suggestions. They aren't practical and not really germane to making a repair. I do believe the only technical solution is a replacement display but have no idea where to get one.So I am stuck either living with a broken display (not terrible) or buying an entire new oven. A third option is to find a free one on Craigslist and use this one as a basis for one of those DIY spot welders. I just made a list of my projects and there are more than a dozen items, repair and construction, so I probably never will go the latter route.I seem to recall paying around $100 for this one new, many years ago. I wonder how much they go for today. I use it enough that I probably should replace it. Amazingly, the lamp has never burned out nor, actually, has anything else gone wrong with it. It's a Sharp and I guess they did a pretty good job.I read an article in an engineering magazine about a new kind of oven but, like so many articles, it gave no clue as to how this magical new device worked. Most of these articles are nothing more than advertisements.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2019, 04:33:22 pm
content: It's extremely unlikely that you will ever find a replacement display other than taking one from another oven. You might be able to buy the whole control board but it will almost certainly be prohibitively expensive. The display itself is going to be a custom part in most cases and spares will not be available. Your only options are either fix it, engineer a hack to replace it (which is cost effective if it's fun to do) or live with it.

author: bob91343
author_rank: Super Contributor
timestamp: July 04, 2019, 09:19:47 pm
content: James, your assessment is spot on. At this point I intend to live with the bad display but keep my eyes open for a free oven from Crakgslist. Some day I may decide I can repair it but for now I'm going to do more important things. The bad part is that the oven has no other issues and is an important part of my kitchen. If it would break, then I'd have more justification for replacing the entire unit.My daughter also uses it and tells me not to worry about it, that she is fine with a bad display.Thanks to all who contributed. If I have more news I will post it.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 04, 2019, 11:27:36 pm
content: The Sharp Carousel 'Sensor' microwave oven here "Rxxxx"? has the exact same issues/series of events as OP bob91343 describedThey must have produced a batch of display duds that got shipped here as well as the US.Or do these displays die anyway as they do on some other devices ? (>>)I've been meaning to pull it apart and take a gander, but haven't bothered as the unit works well with the keypad, with just enough crippled digit readout to get the job doneI wonder if any game (or peaved) Youtubers have had a go at one yet?A teardown and repairor FAIL and shooting range finaleshould score some views

author: xavier60
author_rank: Super Contributor
timestamp: July 04, 2019, 11:44:04 pm
content: A long discussion here,https://forums.whirlpool.net.au/archive/508055"The problem is that flat cable going to it is glued to display board and eventually makes bad contact."

author: Electro Detective
author_rank: Super Contributor
timestamp: July 05, 2019, 02:31:12 am
content: Quote from: xavier60 on July 04, 2019, 11:44:04 pmThanks for thatwill be the first item on the hit list when I get around to pulling it apart..picture hard rain outside, computer clone/backup in progress, crap on the TV (nothing new..)and by chance the main tool box sitting at the kitchen door)That forum discussion is a good read, it seems there are a lot ofShaftSharp microwave users flying blind in the kitchen

author: bob91343
author_rank: Super Contributor
timestamp: July 05, 2019, 06:50:21 am
content: In mine the flat cable isn't glued. And I played with it without being able to change anything. I was going to resolder it but it doesn't appear that the soldering is the trouble.I think it's true that the display module itself has died. And we all feel that getting a new one isn't possible. Maybe we can get Sharp to recall the model and offer a free replacement display. I am not holding my breath.It is, however, reassuring to note that someone else has the identical issue. Makes me think that I wasn't the one who screwed it up.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2019, 11:06:10 pm
content: Is the display anything other than a bare glass LCD? There is not much to go wrong with those other than bad connections. I suppose it's all integrated with the MCU driving the display directly too which makes hacking in something else a lot harder. I miss the days when stuff had a separate display driver IC with a SPI bus. Then you could pick off the data with a microcontroller and drive any kind of display you wish. I suppose you could try building a whole new control board from scratch, atmega328 or whatever MCU of choice, TFT or OLED display or something cool like Nixie or planar neon. Open source microwave oven project, seems like it would be an entertaining "because it can be done" sort of thing.

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 12:21:47 am
content: The display is just the LCD, no electronics. However, it's complicated by the fact that it's more than numerals. It's four digits of numerals, a colon for the space between the minutes and seconds (or hours and minutes when it's a clock) and a few legends such as lb, pct, end, or similar stuff for different selections of power level and number of potatoes and pounds of meat and all that.It's connected with a ribbon cable that I have checked for integrity. The board it's connected to seems to work okay, as it lets me set the clock and the times and all the other stuff. The display tries to indicate my selections but just can't seem to display the appropriate legends. I watch the countdown during cooking and it weakly shows the numbers but you can't really read them. You can press on the panel and it changes a bit. I'm pretty sure it's toast.So what I need isn't a controller or timer or the like. I just need the display. Look above at the diagram I uploaded and you can see how many connections it has. It's Sharp model R509BK and you can download the instructions which show what the display indicates.

author: fzabkar
author_rank: Super Contributor
timestamp: July 06, 2019, 02:50:12 am
content: Just FYI ...https://www.searspartsdirect.com/model/4d63d977pp-000958/sharp-r-509bk-countertop-microwave-parts?page=2

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 03:49:59 am
content: Thanks, I have all that. I am convincing myself that the display has deteriorated. It does have the other stuff. But ignoring that detail, I suppose I could find a generic display to at least show me the numbers. Any suggestions? Are these displays all the same basically?

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 04:13:36 am
content: I wonder if this can be adapted to replace the display.https://www.ebay.com/itm/272893019103

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 04:16:12 am
content: Or perhaps this one:https://www.ebay.com/itm/LCD-1602-Blue-screen-with-backlight-display-1602A-5v-module-for-arduino/382921423682

author: james_s
author_rank: Super Contributor
timestamp: July 06, 2019, 04:36:46 pm
content: The problem is that the display is controlled directly by the main microcontroller that does everything, there is no separate display controller so you cannot pick off the signals that drive the display unless you use something with enough IO to read all of those wires. It is further complicated by the fact that it is almost certainly multiplexed so even if you did hook up a CPLD or micro with sufficient IO there's an added step of figuring out how the multiplexing works. In order to adapt any kind of off the shelf display other than a similarly configured bare LCD segment display you're going to need to decode the signals to the original display and given the way that is designed it's going to be easier to design a whole new control board.

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 05:08:31 pm
content: I find your comments disheartening, James. I did suspect this was the case but figured that since I don't really understand this stuff I might as well ask the group.So I will try to live with the situation. If a freebie comes my way I can consider replacing it but for now we can still use the machine. Who knows, perhaps an identical model will appear and I can either swap displays or swap ovens.Thanks for your thoughts.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 07, 2019, 10:59:48 am
content: Tech futility thoughts aside, once the display on mine goes totally south, I will have a go at both the connector jazz, refresh any 'good looking' soldering, chase down any caps in the mix,and check if the shielding to the display and it's cabling needs attention or an update.I've been down this path before with other white goods that were doomed to landfill duty,and after a couple ofmoments at the manufacturing balls ups and or cheapassery,some dusting off, jiggling, soldering, crimp re-work and contact spray ups, those devices are working flawlesslyand from customer feedback'better than new'i.e. it will either be win win fix and save money on a new food zapperor lose win and scavenge some useful parts from the clapped Sharp

author: bob91343
author_rank: Super Contributor
timestamp: July 08, 2019, 12:01:29 am
content: I have lost hope. I have done everything I can think of to make this display work and no progress. So getting a new one is my next thought.I am off to Google and ebay. The main constraint is that it fit under the cabinet so its height is an issue.

author: xavier60
author_rank: Super Contributor
timestamp: July 08, 2019, 12:08:37 am
content: Has anyone ruled out the possibility of a polarization problem?I have seen this problem only with LCDs that are exposed to high temperature and sunlight such as LCD tractor instruments which gradually fade out, yet can still be clearly read while wearing polarized sun glasses.

author: bob91343
author_rank: Super Contributor
timestamp: July 08, 2019, 01:40:42 am
content: I have no idea what to do to either detect the problem or correct it. I don't think I have any polarizing plastic.

author: xavier60
author_rank: Super Contributor
timestamp: July 08, 2019, 02:01:40 am
content: Quote from: bob91343 on July 08, 2019, 01:40:42 amSome if not all LCD calculators have a loose fitted strip of polarizing film that could be easily removed.https://newatlas.com/stealth-computer-display-lcd-polarizing-glasses/29700/TV and computer monitor LCD panels also have a polarizing film but it is stuck to the glass.

author: bob91343
author_rank: Super Contributor
timestamp: July 08, 2019, 03:52:48 am
content: I have learned a bit from all of this. However, my options have been reduced to just one - buying a new oven.Having made that decision, a whole new can of worms has been opened. Finding one that will replicate the performance of this one is not a trivial exercise. There seem to be no ovens of similar specifications anyway.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 09, 2019, 12:21:37 am
content: Don't rule out a pre-loved one, same model, with a working displayLots of casual users out there that flog them off cheap on a house move,or new kitchen updateeye candy fueled money pitPersonally I prefer the Sharp user layout, and would most likely go with another in the future,as long as some twat at Sharp HQ with a CAD toy program doesn't don't go changing the design for the sake of change (or just to annoy old skool Sharp Luddites)With most Sharps used over the years, I can cover up the food/beverage to be warmed, punch in the exact or 'good enough' time and power setting I need,and you'd never know it was re-heated (or un-staled) in a microwave chamberand it's easy to add or subtract a few seconds to compensate for different wattage amongst different powered models (800, 900, 1100w etc)fwiw I am NO fan of the recent inverter style ones where the fan keeps running long after after the job is done and door closed.I can smell some penny pinching'Progress'on that design

author: bob91343
author_rank: Super Contributor
timestamp: July 09, 2019, 02:43:17 am
content: Well I bit the bullet and bought a new unit, this time GE. Used it once and it seems fine.I am on the fence regarding what to do with the old one. If I could get parts for very little, I'd repair it. Else I may try building a spot welder. Too bad, as nothing is wrong except the display.The new one is slightly smaller but certainly big enough for my needs. I tried three stores and online before picking this one up at Lowe's. They had it marked down and gave me my veterans discount so my cost was low.

author: james_s
author_rank: Super Contributor
timestamp: July 09, 2019, 04:31:39 pm
content: I'm sure it'll be fine, microwave ovens are generally inexpensive and fairly reliable.I think the only time I've ever actually paid anything for one is the early 80s Amana Radarange I bought used for our cabin, I'd have preferred one of the 70s models with the illuminated chrome dials but haven't been able to find one. This one is over 35 years old and still going strong.

author: bob91343
author_rank: Super Contributor
timestamp: July 11, 2019, 08:54:24 pm
content: So I purchased a GE unit to replace the Sharp and am satisfied with it.I tore apart the Sharp and have the magnetron in hand in case anyone wants it. But what I like is the transformer, which puts out 2200 VAC unloaded with 120 VAC input. I assume it's capable of perhaps 800 millismperes load current. The main fuse is 20 A. Not sure what I want to do with this. Oh and I have the high voltage rectifier which appears to be a molded assembly of several diodes in series.There is a tertiary winding on the transformer core the function of which eludes me.Perhaps I could make a Hipot tester or a spot welder. Ideas, anyone?

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2019, 09:15:28 pm
content: QuoteWell it's a pretty handy way of killing yourself!Seriously though, itwilltry to kill you given enough time and opportunity. The output current is far too high for a Hipot tester and the voltage too high for anything else. The tertiary winding is for the magnetron filament.Some folks cut (chop) out the secondary winding*and wrap a couple of turns of very heavy cable instead to use as a spot welder. There are plenty of hits on the web.Aside from that, it's a pretty poor quality high power transformer that runs very close to core saturation and has welded laminations, making it virtually impossible to take apart and put back together successfully.P.S.*Damage the primary winding or its insulation and you're done.

author: bob91343
author_rank: Super Contributor
timestamp: July 12, 2019, 06:19:13 am
content: Thanks for the tips. I made some progress and mounted the transformer in a box with a variac. I need a fuse and meter and terminals. I have the parts, just have to decide where to put them.Oh the magnetron filament. What voltage and current would that be? I haven't found a spec sheet.The box I am using was originally a WW II command set, the one with the 1629 tube and little mirror but all that's been gutted a long time ago. There is a 3A variac on the front where the dial was. A pilot lamp, AC outlet, and a switch. I intend to severely current limit the output so that there can't be much current flow. Just how much I have not decided.The welder is not off the table, although the you tubes I have seen generally call for two transformers. Maybe 1 kW isn't enough for spot welding.None of this really matters. I am just playing. And yes I am seriously aware of the hazards and intend to take all due precautions.

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2019, 09:26:38 am
content: Personally, I'd keep the current limit really low, probably a couple of mA max - even that will bite. The highest current range on my own Hunting Hivolt Hipot tester goes to a maximum trip current of 100uA at up to 10kV. A decent value high voltage series resistor (or string) would be a good first step.You didn't mention it, but you've got a high voltage multi uF capacitor hanging around there too. Ditch that and use a single digit nF range high voltage ceramic if you're still planning on using it for insulation testing.As for the filament winding, I'm not too sure but I think it will probably be a couple of volts at maybe 1A.I've seen spot welder designs using single transformers,1kV1kW ought to be enough small stuff, I'll see if I can find the link.EDIT: This is the one I was thinking of...P.S. That microswitch arrangement could use some work.

author: bob91343
author_rank: Super Contributor
timestamp: July 12, 2019, 05:53:32 pm
content: Interesing. Maybe I will see about going that route. Most of the descriptiiin was on sawing wood. No diagram is provided but I can puzzle that out.I was planning to use ac for my hipot tester so no capacitor needed. Maybe that's a bad idea.If I add the filament power to the output power this transformer comoes out to over a kilowatt.Like most of my projects, this one will sit waiting for me to get back in the mood.

author: bob91343
author_rank: Super Contributor
timestamp: July 12, 2019, 10:45:48 pm
content: So now I have up to 2 kV ac when I bring up the variac. Actually it goes higher but the meter full scale is 2 kV. What's the voltage rating of a carbon composition 2 W resistor? I am using two 1 Megohm in series as voltmeter multiplier but have a hunch that I am exceeding rated voltage. If so I will take them out and put in a string of as many as required. What's more, the resistors are now drawing rated power at 2 kV out, not a good idea. I will put in maybe five 220k or so, 2 W each.No current limiting yet. I am debating as to just how to accomplish that, probably just a series resistor. I wonder if I should monitor output current. I also will put in a fuse, probably an amp or so, in the primary. How much output current should I limit to? Perhaps 100 microamperes? That requres resistance of 20 Megohms.I checked the magnetron filament. It draws 10A at 3V. I suppose if I can light the filament while it is raised to 2 kV I will have a 2.4 GHz transmitter, one kilowatt. I don't think I want to do that. The magnetron connections are simple; the case is ground, the filament is also the cathode. The magnets are a part of the magnetron sturcture so there is no messing with those. There are cooling fins but that's for fan cooling while cooking for long periods up to an hour or so.

author: james_s
author_rank: Super Contributor
timestamp: July 12, 2019, 10:57:33 pm
content: The magnetron will need fan cooling to run for more than a few seconds, I don't know how efficient they are but remember that you're pumping ~1kW into it and getting somewhat less than that out along with a lot of heat. There's rather little thermal mass so for a given amount of airflow the temperature will reach equilibrium fairly quickly.I knew a guy who used a magnetron as a hand warmer, I don't recommend it though. I've always been tempted to mount one in a horn and see if I can make plasma balls outside of an oven.

author: bob91343
author_rank: Super Contributor
timestamp: July 12, 2019, 11:03:06 pm
content: A better idea might be to mount the magnetron in a horn and use it to beep at bad drivers.