Microwave oven repair

[bob91343]

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?

[Psi]

I would check the bias voltage on the LCD

[bob91343]

I didn't know there was a bias voltage.  Can you direct me as to how to measure and what I should read?

[fzabkar]

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

[bob91343]

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.

[fzabkar]

This application note describes how the LCD segments are driven:

http://ww1.microchip.com/downloads/en/AppNotes/doc2569.pdf
http://www.cloverdisplay.com/pdf/S5080.pdf

[bob91343]

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.

[bob91343]

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.

[fzabkar]

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.

[Gyro]

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.

[bob91343]

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.

[bob91343]

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.

[james_s]

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.

[xavier60]

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.

[NiHaoMike]

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.

[bob91343]

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.

[james_s]

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.

[bob91343]

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.

[Electro Detective]

The Sharp Carousel 'Sensor' microwave oven here "Rxxxx"? has the exact same issues/series of events as OP bob91343 described 

They 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 done

I wonder if any game (or peaved) Youtubers have had a go at one yet?

A teardown and repair or FAIL and shooting range finale    should score some views 

[xavier60]

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."

[Electro Detective]

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."[/b][/i]

Thanks for that  will 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 of Shaft Sharp microwave users flying blind in the kitchen 

[bob91343]

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.

[james_s]

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.

[bob91343]

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.

[fzabkar]

Just FYI ...

https://www.searspartsdirect.com/model/4d63d977pp-000958/sharp-r-509bk-countertop-microwave-parts?page=2