Sharp combo microwave model R-861SLM- electonics dead so looking for help in fixing

[jcyeandle]

Hello – first post and so apologies if I fail to comply with every protocol.
Our Sharp microwave/convection/grill oven died whilst doing some repeated short microwave warmings (repeated start button pushing).

I am not a tradesman so my appliance fixing experience is limited to what we have and use.

I have some limited experience in electronics and the bottom line is should I continue to seek and effect a repair at the component level or spend £78 with Kenneth (parts dept. – very helpful guy), purchase a new appliance for £150 or is this model so unreliable that I go elsewhere? It is generally a nice microwave (we don’t use the other heating options) being a drop down door and easy clean-up appliance.

I guess it would be nice to find someone here with prior experience of the Sharp R-861SLM repair or a service manual with full circuit diagram:-) Alternatively some help on Sharp microwave control system boards in general.

The story so far and bear in mind that my use of ‘dead’ above refers to no operation, no display and no response to any button: –

1-all fuses are ok
2-all interlock microswitches and thermistors have checked out ok
3-the filter board (first thing in line with the incoming mains L&N) is ok and passing power onwards
4-the microwave generating side of things checks out ok for continuity and resistance of the HV transformer and associated components
5-L&N are appearing ok at the main control/display/relay PCB.
6-after separating and removing the control PCB from the cabinet there is no apparent damage to see under a stereo zoom microscope, either components looking cooked/leaking or foil/track/pad damage. Dissapointing as usually I can find 90{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} of ‘electronic’ faults this way but!
7-After hooking up the control PCB assembly to an external L&N supply, there is still no life on board and so I’m assuming that the oven would work if the board functioned.
8-I’ve no PCB circuit diagram and can’t find one on the web so I have traced the HV side of things – see attached.
9-there is a varistor accross the L&N with a series component which may or may not be some form of fuse which would fail or go high resistance as the varistor goes low impedance in the presence of an overvoltage on the L&N. The varistor measures several megohms (as I believe it should) and the fuse device (which maybe an inductor) measures approximately 0.3ohm and I believe this to be as it should be also.

There is a ‘trick’ looking IC (LC1206A) involved with this HV side of things and from a Chinese data sheet (my only guidance) could be a PWM involved in the mark/space control of the heating/power functions -but I’m not sure!
One side of the LC1206A also appears at the second of the transformer primaries and is also involved with an opto-isolator thereby completing the HV side isolation (the opto-isolator and the transformer).

The isolated and LV side of things comprises of a centre tapped pair of transformer secondaries, a couple of diodes and some electrolytic’s feeding a standard (surface mount) PBM05 5 volt regulator and thence onwards to the control/management section with presumably a microcontroller, display driver/key matrix and then an output control side consisting of a ‘driver’ IC and a bunch of relays.

10-As far as tracing voltages goes: –
-the HV rectifier is generating the expected 1.4xline voltage. The transformer primaries and secondaries are all in the low ohms range and so I assume that it is primarily an isolation transformer?
-The secondary side is apparently not giving enough output voltage to enable the 5V regulator to do its thing (ie 12-18v DC?) so I suspected the transformer or the LC1206A. I sourced the LC1206A from China but replacement did not get a result. I find no info on the transformer so it’ll be a case of removing from the board and given the lead-free issues I’m holding back on this for now.

11-Without 5V on the control side nothing is going to work but I’m faced with no obvious supply of a replacement transformer (GAL-TR009(galanz)) and no idea of the correct primary/secondary voltages.
12-I’ve yet to trace the LV circuit but it looks fairly standard stuff as far as the supply goes
13-I’m assuming that the opto-isolator (Sharp-PC817) is doing a check on the HV powering up and stable before the microcontroller starts to do things but this wouldn’t affect the 5V supply working?

Thanks in anticipation,
Jonathan

[electrofix]

sounds like your digging too deep

the fault here sounds like a board power supply problem

the board needs 12v or 5v or whatever voltage the design requires. you need to find out where that is first

sounds like standard transformer, rectifier plus asmoothing so should not be hard to find

Dave

[jcyeandle]

Hello Dave and thanks for taking time to reply and I am guilty as charged!

I concur, the board needs its 5v for the control part to function.

I’m struggling to work out what and how the primary windings of the transformer are being driven and it doesn’t appear to be ‘standard’ ie. line and neutral supplying the necessary AC being transformed/stepped down to say 12vAC. The only direct(ish) connection to the primaries is via the LC1206A which presumably outputting some form of digitally constructed and maybe shaped square wave. Judging by the size of the transformer (tiny/board mounted) it is being driven at far more than 50HZ, but I could be wrong. Probably am.

I failed to upload my de-constructed circuit here and I see that there is very small size limit. I have no/do not use web storage but I could email it somewhere?

I could easily lash up a 5V DC supply off board but don’t feel that is the fix that I need. Neither have I been able to find an equivalent transformer (tried the Galanz web site and others).

Maybe this HV circuit design is too obtuse.

Kind regards,
Jonathan

[electrofix]

ah its a switch mode power supply

power comes in. get rectified and smoothed to approx 300v DC. power normally comes in via a resistor with a low value normally 100 ohm or lower

the chips job is to create a high frequency sine wave to lower the voltage

you need to check all this works first and you have this voltage

also check the diode after the transformer. they are shottsky diodes and we get lots failing

check voltage on 5v side. look for a voltage that rises and falls and let me know

dave

[jcyeandle]

Hello Dave,
thanks for looking at this.
Yep, that sounds about right although the IC is possibly doing a little more than just doing the high frequency chopping job.? The opto-isolator goes directly to pin 5 of the LC1206A and is possibly a not-enable line from the controller?

I do get the 340vDC or so.

The shottky diode is a very good lead – thanks. One of them conducts both ways:-) Hopefully I’ll be off and running once I track some replacements down.

I’m in the middle of some plastering right now but hopefully get some time at the weekend or evening and I’ll update you.
Good man,

Kind regards,
Jonathan

[jcyeandle]

Two replacement shottky diodes now ordered and fingers crossed for a funtional microwave for Xmas.

I’ll report later…

Jonathan

[jcyeandle]

Hello, it is with much thanks and satisfaction that the replacement shottky diode (LV side of control board) as suggested by Dave of Electrofix, has brought the control board back to life and we will now have a functioning microwave oven for Xmas

I really appreciate all of the help that this site can provide folk like me. In particular, Dave’s freely given assistance has enabled me to save a perfectly good appliance from the tip and given me enormous satisfaction (and saved me some cash).

Of course, I’ll get some kudos from the family and we’ll all get the opportunity of warm mince pies:p

Many thanks Dave and a very merry Xmas to you and yours, and to all of the helpers here,
Kind regards,
Jonathan

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