• ** REMEMBER! **The microwave can still shock you even unplugged!!

    ALWAYS discharge the high-voltage capacitor first if you even think your hands will come close to any HIGH VOLTAGE components.

    Jeff mentions this: Anything in the high voltage ( magnetron, capacitor, diode, wires to and from ):
    ...Use a metal ( not the shiny chrome type ) screw driver with a insulated handle to short across ( touch both at the same time ) the terminals of the high voltage capacitor to discharge it.

    From Jeff's site: http://www.applianceaid.com/component-testing.php

    Jake

FIXED Sharp Carousel Convection oven intermittent power problem

nyeurt

Premium Member
Joined
Apr 21, 2015
Messages
8
Location
Fiji
Model Number
R-9H55
Brand
Sharp
Age
More than 10 years
Hello,

Since 1998 we have been using this 'workhorse' model which worked very well until now; the last (and only) major fault was a shorted HV capacitor back in 2012. Presently, when fired up we can hear a repeated 'clicking' sound coming from the right hand side, and the display intermitently lights up. Sometimes after a while it fires up normally, then suddenly stops in the middle of heating. When this happens, the display goes dim or off (keys still sometime beep on a low volume) and the oven light is off when the door is open. I have checked both the HV capacitor and HV rectifier unit which appear both fine, as well as the HV transformer although am not entirely sure on the last one. The main ceramic fuse does not blow. What is strange is the intermittent nature of the fault, as if power was trying to get through but something is hindering it. It sounds as if the relays do not get enough power to stay on.

Any ideas would be very welcome. This oven is part of the family for over 14 years and very easy to use, and I would really like to keep it working. The Magnetron seems fine (heats very well, when the unit fires up, which it does very rarely now).
 

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jeff1

Appliance Tech - Moderator
Staff member
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Messages
23,618
Location
Ontario, Canada
Hi,

Loose fuse holder, bad noise filter ( if used ) loose/burnt wire, loose/burnt thermostat would be prime suspects....failing those, bad board would be next.

jeff.
 

nyeurt

Premium Member
Joined
Apr 21, 2015
Messages
8
Location
Fiji
Hi Jeff,

Thank you, will give those parts a thorough check could perhaps be also some dry joints of the relays on the main board. As far as I can tell fuse holder is ok and there is no noise filter. Will try and have a go this weekend and update on the forum what I find.

Regards, Tony
 

jeff1

Appliance Tech - Moderator
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Messages
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As far as I can tell fuse holder is ok
Pull the fuse out and check for any small arc marks on the fuse metal ends.

jeff.
 

nyeurt

Premium Member
Joined
Apr 21, 2015
Messages
8
Location
Fiji
Latest update: checked the transformer, primary (1.3 Ohms) and secondary (72 Ohms) as well as filament winding (0 Ohms? service book says less than 1 Ohms...) seem ok. All fuses also (thermal, cutout and ceramic). This time when turned on again, the display flashed normally for a few seconds, then went blank (also oven light when door opened). After a short while some burning smell emanated from the control unit, apparently from Relay 1 (power). Could it be a burnt-out relay? Would the previous intermittent symptoms point to that? No fuse blown. Next step: will try and remove control unit and check the board for burnt components.
 

jeff1

Appliance Tech - Moderator
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Messages
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After a short while some burning smell emanated from the control unit, apparently from Relay 1 (power). Could it be a burnt-out relay?
Sounds plausible.

jeff.
 

nyeurt

Premium Member
Joined
Apr 21, 2015
Messages
8
Location
Fiji
Final update: checked the control unit PCB, and on component side noticed arcing and heat marks around Relay 1 (RY1) and resistors R1 and R2 flanking transistor Q1 (attached image). On the foil side, the solenoid pins of RY1, RY2 had dry solder joints and black arcing marks were obvious around the power pin of RY1, which was loose and dryly joined. The emitter pin of transistor Q1 also appeared to have a dry joint. After resoldering all the dry joints, cleaning and reconnecting the control unit board, the unit was found to fire normally, with no clicking sound, no hum or any smell of burning ('smoke test'). I am pleased to say that the microwave has now a new lease of life after 14 years of loyal service!

In restrospect, the cause of the dry joints might have been ageing of the solder, plus the fact that while in operation the HV relays are subject to vibration which can transfer to the pins on the PCB, increasing the chances of them becoming loose. The intermittent contacts then lead to arcing, heating of R1/R2 and eventually burning. While they are still functional, R1 and R2 (and possibly Q1) have been heat stressed and may need to be replaced to prolong the life of the control unit.

Hoping that this experience will help anyone facing a similar problem with an ageing microwave over ten years old. In those days they were built really solid with quality, serviceable components, so the motto 'old is gold' is worth considering before thinking to get rid of them!

Regards, Tony


Relay_Faults.jpg
 

jeff1

Appliance Tech - Moderator
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Messages
23,618
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Thankx for the update :)

jeff.
 

nyeurt

Premium Member
Joined
Apr 21, 2015
Messages
8
Location
Fiji
Thank you; just a further update that after a while, the turntable stopped working and also the heating (microwave). I assume that it has to do with transistor Q1 and resistors R1 and R2 in the power circuit that were heated when the relay joints were faulty and have now failed; will check those / replace and report back. I will also check the lower latch switch. The fan and control display still work fine during the cooking cycle, there is no more clicking noise from the relays. Back to the work bench! :)
 

nyeurt

Premium Member
Joined
Apr 21, 2015
Messages
8
Location
Fiji
Final update: After locating the required parts overseas (one transistor 2SB793 and two resistors, 330 Ohms and 470 Ohms 0.5 Watts) that are linked to RY2 on the control unit (regulating the turntable motor and HV transformer) good news is that the oven is functioning normally. Actually, this exercise allowed for a thorough cleanup and servicing of all joints and connectors, as well as motors and fans so the unit looks and sounds 'like new' despite its 15 years of duty time. Lesson learned: after some years of operation do a routine servicing to check for any dry joints on the control unit PCB relays, this could save a lot of trouble later! :)

Replaced transistor and resisters.jpg
 

jeff1

Appliance Tech - Moderator
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Thankx for the update!

jeff.
 

nyeurt

Premium Member
Joined
Apr 21, 2015
Messages
8
Location
Fiji
You're welcome. I am pleased to report that several dinners have by now successfully been cooked in the oven, which passed the smoke test (literally!). Total cost of the repairs (components) was $2.50 USD. This unit had been in daily use since 1998.

Sharp R-9H55 repaired in 2015.jpg
 

jeff1

Appliance Tech - Moderator
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Messages
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Ontario, Canada
Good job :)

jeff.
 
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