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Sharp Carousel II Microwave Convection Oven quits convection mode after 3 minutes

Robunhood

Nov 27, 2020
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This is an oldie but a goodie, unfortunately it has developed a fault mode in the convection mode where it will heat for about three minutes and then drop out i.e display still on but no turntable or fan upon which if the start button is pressed the system starts up again and will run for another three minutes and repeat ad infinitum. I can't find schematics but it appears to me that the power to the relays is interrupted and then they re-latch once the start button is depressed. I would appreciate any help resuscitating this old workhorse:)
 

Robunhood

Nov 27, 2020
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I have had some success, found the repair manual online, trouble shooting guide therein indicates a failed thermistor. I am know looking for a cross reference since the original part is no longer available. Any one know the resistance range of Sharp Part # FH-HZA010 WRE?
 

Bluejets

Oct 5, 2014
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Care to show us what you found and save us trying to find it..?
What makes you think the thermistor is not working, other than via trouble shooting guide..?
Did you test it in some way, and if so , how?
 

Robunhood

Nov 27, 2020
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So, after much searching I downloaded a copy of the service manual in which troubleshooting grid the 2 minute timeout was listed and the only potential fault was the convection thermistor. So apparently if the CPU sees no input from the temperature sensor after two minutes it shuts down. Remarkably, although multiple sites list the part as out of stock or unavailable, I found one for<18$ shipped. Oh yes before placing the order I pulled the thermistor and tested it with an ohmmeter, open circuit.
Hope this helps someone else down the line.

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Bluejets

Oct 5, 2014
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Normal room temp resistance can be as high as 10k and most ohm meters on standard scale will not measure that high.
 

Robunhood

Nov 27, 2020
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Normal room temp resistance can be as high as 10k and most ohm meters on standard scale will not measure that high.
Thanks for the reply, my Fluke 75 has a megohm scale so I believe it was accurate, also the oven produces identical behavior with the thermistor disconnected so I feel pretty good about the diagnosis. When the new part is installed I will report back.
 

Bluejets

Oct 5, 2014
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Might pay to take a measurement of the new part just for your own records as thermistors are rarely ever that expensive.
Perhaps it comes with gold leads...:):)
 

73's de Edd

Aug 21, 2015
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As per der bluejetster's . . .Might pay to take a measurement of the new part just for your own records as thermistors . . .
I think that your lifespan is taken care of unless ? ? ? ? happens.
For any OTHERS with a like design Sharp units convection portion that is encountering this same conundrum, you might take both an ambient resistance check and THEN another with the thermistor submerged in a glass Pyrex container of boiling water. That then gives two spread spectrum temp analysis reference points.
Post the values .
If then, an expected real / more realistic . . . mere twee 'mellican dollah . . . .replacement . . .vice 18-40 $ scalper unit is not found.
And research finds a sub unit to not be exactly congruent with that same temp curve, a shunting select value of metal film resistor could swing the response more in line, to a successfull workaround.
BTW . . . can you also confirm the mass of the actual thermistor elements found dimensions as well as if being shaped as rod- bar- disc ?
 

Robunhood

Nov 27, 2020
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