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

[Robunhood]

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

 

[Bluejets]

Model..??
Timer settings..??

 

[Robunhood]

Model..??
Timer settings..??

model# R-9H71, timer settings are anywhere from preheat to 90 minutes doesn't matter as long as the convection mode is selected.

 

[Robunhood]

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]

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]

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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From: Robert Couchman <[email protected]>
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Date: Sat, 28 Nov 2020 16:54:07 -0600
Subject: Re: Sharp Carousel II Microwave Convection Oven quits convection mode after 3 minutes - New reply to watched thread
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[Bluejets]

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

 

[Robunhood]

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]

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]

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]

After patiently waiting the replacement part arrive today! I made three (crude) temp/resistance measurements:.420 megohms@ approximately 30 degrees F, 25Kohms @ 70 degrees F and 2300 ohms @ 270 degrees F . I hope this may be of help to someone else in the future. FYI after much searching I was able to purchase the part online at Encompass
https://encompass.com/?gclid=CjwKCA...329s96-P2yHzQYh8DmUX5LfY4WufGHNBoCo8oQAvD_BwE