Microwave circa 1986/87 part specs?

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bxdobs

Joined May 11, 2014
24
Quasar MQ8895 is a Microwave/Convection oven ... currently, the unit is flashing the Colon at 1 Second intervals (much like it just lost power) but will not accept any user input ... Checked the voltages (S6-S5 2.7Vac rms, S4-S3/1 18.7Vac rms, S2-S3/1 17.6Vac rms) on the control board, tested all the interlocks ... tested all the cross-connections on the membrane keypad (all contact closures range from 140 to 240 ohms) ... attempted to simulate the required startup key closures directly on the keypad connector but nothing accepted ... scoped the Microcontroller which appears to be active (500kHz clock with activity on i/o lines) ... I know the Humidity sensor is broken but that hasn't caused this type of failure ... a failed humidity sensor basically keeps the convection features from running ... I replaced this sensor several years back and the machine was working fine until this sensor failed again ... today I retested all the external components and connections and found that the temperature sensor is open circuit (my initial readings were in the M ohm region suggesting I may have inadvertently been in circuit during those tests) ... suspecting this open sensor is causing the current no go failure.

Neither of these parts/sensors appear to be available from any source ... however, surely there could be an outside the box solution for at least the temperature sensor provided one knew the specs for this sensor; resistance range and coefficient +/- ... as this sensor is in the convection cavity, not certain if it is actually used for microwave operations so looking to do a simulation test.

I have the control cable disconnected and was going to add a variable resistor across the temp sensor input to see if is the actual cause of the current failure but was hoping someone might be able to suggest a starting and or range of resistance I should look for ... there is a pot in series with this sensor currently set to 25k so wondering if I should just add 25k to the other leg of this 3 leg sensor ... Sensor from pin 1 to pin 2, 33k pot from pin 2 to pin 3 (set to 25k) (the numbers on this sensor are 437FV 6574-3E VR)

Unlike newer Microwaves, this machine has only 3 main microwave components simply consisting of a transformer, magnetron and a cap, it is well built and has performed exceptionally well ... both the cap and magnetron were replaced several years back when the original cap shorted out ... even if I was only able to get the microwave working again this machine is worth saving.

I have ordered a service manual from a 3rd party but it is supposedly in the mail ... the expectation is it supposed to include a schematic ... I am already reverse-engineering the control board in case this manual doesn't arrive or doesn't provide a clear enough schema

Thanks in advance for any help you can provide with this
 

Chris65536

Joined Nov 11, 2019
254
however, surely there could be an outside the box solution for at least the temperature sensor provided one knew the specs for this sensor
Can you post a picture of the temperature sensor? Your 25K starting point is as good as any for sensor simulation. I will be surprised though if this is the source of your problem.
 
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