Needing to find part substitutions for a Classic Convection Quasar Microwave

[bxdobs]

Project to repair a beloved Quasar Microwave YMQ8895YW

This unit has been a workhorse since receiving as a gift in 1987 ... Over the years I have had to replace the High Voltage Cap and Magnetron after the CAP shorted out, (replaced the Magnetron at the same time as the cap for safety) and at some other point the humidity sensor stopped working and was replaced.

Within the last year the ceramic temperature sensor has failed (open circuit) along with possibly the humidity sensor? These parts are no longer available from any of the online part sources that I can find ... without these parts working, the convection features of this oven are disabled (understandably for safety) by the firmware ... while the majority of the time we only use the microwave features, the convection features are sorely missed.

So am looking for help to find some part substitutions/suggestions:

I detest throwing away fixable stuff ... in this case, this appliance has been outstandingly simple to use and has always produced perfect results for roasts, lasagna, meatloaf, and much much more ... I had this machine apart for 2 weeks to diagnose and reverse engineer but was forced to reassemble, as is, to quiet the ensuing family revolt

Thanks in advance for any help you can provide with this project

The 40 pin embedded processor used by this oven is a TMS2670 ... the only information I have found for this Processor is a functional listing that didn't include a pinout:

MCU is using a 500kHz Xtal
VCC 9V
ROM 4k x 8 
RAM 256 x 4
J|K 8 (I/P)
I/O 4 (I/P | O/P)
ADR 14 (O/P)
O/P 8 (Latched)
IRQ 1
A/D 4 x 8b (256 bit resolution)
ZCD 1? (Zero Cross Detection)
TMR 1? TMR
ECI 1? ECI? (External Interrupt)?

Expecting due to limited ROM space, Quasar didn't include any diagnostic features in the firmware (none are listed in the Service Manual)

Per the Quasar Service Manual:

ANE605Y2LOAP Element/Oven Temperature Sensor
Appears to be just a thermistor mounted on a ceramic block which is secured physically inside the compartment with the actual heating element (so must endure high temperatures and therefore leads are NOT soldered)

This device has failed/open circuit (resulting in being unable to take any measurements) The Service manual, page 30, point 9, provides the following measurement details ... sensor should measure in the range of 360k to 800k ohms (between 50F and 90F) ... there is no indication of any other readings/specs, including whether this is an N or P coefficient sensor, however, the test details highly suggest the resistance goes UP with temperature

Based on these minimal details, expecting the following specs; Thermistor: 620k ohms at 77F (25C) with a positive coefficient and slope of 7k to 8k ohms per degree F

A replacement should be able to be crimped or welded to the existing ceramic block ... can anyone recommend either an existing consumer part (from another appliance) or a discrete thermistor that might meet these specs?

There is also an external 50k POT in series with this thermistor which is set to 25k Ohms (expecting this is being used to fine-tune the initial temperature reading)... below is a node representation of how this device is wired into the embedded circuit

Temperature Sensor Parts:
IC1 MCU
Z4  5.6V
R17 23k27
R42 10k
R46 910k
C10 .01uF
TV1 Thermister
RV1 50k Pot

Note: as I do not have a pinout for this MCU, I do not know exactly what the MCU pins (specifically Pins; 32, 34, 35, 36 and 37) listed here, are functionally intended for

Node - Descriptions
   1 - 9V DC
       IC1-18
       IC1-36
       TV1-1
       C10-1
       Z4-K (cathode)
   2 - TV1-2
       RV1-1
       RV1-2 (wiper) Set to 25k
       C10-2
       R42-2
       R46-1
   3 - R42-1
       IC1-35
   4 - Z4-A (annode)
       IC1-37
       R17-1
   5 - R17-2
       R46-2
       RV1-3
   6 - GND
       IC1-40
       IC1-2
       
ANE601L-F70  Humidity Sensor
This device has a coil that appears to be a heater with an unknown device in the center (could be a thermister)? I don't recall having made any measurements of this part when I replaced it.

This is a fairly complicated device ... it has a loop of wire which appears to be some kind of heater arrangement (possibly providing a constant/known temperature) that surrounds what appears to be another thermistor ... the loop is still intact but the device in the center is open circuit ... the device is located in an airflow cavity on the side of the oven ... so ... not in direct contact with the Element OR the Magnatron

The main parts associated with this device are as follows:
IC1 - MCU
TV2 - Thermister?
CV1 - Heater Coil
C9  - .01uF
C13 - 1uF 50V
C16 - 1uF
R10 - 10k
R35 - 10k

There are more nodes associated with the two parts of this device but my main focus is to first get the temperature thermistor replaced then determine if this Humidity sensor has actually failed or is just failing because the temperature sensor failed

The service manual doesn't provide much more than one measurement test to only ensure the heating coil is still reading 6 ohms ... then some kludge of an operations test requiring disrupting (cutting/disconnecting sensor wires) the physical sensor circuit then shorting sensor wires together at some point to see if the MW shuts down. Being there are no measurements being requested of the sensor itself, suggests this device may not be measurable (not a thermistor) even though the block and level in the service manual shows a RESISTOR symbol for this device.

Node - Description
   1 - 9V DC
       IC1-18
       IC1-36
       TV2-1
       C9-1
   2 - C9-2
       R10-1
   3 - R10-2
       IC1-32
   4 - CV1-1
       C13-2 (-)
   5 - CV2-2
       C16-1
   6 - C16-2
       R35-1
   7 - R35-2
       IC1-34
   8 - IC1-15
       C13-1 (+)
   9 - GND
       IC1-40
       IC1-2
 
I can attach the KiCad sch file I have made of this machine if it helps

[james_s]

I don't think the pinout of the microcontrollers is going to be of much use.

I suspect the temperature sensor is just a thermistor, I would take an educated guess and buy a few different thermistors in the right general ballpark and try them out, I doubt whatever it had was particularly high precision.

The humidity sensor sounds like a thermal conductivity type, there's an overview here https://www.fierceelectronics.com/components/choosing-a-humidity-sensor-a-review-three-technologies  One of the pictures there looks very much like the sensor I've seen in a microwave oven, so the open device may indeed be a thermistor. You might look and see if you can find a sensor from a similar oven, or maybe you can replace the thermistor that's there? If you're sure the thermistor is bad you have little to lose by cracking it open, you might be able to measure a large enough chunk of intact material to estimate the characteristics.

[bxdobs]

Thanks for the info ... the MCU pinouts would be useful to confirm if A/D or Digital i/p's were being used ... the expectation is A/D ... as the MCU's A/D only has an 8b resolution I would concur with your observation that the tolerances of these devices are most likely very loose ... now that I have made a KiCad schema, my next step was going to involve adding pots in place of the 2 thermistors and basically (carefully) do some active testing and measurements ... there are external Hi-Temp cutoff switches for both the Mag and the Element compartments so wouldn't be bypassing any safety there ... plus we typically use the plugin temperature probe for roasts ... there is an expectation that the MCU may ignore the built-in during probe use ... thinking the element thermistor may just need to be cheated with a resistor somewhere between 360-800k so the MCU doesn't go into full fault shut down

Was also thinking of carefully scraping off some of the hard (ceramic?) coating on the element thermistor to see if I could locate the break then measure it from there ... need to find a time that won't disrupt the appliance use.

[james_s]

If I were doing this, I would probably disconnect the primary wire to the HV transformer, that will allow the oven to run without actually powering up the magnetron and it will make things much safer. I also usually install a clip lead across the HV capacitor, it's cheap insurance against a careless mistake. Microwave ovens are arguably the most dangerous device to poke around inside that you'll find in a typical home so a few precautions are worthwhile.

Connecting a pot in place of the original sensor for testing purposes sounds like a great idea, I don't know why I didn't think of that.