Diagnosing heater control board on Honeywell / KAZ 709

Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
Hello,

I'm tackling a space heater that stopped working. Unfortunately I was lending it to a friend so I don't know exactly what happened.

But the main issue is that the unit won't power on at all.

I have checked the tip over switch and it's working correctly, and the board is getting 120V when the switch is set correctly.

The on/off switch seems to be working (shows continuity when pressed).

The resistors and zener diodes on the 120V board all checked out, and I don't seen signs of any resistors or any diodes having blown.

At the moment I'm thinking the main chip might have gone bad. But while I found a spec sheet for it, I'm not sure how to test it, or if I just need to buy a replacement. I hate to swap it if it's good.

My other thought is the over-temp sensor broke, but I'm again not sure how to test it. I believe that's the part with the blue wires.

Any other suggestions? Fair warning, I'm a mechanical engineer by degree (years ago) but I have been playing with electronic projects the past year.

Thanks!
Ian
 

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Dodgydave

Joined Jun 22, 2012
11,303
You can check the temperature sensor, marked NTC on the main chip board, the Black wires going into the heating system, put your meter across the wires on Ohms, and see what value it is, if you heat the sensor up, it should decrease in resistance. Also check the resistance of the heater coils.

Does the board have any power to it, is it lit up, it's transformerless supply, using capacitors to drop the mains down to lower voltage, you can check the voltages across the black capacitors EC1 and EC2.
 
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Pyrex

Joined Feb 16, 2022
267
hi,
first of all, check two big brown capacitors rated at 250V. Check the capacitance, it shoud be 1.5 uF roughly. And big power resistor marked with orange, orange and brown strips. It should read 330 Ohm. And two electrolytic capacitors EC1, EC2 , as it was noted before
 

MrChips

Joined Oct 2, 2009
30,819
You wrote "But the main issue is that the unit won't power on at all. "
I assume that means that no LED is lit.

I would have guessed that somewhere there is a thermal cut-off fuse that looks like either of the these two.
1704379321134.png

1704379334718.png

Can you show a photograph of the main power cord and where it enters the unit?
 

Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
You can check the temperature sensor, marked NTC on the main chip board, the Black wires going into the heating system, put your meter across the wires on Ohms, and see what value it is, if you heat the sensor up, it should decrease in resistance. Also check the resistance of the heater coils.

Does the board have any power to it, is it lit up, it's transformerless supply, using capacitors to drop the mains down to lower voltage, you can check the voltages across the black capacitors EC1 and EC2.
I checked for continuity across the temp sensor, but I will try heating it and measuring the resistance.

I did measure the resistance across the heater coils, and got 15 Ohms, 22.5 Ohms and 37.5 Ohms depending on which I checked. That seems ok.

The fuse on the power board tests ok.

The board isn't light up at all when I press the power button. No lights, no relay click, nothing.

I will check the capacitor voltages.

Thanks!
 

Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
hi,
first of all, check two big brown capacitors rated at 250V. Check the capacitance, it shoud be 1.5 uF roughly. And big power resistor marked with orange, orange and brown strips. It should read 330 Ohm. And two electrolytic capacitors EC1, EC2 , as it was noted before
Sounds good, I will check those. Thanks!
 

Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
You wrote "But the main issue is that the unit won't power on at all. "
I assume that means that no LED is lit.

I would have guessed that somewhere there is a thermal cut-off fuse that looks like either of the these two.
View attachment 311684

View attachment 311685

Can you show a photograph of the main power cord and where it enters the unit?
You guessed correct, no signs of power at all when I press the power button. I do have power going to the power board, and confirmed the switch closes and makes contact. But nothing happens.

I saw a few other troubleshooting videos and posts that mention the a thermal cutoff like those you show. But this doesn't seem to have one. I can photograph the cord coming in, but it definitely doesn't have one like the bottom, and I haven't seen anything like the top.

Thanks!
 

Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
Here’s a photo of the wires going into the unit. The two black wires coming from the board marked NTC are in a white casing and they go down near the bottom of the unit to measure the external temp. You can see the green end of it in the second photo.
I’m not sure if I should be measuring AC or DC volts so I did both.


EC1 0.53V AC, 0.124V DC
EC2 0.2mV AC. 0V DC

Also the voltage wheee the red line voltage comes into the board vs the black neutral line is 256 mV AC

thanks!

photos won’t attach, I will try again in a bit
 

Dodgydave

Joined Jun 22, 2012
11,303
Sounds like No AC mains coming in to the pcb.
Here's what I think the circuit board is.
Check the mains supply at the Orange Capacitors, and then the DC supply across capacitor EC1 and the Zener diodes, .

17044451300545902521443645444549.jpg
 
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Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
Here’s a photo of the wires going into the unit. The two black wires coming from the board marked NTC are in a white casing and they go down near the bottom of the unit to measure the external temp. You can see the green end of it in the second photo.
I’m not sure if I should be measuring AC or DC volts so I did both.


EC1 0.53V AC, 0.124V DC
EC2 0.2mV AC. 0V DC

Also the voltage wheee the red line voltage comes into the board vs the black neutral line is 256 mV AC

thanks!

photos won’t attach, I will try again in a bit
Here they are:
 

Attachments

MrChips

Joined Oct 2, 2009
30,819
Unplug the unit from power source.
Measure for conductivity across this switch. It looks like a thermostat.

Also measure across the thermal cut-off fuse inside the white high temperature braid.

AAC heater thermal switch.jpg
 

Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
That's the anti-tip switch. I'm making sure it's got a good connection when doing my testing.

I can measure the temp sensor, I believe I measured that it has continuity, but didn't measure the resistance.
 

MrChips

Joined Oct 2, 2009
30,819
Look at the white high temperature braid held down by two black metal hooks.
Inside the braid is a thermal cut-off fuse. I bet all my money that the fuse is blown.
 

Thread Starter

autoarcheologist

Joined Jan 1, 2024
12
Look at the white high temperature braid held down by two black metal hooks.
Inside the braid is a thermal cut-off fuse. I bet all my money that the fuse is blown.
I felt all along the braid and can’t feel any bumps like a fuse. I measure 8-12 kOhm across the temp sensor
 
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