I have an expensive toaster that will not stay down and after testing found the voltage to the electromagnet does not change. This has led me to believe the pcb component(s) are faulty.


I have done a fair bit of electronics work but this is my first-time attempting PCB repair. My first and foremost question is if I replaced all the components would it be guaranteed to work or is there more to it than that? Second most, I am not sure which are the power pins to the electromagnet, so I just recorded all measurements and found that both DC and AC voltages are read from my multimeter. How/why?

Here is the data I have collected thus far. The following are the voltages for the electromagnet initially and after 5 minutes on the lowest toast setting. I would anticipate the electromagnet to be 12vdc so I am unsure if I am recording what should be the off voltage or on voltage.

Pins 1 and 2: 31.4v AC/0.271v DC -> 31.4v AC/0.276v DC

Pins 2 and 3: 1.92v AC/8.71v DC -> 1.90v AC/8.78v DC

Pins 1 and 3: 29.9v AC/8.80v DC -> 29.9v AC/8.97v DC

The zener diode in the top left of the first image (ZD1) has a 0.512v forward bias and 0.729v reverse bias with my multimeter. Furthermore, the resistor R5 (top right of first image) is reading 0.5 ohms which should be in the megaohms if I read it correctly. This is a 4 slot toaster which has one of these circuits for each two slots. The issue occurs for both with the same faults found. I have not tested the capacitors as I do not have a capacitance test on my multimeter. Likewise, I do not know how to test or read the thermistor; it has 404 written on it and is reading 0.384 megaohms at room temperature.

 

Replacing components with the hope of fixing the fault is a poor trouble-shooting approach.

Try to figure out what it is doing and what it is supposed to do. That is the way I would approach the problem.

If it does not stay down, look for a mechanical issue. There is usually a hooking mechanism that holds it down.

 

Replacing components with the hope of fixing the fault is a poor trouble-shooting approach.

Try to figure out what it is doing and what it is supposed to do. That is the way I would approach the problem.

If it does not stay down, look for a mechanical issue. There is usually a hooking mechanism that holds it down.

I would like to do so but I do not have enough knowledge currently to paint much of a picture. Moreso, I do not know if my findings are correct. Overall, it should just be a timing circuit with some AC to DC conversion for the electromagnet. Though that does not explain why I am getting AC and DC output for it; don't even know if that should be happening or if it's a symptom of the fault(s).

Here are my thoughts so far:

1. Electrolytic capacitors dry out which will be expedited by the high temp of a toaster.
2. I see the thermistor is touching the large resistor on the left side. I suspect this plays a key role in when the electromagnet is switched off
3. I suspect the transistor is what actually turns off the electromagnet, but it tested fine.

The toaster is also 15 years old at this point so simply age could be the culprit. Though I am intrigued that both circuits have the exact same issue. That all said, what should I be testing/measuring to better understand?

 

Hi,
Nice pictures.
Q1 requires moving away from that resistor. Have you checked if Q1 has survived cooking.
Don't take this personally, is the toaster loaded when the lever is pressed down?
Good luck.........

 

Hello,
Just a thought, before diving into the electronics, these devices have strong springs which are stretched down hard when the bread is inserted and the lever is pushed down,then when the bread is toasted they shoot the toast up with force.

Ok bear with me, there is usually a pair of simple but strong contacts that make a connection when the device is activated (bread in, lever down), they power the board and the nichrome heating wire. (see attached photo of one)
Check to see if these contacts are actually making a connection, it is a common fault, they are subject to a lot of vibration and wear from arcing.

Is the arm on the side of the toaster going all the way down?, and the solenoid clean inside, any crumbs stopping it reaching the bottom.

 

I agree with the sentiment of checking the mechanicals before digging deeper into the electronics.

That said, I notice the capacitor leaning towards the jumper between D2 and Q1. It could just be the picture but the cap looks awfully close to touching, which of course would be a problem.

Replacing the two electrolytic caps is not a crazy move. They do age, and are a common failure point, especially in hot applications. I'd look for replacements with high temperature ratings.