Malfunctioning Popcorn Machine (burning resistor issue)

[maker_ye9txivy_1663341611]

Hello everyone

My name is Dustin and i'm new to eletronics repair so this is a learning experience for me.

I have a popcorn machine that was made by "vintage appliances" when you plug it up and turn the power switch on the 1k ohm 2w resistor (circled in red) starts to burn, indicating that it is recieving too much power?

I removed a zener diode (circled in yellow) tested on and off the board it is sending power both directions wich if i understand means its faulty?

But I'm failing to wrap my head around how the power flows on this board.. could it be just that zener diode that is causing my problem? I feel like im missing something else. This thing plugs into 120v wall outlet but most of these parts operate at much lower voltage or wattage.. What is it that lowers the power on this board so that it doesnt melt everything..

Thank you for any helpful information!

 

[Tha fios agaibh]

Think current instead of Power.

A zener will pass current in either direction.
Best way to test is with a variable power supply and a limiting resistor in series.
You could alternately use a 9v battery and about 1k ohm resistor. Then measure the voltage across the zener with a volt meter. It's probably a 5.3v diode. Which your meter should show. If much higher or lower it's bad.
You can also check the resistance in both directions. If it almost 0 ohms in both directions it's bad.

Also check the rectifier diodes D1, D2.

The biggest voltage drop in this circuit is across capacitor C6.

It is normal for that 1k resistor to generate a bit of heat but obviously not smoke.

Good luck.

 

[Bluejets]

1k ohm 2w resistor

Red Blk Blk looks like 20R to me.....OR brown blk Blk..... 10R.
Either way, it's a fusible resistor intended to protect the circuit.
If it's burning there will be some kind of short or excess power draw.
Looks like a fairly crude power supply, non-isolating.

 

[maker_ye9txivy_1663341611]

Thank you both for the helpful advice I will do some tests and report back what I find out!

 

[maker_ye9txivy_1663341611]

OK so D1 & D2 I get .950 only one direction, changing the leads around I get nothing.

The Zener got 9.5v from a 9v battery with a 1k ohm resistor but when using the diode setting on my meter I get .013 and it beeps like a short and this happens either way you place the leads.

Also C6, big yellow capacitor? When the machine is plugged in this has 120v across it.. Is that normal?

Thanks again!

 

[Bluejets]

The Zener got 9.5v from a 9v battery with a 1k ohm resistor but when using the diode setting on my meter I get .013 and it beeps like a short and this happens either way you place the leads.

Suggest you do a Google on how to test a zener diode with battery.

The low reading on diode test on the meter, zener appears as though it is faulty.

Capacitor will be mains rated, possibly X2, could have that voltage BUT not the way to test it.
Lift one leg and test with a capacitor meter.

Diodes likewise. Readings can be incorrect if left in circuit.

If you are going to work on electronic fault finding, best to invest $30 on a component tester from one of the online suppliers.

 

[maker_ye9txivy_1663341611]

So I replaced the components in green because they were testing as faulty when I took them off the board and tested them..

Now the machine comes on and works properly when you push the button on the board to turn it on.

BUT when left plugged in and not running, the 1k ohm 2w resistor is still heating up and trying to burn.. Any suggestions?

How can I figure out which way the current is flowing across this resistor? I feel like if I knew which way it was going I could trace back and and find what is putting out so much current.

 

[maker_ye9txivy_1663341611]

As far as I am able to tell the 2 big caps C6 and C7 are functioning in spec.. Removed from board and tested, I'm getting 6 watts (0.05 amps) across the resistor that is burning up.. Can someone tell me if my resistor is just too small or am I missing a short somewhere?

Could I safely link 3 resistors in series so they would handle the wattage without burning up? Or would that burn everything else up?

 

[Bluejets]

Only 1k resistor I see on your board is the LED resistor, no problem there.
Couple of parts which are a bit vague are just how you are testing the caps which you say are "functioning in spec"
Resistors in series do not increase the wattage handling of a resistor.
Other parts of the circuit do not "put out large amounts of current".
Circuits draw what is required to operate and if excessive , it will be due to a fault.

Did you test the 0.1uF yellow capacitor with a capacitor tester?
They are x2 mains cap and responsible (in a non-isolated supply) for working like a mains resistance due to their capacitive reactance.
In my experience they are responsible for (at least the beginning of ) faults in this type of supply.

Once again, these are non-isolated power supplies and extremely dangerous to work on live.

 

[maker_ye9txivy_1663341611]

I used the capacitor testing function on my multimeter to test the capacitance of the capacitors.

Thank you for the information I'm not familiar with the term non-isolated but I will Google it, this is my first attempt at any kind of circuit repair..

 

[maker_ye9txivy_1663341611]

Fixed it.. Works perfect now.. And no more burning resistors lol

 

[Bluejets]

Going by the value of your 4 series resistors, I wouldn't bet on it being fixed.

 

[maker_ye9txivy_1663341611]

An explanation as to why you think that, would be helpful.

 

[Bluejets]

1 x 10 or 20r is not equal to 4 x 1k in series or 4kOhm.

 

[maker_ye9txivy_1663341611]

1 x 10 or 20r is not equal to 4 x 1k in series or 4kOhm.

Your explanation doesn't make sense, 1 x 10 = 10 & 1 x 20 = 20 that is correct.

But where does that have anything to do with what I have.

Those are indeed 1k ohm 2watt resistors I have stacked, Not 10 or 20.

 

[Bluejets]

Lets see if the penny drops.........
Brown black black gold on original is 10 ohm....... (10r) ... (1 x 10r)

Your arrangement is 4 series 1k resistors (4 x 1k0) ....total 4k0 or to make it clearer 4,000r or 4,000 ohm (thats four thousand ohm)

 

[maker_ye9txivy_1663341611]

Those lines were not originally both black, they are red and black, the reason they look like 2 black lines is because the resistor is burned.

Originally it had a 1k ohm 2watt resistor there which burned up & shorted out I replaced that resistor only and found there were several other components on the board that were not working correctly.

I have replaced those components but still the resistor was burning up.

I failed to determine why it is burning up.

My guess is that somehow C6 is passing too much current which burned up the risitor which burned everything else up when it shorted.

I really have no idea I'm just guessing at this point because like I said this is my first time working on a circuit board.

Anyway my solution might not be the current one but it did stop my resistor from overheating and the machine pops popcorn currently again.

If you know how to help me fix it correctly please instruct me so I can learn from this experience.

Thank you

P. S. The attached photo is the original resistor before it burned up the first time. Taken from a video of someone working on the exact same popcorn maker, but he wasn't having the same problem as I was his resistor was not burning up.

 

[Bluejets]

Those lines were not originally both black, they are red and black

Nope...look again.