LG LST5651SW stove: oven not heating properly

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

Thanks for all the help and hanging in there! It took a while, but I learned some basics. If I had to do it over, or next time, this would go way quicker.

I'm pretty confident it's something on the main board. I'm not gonna start trying to replace components on the board. A replacement board is $300 -- not worth it.

Cheers!

[petehall347]

Re: LG LST5651SW stove: oven not heating properly

could q205 be bad ? i know transistors generally short e to c but on the odd occasion its b to e short .

[R_J]

Re: LG LST5651SW stove: oven not heating properly

Checked in place. Both it and the one above are 1.98. No signs of damage.

Oh well it was worth a try.

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

After a closer look Check that resistor R206 just in case it is open. It is just between J109 and R205
Maybe this resistor is open?
It looks like the same value as R205
Alternately you could check the voltage on both sides of R206

Checked resistance in place. Both it and the one above are 1.98 ohms. No signs of damage.

[R_J]

Re: LG LST5651SW stove: oven not heating properly

After a closer look Check that resistor R206 just in case it is open. It is just between J109 and R205
Maybe this resistor is open?
It looks like the same value as R205
Alternately you could check the voltage on both sides of R206

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

So Q205 is being turned ON but Q36 is not, the signal on J206 never goes high (4,83)
Unless something else is preventing the microprocessor to output a high to Q36, maybe some safety feature we don't know about?
I would try and contact L.G. maybe they will replace the board, being that you are so disappointed with there product and would not....

So that's the problem found? I'll try getting in touch with LG. Otherwise, it would mean replacing that main board?

[R_J]

Re: LG LST5651SW stove: oven not heating properly

So Q205 is being turned ON but Q36 is not, the signal on J206 never goes high (4,83)
Unless something else is preventing the microprocessor to output a high to Q36, maybe some safety feature we don't know about?
I would try and contact L.G. maybe they will replace the board, being that you are so disappointed with there product and would not....

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

You can install the board back into the stove, then if you look at the second part of the picture, there are two jumper wires on the board, these have the logic from the microprocessor and are directly connected to the two transistor bases.
The voltage on these two jumpers will be low, around 1~5 vdc, so all you need to do is place your meter ground on the small heat sink and check the two jumpers J206 & J202, note the voltage on each (this is done with the oven in bake mode)

Between clicks (I guess the relay opening and closing) they were alternately both 0.00 or

J202 4.83
J206 0.88

Tested from the component side, not the solder side, during the first five minutes of the BAKE preheat. The 0.00 periods were much longer, maybe 30 seconds or more, than the periods lasting maybe 15 seconds that showed current.

[R_J]

Re: LG LST5651SW stove: oven not heating properly

You can install the board back into the stove, then if you look at the second part of the picture, there are two jumper wires on the board, these have the logic from the microprocessor and are directly connected to the two transistor bases.
The voltage on these two jumpers will be low, around 1~5 vdc, so all you need to do is place your meter ground on the small heat sink and check the two jumpers J206 & J202, note the voltage on each (this is done with the oven in bake mode)

[redwire]

Re: LG LST5651SW stove: oven not heating properly

With the stove powered up, the BAKE heating element turned on, put your multimeter on DC Volts and (-) probe touching the heatsink ground (or its pads), and (+) probe each of the blue and yellow marked spots in the picture in Post #64.

You can access these points from either side of the board. I thought the solder-side was the front with the LED display and not normally accessible. It's safest if the board is mounted and does not move around flipped down.

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

If you like you can check the logic that controls the two transistors, The main control board can be installed and you can access the two test points I marked in Blue J206 and Yellow J202. These two test points are the base of Q206 and Q36 which are directly controlled by the microprocessor ic (probe carefully) I would guess a voltage of between 1~5v for a high.
The small heat sink can be used for your meter ground

In the photo, I see the yellow and blue lines and the ground. Not sure how to test. The stove is live, oven set to bake preheat? Testing from the solder side. Where do I position the probes, one on one of the ground points and the other to each of the yellow and blue solder points?

[R_J]

Re: LG LST5651SW stove: oven not heating properly

The board can be tested when it is mounted in place, I marked the two test points so it would be easy and safe to test.
I'm sure the op skill level can handle checking two voltages on the control board as he already checked the three voltages on the relay board. The relay board also has hazardous voltages which did not seem to be an issue

[redwire]

Re: LG LST5651SW stove: oven not heating properly

It might not be easy and OP's skill level might be a concern, so I did not suggest it.
The oven control board has hazardous live wires and parts, and I'm not sure if you can use the keypad to active BAKE and then somehow probe the PC board face. Can you operate it with the board folded down?
I use wood/cardboard to prop up a board away from any metal (chassis) so there are no short circuits and kaboom.

There are some parts and jumpers on the backside which could instead be probed (they connect to the transistors). Like jumpers J202/J160, J206, R206 DCV readings would tell us everything.

Q206 might be suspect but again, we don't know its marking code or Q36 as well.

[R_J]

Re: LG LST5651SW stove: oven not heating properly

You didn't check the transistor base voltages which would have confirmed the fault is with the main microprocessor and not just a faulty transistor.

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

To summarize:

1. Oven heater element: continuity and resistance OK and to spec; visual inspection showed no flaws.

2. Sensor continuity and resistance OK and to spec.

3. Relay board: no 240 current from bake relay (live), diodes OK.

4. CN8 to CN7 wires from relay board to main control board continuity OK. Wires and solders in general look OK.

5. Main board: transistor checks, 1 OK, 1 inconclusive.

As it stands, stovetop works, oven works to about 360 heated by broil element alone.

I think that covers it. What's the conclusion, unrepairable main board problem?

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

Sometimes there is a slimy oxide layer that makes readings tough or wander.
On Diode-Test, when multimeter leads are not touching anything, should see "OL" or "1---" or "OPEN"
On Diode-Test, when multimeter leads are shorted together, should see "0.00"
So I'm a bit confused about your 0.00 readings

Thanks. Learning as I go:

In diode test mode, based on the color-coded drawing above:

Top transistor (in photo)

PINK 0.65 OL
BLUE OL OL
GREEN 0.65 OL

Bottom transistor

BLUE 0.88 OL
PINK 0.98 OL
GREEN 0.87 0.55

[redwire]

Re: LG LST5651SW stove: oven not heating properly

Sometimes there is a slimy oxide layer that makes readings tough or wander.
On Diode-Test, when multimeter leads are not touching anything, should see "OL" or "1---" or "OPEN"
On Diode-Test, when multimeter leads are shorted together, should see "0.00"
So I'm a bit confused about your 0.00 readings

Transistor Q206 I don't know it's number (and there is no base resistor which might be a design flaw) so I can't confirm the readings you have. It could be a cracked resistor R206 (1000Ω?) you could measure ohms across it. Transistor Q36 readings look OK.

Beyond this point, I would say there is a bad connection (cable/connector/transistors) cracked soldering joint to look for using an eye magnifier.
Or the microcontroller output line is not working (which means bite the bullet and replace the oven control board).
LG had the recall on these boards turning on the range top elements, you could talk to an LG tech and see if you qualify for a board swap.

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

OK, if you can take some multimeter Diode-Test readings (power off) on the oven control board to see if the two (bake) transistors look to be reasonable.
Each transistor has three pins, so that is three readings across each pair, but you also need to flip the test lead polarity so that is another three readings, total six per transistor.
I tried to draw that in different colours, so you know which two pins to take a reading on, and then reverse the test leads and take that reading as well.
If you can do that and write down and post the results here, we can see if the board has an easy fix. Beyond any bad solder joints, these two transistors are the only replaceable parts on the control board right now that I know of.

Wild guess I would expect the green readings (emitter-base) to be wonky.
Still need a magnifier and read the numbers on the little parts too.

Not sure of these readings. Had to move the probes around to get anything, wasn't just touching the solders and catch a reading. I did manage to get all of the readings below more than once.

In diode test mode, based on the color-coded drawing above:

Top transistor (in photo)

PINK 0.65 0.00
BLUE 0.00 0.00
GREEN 0.65 0.00

Bottom transistor

BLUE 0.88 0.00
PINK 0.98 0.00
GREEN 0.87 0.55

[Curious808]

Re: LG LST5651SW stove: oven not heating properly

Continuity test OK for all 6 wires in relay CN8 to main board CN 7.

[R_J]

Re: LG LST5651SW stove: oven not heating properly

No Sabbath mode: long press BROIL does nothing.

START is oven lockout, it's marked on the pad.

Long-pressed other main controls. Ones that did something:

BAKE -> Adj
COOK & WARM -> Opt

This option is to change the clock from 12H to 24H (press 1 to change, START to accept)

If you like you can check the logic that controls the two transistors, The main control board can be installed and you can access the two test points I marked in Blue J206 and Yellow J202. These two test points are the base of Q206 and Q36 which are directly controlled by the microprocessor ic (probe carefully) I would guess a voltage of between 1~5v for a high.
The small heat sink can be used for your meter ground