Need help with Samsung NZ64H57477K Induction Hob repair

[wraper]

Did you attach the load after replacing the diode?

[flattermann1822]

Just curious what the pinout on the LVR you soldered in was.

TO220, more see attachment

Did you attach the load after replacing the diode?

You mean the original load (system PCBs)? Yes i did attached it. But hob does not seem to work again. I did not connect sensors and coils at this time.

[flattermann1822]

I did some further test with the desoldered LDO. In my opinion it still works.

[Shock]

This is what I would do. I'd not connect the actual load until the supply looked functional (it may be damaging the supply). The resistors however, as long as low current is the safer route.

I'd replace the LVR and order a couple of the proper LDOs (as they fail in stupid ways).
Resolder that transformer connection closest to the 5V input. Clean up the solder splashes and flux.

I'd decide it if it was worth physically isolating the 5V and 12V from the feedback/error circuit and make sure it's perfect by pulling and checking the small caps for capacitance and DC resistance.

With the 5V regulator and the 12V dual diodes out I'd measure resistance between the 5V input and ground. Swap the leads over and measure ground to 5V input. Then do the same on the 5V output to ground (both ways) and then check the 12V to ground in both ways. If the feedback/error circuit is isolated as well this should build confidence there is no weird stuff going on in the secondary (and you can always apply a DC voltage source to the PCB and test it).

Then recheck the previously replaced diode and test the dual diodes.
At this stage if nothing conclusive I'd install the new LDO and dual diode again and power the supply and take another scope measurement unloaded and loaded with a small current.

If still getting 6V on the output of the 5V LDO with a small load I'd check the Y suppression caps between the primary and secondary. Then lastly start checking the components in the feedback/error circuit.

An ESR meter (even a cheap one) would be handy to know how bad things were on those replaced caps.

[flattermann1822]

This is what I would do. I'd not connect the actual load until the supply looked functional (it may be damaging the supply). The resistors however, as long as low current is the safer route.

I'd replace the LVR and order a couple of the proper LDOs (as they fail in stupid ways).
Resolder that transformer connection closest to the 5V input. Clean up the solder splashes and flux.

I'd decide it if it was worth physically isolating the 5V and 12V from the feedback/error circuit and make sure it's perfect by pulling and checking the small caps for capacitance and DC resistance.

With the 5V regulator and the 12V dual diodes out I'd measure resistance between the 5V input and ground. Swap the leads over and measure ground to 5V input. Then do the same on the 5V output to ground (both ways) and then check the 12V to ground in both ways. If the feedback/error circuit is isolated as well this should build confidence there is no weird stuff going on in the secondary (and you can always apply a DC voltage source to the PCB and test it).

Then recheck the previously replaced diode and test the dual diodes.
At this stage if nothing conclusive I'd install the new LDO and dual diode again and power the supply and take another scope measurement unloaded and loaded with a small current.

If still getting 6V on the output of the 5V LDO with a small load I'd check the Y suppression caps between the primary and secondary. Then lastly start checking the components in the feedback/error circuit.

An ESR meter (even a cheap one) would be handy to know how bad things were on those replaced caps.

Thank you for your reply. i will order new LDOs, a cheap ESR meter and try to work through the points.

[wraper]

I don't think there is any issue with LDO. The problem was with crappy measurement, not IC.

[Shock]

I don't think there is any issue with LDO. The problem was with crappy measurement, not IC.

I don't think the LDO is necessarily going to fix it. It could be the 5V input is voltage/ripple, or current sensitive and cheap to get a couple and use one as a replacement.

The question is what shorted the diode. It appears from the oscilloscope measurement it's switching at around 8KHz with current issues.

But anyway continue on Wrapper with what you think should be done next, I was only suggesting what I'd try.

[flattermann1822]

I'd replace the LVR and order a couple of the proper LDOs (as they fail in stupid ways).
Resolder that transformer connection closest to the 5V input. Clean up the solder splashes and flux.

I resoldered transformer connection. Clean up the pcb and have some new LDOs here

Then recheck the previously replaced diode and test the dual diodes.
...
An ESR meter (even a cheap one) would be handy to know how bad things were on those replaced caps.

dual Diodes (DS01) and DS2 are fine and I have a cheap ESR Meter here.

With the 5V regulator and the 12V dual diodes out I'd measure resistance between the 5V input and ground. Swap the leads over and measure ground to 5V input. Then do the same on the 5V output to ground (both ways) and then check the 12V to ground in both ways. If the feedback/error circuit is isolated as well this should build confidence there is no weird stuff going on in the secondary (and you can always apply a DC voltage source to the PCB and test it).

Did you mean that I now desolder all caps, RS05,RS08 and RS11 to check that there is high resistance between 12V/GND and 5V/GND?

I tried to create the circuit diagram of the secondary. Not sure if TR1 and ICS02 have the correct symbols. At the moment ICS01 and DS01 are not mounted.

[Shock]

Last diagnosis was it looks to be either the circuits being pulled down or still suffering from a regulation issue. The 6V out of a 5V regulator is a concern because between ground and out should read 5V.

By measuring between the rails and ground you can determine if a low resistance path exists which could be loading the output. The other idea is to isolate the transformer and feedback circuit and inject DC or AC to check the regulation, in a similar way that you did it out of circuit. Or check component by component. When the regulators or diodes are out it allows you to easily measure the input side of the regulator to ground or the output side to ground.

To put it clearer, not a lot of components in that circuit should be drawing sizable current to ground, only the regulator which has to do it in order to lower the voltage. Aside from the diodes it's just the resistors in the feedback circuit.

Check the Y suppression caps (marked CY01 CY02) as well.

Confirm DC regulation is perfect, confirm secondary components not applying adverse load or affecting feedback, confirm optocoupler is functioning correctly and affecting switcher. Confirm switcher and flyback is performing within spec.

[flattermann1822]

So I think the 6V never exist. My scope showed a DC-offset which is not real. The waveform below shows the actual measurement

[Shock]

Assumed you put the load back on both outputs for that reading as the 12V is showing pulled down or weak. So the 5V is actually low in that case. Same deal if you want to check everything on the output.

Please also pull and measure the current sense resistor on the primary RP03 - 0.7 ohms 1%, then do the snubber resistor RP01 - 4.7k Ohms 5%? If I have read those correctly, which sounds about right.

Your multimeter might have problems with the first one as it's a low resistance so you could try the ESR tester. If those resistors drift it can cause output issues.

[larsmtb]

Hello,

I've got the same induction hob (NZ64H57477K) and it won't power on too. Unfortunately I don't own an oscilloscope but a quick measurement with my multimeter showed 0V at the 5V output voltage of the power supply pcb and 10V at the 12V output. But everything without load.

@flatterman1822 did you successfully fixed your hob?

Greetings from Germany

[flattermann1822]

no, i didnt

[larsmtb]

Too bad! What else have you tried?
I put a load to the 5V output but also no voltage and no current. The 12V output collapses under load.

PS: Grüße 

[larsmtb]

So I replaced the diode with this one:
https://www.reichelt.de/schottkydiode-100-v-2-a-do-41-stps-2h100-p95386.html?&trstct=pos_0&nbc=1

and CS03 with this one:
https://www.reichelt.de/elko-radial-100-uf-35-v-105-c-low-esr-aec-q200-rad-fc-100-35-p84619.html?search=rad+fc+100%2F35

and now everything is okay! I've got clean 5V and 12V at the output and after putting it back into the Induction Hob, everything is fine.