Electronics > Repair
Miele elp165-s motherboard after tny278GN replacement not starting
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PiotrMisiuna:
I'm braking my head over this one for weeks now. Maybe someone had similar case.
Miele washing machine main board. They come in many different flavors and most of their PS are build around some version of TNY.
I replaced the chip as well as the 120Ω resistor, measured every single component on the board and nothing seams to be out off.
TNY in this configuration is driven without an optocouppler,
I get mains passing filters to the main DC cap
TNY produces internal 5v so I assume is good.
on the output of the transformer after rectification I get around 1V. only 0.3v gets to the pin 1 of TNY
I checked with thermal camera, nothing gets hot
I applied 12v to the KF50, draws 0.025A, regulator works fine giving 5V on the output.
Any idea what can cause it to work this way?
timeandfrequency:
Hello PiotrMisiuna,
I had a look to the TNY278 datasheet and application note.
IMO, PI devised a really complicated chip :
- EN/UV (pin nr 1) has dual function
- BP/M pin (pin nr 2) has 3 simultaneous functions :P
Among 75 pages of documentation, the sole clue about an implementation without an optocoupler is Figure 28.
Miele probably designed his own schematic around the TNY278 without an opto on the BOM : 20 Cents saved per board :clap:
This would be more than a day harship to fully understand how this chip works and what are every possible reasons that avoid the full start, or entering into protection or hiccup mode.
--- Quote from: PiotrMisiuna on December 12, 2024, 09:06:47 pm ---TNY produces internal 5v so I assume is good.
--- End quote ---
Not sure : internal regulator (pin 2) should provide 5,85 VDC and below 4,9 VDC it triggers shutdown.
If you mesure 5 V, it's weird.
Q1) Check again the voltage at pin 2
Q2) Try to mesure VBias (see Figure 28)
Q3) Can you probe the drain (pin 4) with an isolated scope or differential high voltage probe to check if the IC hiccups ? (normal mode : switching around 130 kHz)
Q4) Did you check if the board behaves differently when inserted in the appliance ?
Q5) Is the UL339 (LM339) somewhat involved in monitoring/controlling the TNY278 power supply ?
(follow the tracks to see how it is interconnected)
Q6) Are there parts on the other side of the PCB ?
Q7) How many windings does the ferrite transformer have ?
Q8) Did you ring it ?
I'm wondering if would not be easier to bypass the TNY278 power supply by using an external 12V SMPS.
Q9) Can you try to check if the appliance works when feeding the board with external 12 VDC ?
Using a lab power supply to perform the test would also allow to mesure the current drawn. Table 16 in the AN says 21,8 W max for the TNY278. A 2 A SMPS should be sufficient.
Some external 12 VDC 2 A SMPS :
https://www.reichelt.de/de/en/plug-in-switching-adaptor-24-w-12-v-2-0-a-hnp-24-120l6-p177030.html
https://www.reichelt.de/de/en/switching-power-supply-25-2-w-2-1-a-12-v-snt-rs-25-12-p137085.html
PiotrMisiuna:
I'm trying to reverse engineer the schematic but I'm a novice. The figure in the datasheet you pointed It is most probable application here, but I still don't see the resemblance in the pcb, It
I will post here anything meaningful.
LM339 seams to only be related to the signals from other components.
When inserted into device it remains in the same state. This model has a soft switch and I took it into account. It seams to me, thou the tny278 psu circuit wold be required to run for the micro controller to receive the signal through an optocouppler. (bottom left).
The only scope I have is a joke - DSO shell, it is isolated, but I have zero confidence in it. I didn't made a ring tester yet. Before Digging into the transformer is a bit intimidating for me, I probably will do it eventually. First I want to learn as much from it as possible.
Nothing on the other side. I made some better photos of both sides, I will post it with my schematic
Bypassing this psu is indeed what I consider as a final solution.
I do not rush with this repair.
timeandfrequency:
Hello PiotrMisiuna,
--- Quote from: PiotrMisiuna on January 04, 2025, 09:17:16 am ---
When inserted into device it remains in the same state. This model has a soft switch and I took it into account. It seams to me, thou the tny278 psu circuit wold be required to run for the micro controller to receive the signal through an optocouppler. (bottom left).
--- End quote ---
It is reasonable to think that this PS is always active, just waiting to wake up the microcontroller.
Great, continue posting at your own pace, we're not in a hurry.
Yes, there are two optocoplers at the bottom of the board : possibly PC813 or PC817.
As you rather often dig into live circuits repairs, using a truly isolated oscilloscope would definitely keep you on the safe side, and this is far preferable than a vanilla scope + high voltage differential probe.
Unforunately, such TE is really expensive and good second hand stuff is not easy to procure. So, it's truly an investment for safety and work efficiency.
The less expensive without any serious flaws are probably those from Siglent. They come with two isolated probes (*).
(*) Probably something similar to this one.
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